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Musings of an Energy Nerd

Cut-and-Cobble Insulation

Does it ever make sense to cut rigid foam into strips and insert the strips between your studs or rafters?

Image 1 of 4
This cut-and-cobble job was performed from the exterior. The rigid foam is recycled polyiso reclaimed from a commercial roof. [Photo credit: Michael Brahmey]
Image Credit: Michael Brahmey
This cut-and-cobble job was performed from the exterior. The rigid foam is recycled polyiso reclaimed from a commercial roof. [Photo credit: Michael Brahmey]
Image Credit: Michael Brahmey
The rafters of this cathedral ceiling were completely filled with XPS, using the stack-of-pancakes method.
Image Credit: DC
A neat cut-and-cobble job using XPS foam.
Image Credit: www.diychatroom.com
GBA reader Erich Riesenberg installed at least 4 inches of polystyrene insulation in his rafter bays. He reported that the work took him "2.5 months of 4 to 6 hour days to complete."
Image Credit: Erich Riesenberg

Here at GBA, readers regularly ask about the best way to install rigid foam insulation between studs or rafters. A typical question might go like this: “I’d like to insulate between my studs with strips of 2-inch-thick polyiso. I plan to cut the rigid foam pieces a little bit loose, and seal the edges of the polyiso with canned spray foam. Will this work?”

Here’s my standard answer: “If you want to insulate your walls with rigid foam, you shouldn’t cut the foam into thin strips. Instead, you should keep the sheets of foam intact and install the foam as a continuous layer on the exterior side of your wall sheathing. That way, the foam will interrupt thermal bridging through the studs.”

Sometimes, I also point out: “Although the method you suggest, informally known as ‘cut-and-cobble,’ is often used by homeowners, it is such fussy, time-consuming work that it is never used by insulation contractors.” (The first person I heard use the term “cut-and-cobble” was Dana Dorsett, a regular contributor to the Q&A column on GBA. Dorsett first used the phrase in a web forum post in April 2012. “I’m not entirely sure if I was the first to use that term in a rigid insulation context, but I might be,” Dorsett told me.)

Time for a confession

If I had to summarize the theme underlying my cut-and-cobble advice, it would probably be, “Don’t do it.” But this advice leaves me feeling somewhat guilty. It’s time to come clean, and, like a newcomer at a 12-step meeting, announce: “My name is Martin, and I have cut and cobbled.”

Yes, I’ve done it — for the same reasons that lots of other people have done it. Sometimes, cut-and-cobble makes sense.

Cut-and-cobble is cheaper than spray foam

Cut-and-cobble is a well-established method of insulating rim joists. When used for insulating above-grade walls, the cut-and-cobble method is usually chosen by homeowners or owner/builders who are leery of fiberglass batt insulation. Although fiberglass insulation is inexpensive, it is air-permeable, hard to install well, and attractive to mice. Homeowners who prefer to install foam insulation have three choices:

  • Hire a spray foam contractor;
  • Purchase a two-component spray foam kit at a building supply store; or
  • Install strips of rigid foam using the cut-and-cobble method.

Cut-and-cobble can be used in a way similar to flash-and-batt (by installing a layer of rigid foam against the wall sheathing, and filling the rest of the stud bay with fluffy insulation); or cut-and-cobble can be used to fill the entire stud bay with rigid foam (an approach that I call the “stack of pancakes” method).

Cut-and-cobble has some disadvantages:

  • To address thermal bridging, it’s always better to put the rigid foam on the exterior side of the sheathing. However, in an existing house, homeowners are rarely willing to demolish their siding just to install a layer of rigid foam.
  • This insulation method is very time-consuming. If you want to do a good job, it will take longer than any other insulation method.
  • Wood framing expands and contracts with changing humidity levels, raising the possibility that attempts to seal the perimeter of the rigid foam (whether with caulk, spray foam, or tape) will fail over time. Anecdotal evidence suggest that this danger is real, especially for cut-and-cobble cathedral ceilings. (A GBA reader recently posted an account of a cut-and-cobble roof insulation job gone wrong. Because of air leaks through cut-and-cobble cracks, the reader’s flat roof is now raining condensation. Another report of a failed cut-and-cobble roof assembly is provided by building scientist Kohta Ueno in Comment #7, posted below.)

In spite of these disadvantages, cut-and-cobble sometimes makes sense. It can be used:

  • When a homeowner can’t afford the cost of a spray foam contractor or a two-component spray foam kit. (As Richard Briede posted on the Q&A page here at GBA, “Please help with a poor man’s insulation technique. I know closed-cell spray foam would be the best for this application, but I simply can’t afford it.”)
  • When a homeowner has access to leftover or recycled pieces of inexpensive rigid foam.
  • When a homeowner is leery of the “lingering odor” risk associated with spray foam.
  • When a homeowner or builder wants to create a WRB in the stud bays of an older house that lacks sheathing.
  • When a homeowner wants to tackle a large job in phases, and can’t manage to have everything ready for a spray foam crew all at once.
  • When sheathing is too cold to be used as a substrate for spray foam.
  • When a homeowner wants to insulate an unvented cathedral ceiling without using spray foam or adding exterior rigid foam. (Note that this approach only applies to homeowners who are willing to disregard reports of problems that arise when the cut-and-cobble method is used for cathedral ceilings.)

Cut-and-cobble, step by step

If you want to use the cut-and-cobble method to install rigid foam in your walls or ceilings, what are the necessary steps?

  • The first step is to calculate the minimum necessary thickness of your rigid foam. Here is a link to an article that tells you how to figure this out: Calculating the Minimum Thickness of Rigid Foam Sheathing. Note that minimum foam R-values are generally higher for cathedral ceilings than for walls. (You can skip this first step if you don’t plan to install any fiberglass insulation, but intend instead to use the stack-of-pancakes method.)
  • The second step is to choose which type of rigid foam to use. Polyisocyanurate is considered the most environmentally friendly foam. If you plan to use the cut-and-cobble method to insulate an unvented cathedral ceiling, some experts say that EPS or fiber-faced polyiso are preferable to foil-faced polyiso or XPS, because EPS and fiber-faced polyiso will allow the roof sheathing to dry (very slowly) to the interior if necessary.
  • The third step is to decide whether to push the foam as far as possible toward the exterior side of the framing cavity — the method that is usually used for walls with solid sheathing or for unvented cathedral ceilings — or whether you want to leave a gap on the exterior side of the framing bay — the method that is used for walls that lack sheathing or for vented cathedral ceilings. If you prefer to have a gap on the exterior side of the foam, you’ll probably want to install 1″x1″ or 1.5″x1.5″ sticks at the corners of each framing bay to maintain the desired gap.
  • Most cut-and-cobble veterans suggest that it makes more sense to cut the rigid foam for a loose and sloppy fit rather than for a tight, perfect fit — especially if you plan to seal the perimeter of the rigid foam with canned spray foam. After all, the spray foam nozzle needs a gap of at least 1/8 inch for proper insertion. Others, however — especially those who don’t want to use canned spray foam — advise cutting rigid foam on a table saw and aiming for a tight fit.
  • The last step is to seal the perimeter of each piece of foam to prevent air leaks. Most cut-and-cobble veterans prefer canned spray foam for this step, but if your cracks are small, caulk will work. There is some evidence that canned spray foam does not create an airtight seal; if you are a fanatic for air sealing, you may prefer to invest in a European tape to seal these cracks.

A variation on cut-and-cobble

What if you want to fill your stud bays all the way full with spray foam, using a two-component spray foam kit? (This kits are available at building supply outlets for $300 to $600.) If you do a little math, you’ll realize that the price of these kits is so high that they are really too expensive to fill stud bays; that’s why they are usually used for air sealing, not insulating.

Some homeowners have figured out, however, that if they can fill most of the volume of their stud bays with rigid foam scraps, the amount of spray foam required to fill the remainder of the stud bays is quite small. If you tack up the scraps in the stud bays like jigsaw puzzle pieces, all you have to do is seal the edges of the rigid foam and then install a skim coat of foam on top of everything. I hereby dub this method the “peanut brittle” method.

The peanut-brittle insulation method is similar to the old hippie trick of throwing some big stones into foundation forms while the Ready-Mix truck is discharging its load of concrete. It’s a way of stretching expensive store-bought material with something cheap.

What does the building code say about cut-and-cobble for cathedral ceilings?

The 2009 International Residential Code (IRC) provided rules for creating an unvented insulated sloped roof assembly. Unfortunately, the language was flawed.

This type of insulated sloped roof assembly occurs in at least two locations: cathedral ceilings and unvented conditioned attics. The language used in section R806.4 of the 2009 IRC clearly applied to unvented attics, but never addressed cathedral ceilings.

The section of the 2009 IRC that can be interpreted as supporting the use of cut-and-cobble is section R806.4.5.3: “Air-impermeable and air-permeable insulation. The air-impermeable insulation [e.g., rigid foam] shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation [e.g., fiberglass batts] shall be installed directly under the air-impermeable insulation.” The table calls for a minimum of R-5 foam for Climate Zones 1-3, R-10 for Climate Zone 4C, R-15 for Climate Zones 4A and 4B, R-20 for Climate Zone 5, R-25 for Climate Zone 6, R-30 for Climate Zone 7, and R-35 for Climate Zone 8.

In the 2012 version of the IRC, the language was corrected to include cathedral ceilings. The relevant section of the code (Section R806.5 of the 2012 IRC) reads, “Unvented attic assemblies (spaces between the top-story ceiling joists and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters [otherwise known as cathedral ceilings]) shall be permitted if all the following conditions are met…” The required minimum R-values for the foam layer haven’t changed.

Thermal bridging still matters

Before I end this article, I’d like to come full circle, and return to the advice I gave at the beginning. Now that we’ve explored all of the reasons why someone might use the cut-and-cobble method, it’s important to emphasize that thermal bridging still matters.

So if you possibly can, try to install a continuous layer of rigid insulation on the exterior side of your wall sheathing. Exterior insulation is better than cut-and-cobble in all respects.

Martin Holladay’s previous blog: “All About Embodied Energy.”

Click here to follow Martin Holladay on Twitter.

121 Comments

  1. User avater
    Greg Labbe | | #1

    Cut'n Cobble
    I love the name! I've seen this done and its rarely done well. The best was seeing a basement framed out in steel studs with lose board-stock XTPS roughly placed between steel members... Sadly, it was an engineer who prided himself on his DIY job.

    Thanks for the chuckle!

  2. User avater
    Armando Cobo | | #2

    Options
    Cut-and-cobble do makes sense in many remodeling applications. When the cost of removing an existing cladding is too high (e.g., brick, stucco, etc.) or when Historical Districts do not allow for any cladding removal to install rigid foam on the outside of the wall.
    Installing a ½”+ spacer between the sheathing and rigid foam creates a “venting” cavity that allows the sheathing to dry to the outside. One could install a permeable (e.g., wool) insulation in the cavity and continuous rigid foam on the inside of the wall, allowing the wall to dry to the outside.

  3. Nick T - 6A (MN) | | #3

    alternatives interior?
    Lets say you have a unfinished walkout basement (like myself)- where the above grade areas are 2x6 FG insulated and poly'd. I had contemplated bumping up insulation with a Mooney wall and cut/cobble between with foam.

    But more and more i am reading of success stories of using XPS over studs, and then drywall right over the top. from sites like contractortalk. Just would need extended boxes for elec.

    Thoughts on beefing up this code minimum wall without the wife causing a ruckus? ($$$) (we'll just say she doesn't know about all the LED's on our main level) LOL

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Nick T
    Nick,
    You forgot to tell us where you live. Unless you live very far north, the polyethylene is a mistake.

    Since you describe your basement as "unfinished," I assume that your walls have no drywall. That means that your polyethylene is also a fire hazard.

    The first step will probably be to remove the polyethylene. If you want your walls to perform well, the next step will be to remove the fiberglass batts and put them in a dumpster. That will give you access to the back of the stud bays so that you can perform air sealing work. Once that's done, there are a great many ways you can proceed to insulate your walls.

    If you aren't willing to remove and dispose of the fiberglass batts, you'll have to accept all of the performance problems that come with the batts.

    It is possible to install interior rigid foam if you want. The disadvantage of this approach is that it doesn't address the "cold sheathing" problem. If you have well-ventilated siding (vinyl siding or wood lap siding installed over a rainscreen gap), the approach is less risky than if you have poorly ventilated siding (for example, stucco).

  5. Seth Rutledge | | #5

    option for floor joist retrofit
    I am considering this option for between my floor joists. Fiberglass would be pretty much useless considering air flow, and spray foam is very expensive and might require a thermal shield; and going over the joists is impossible because of all the wires and pipes.

    Of course most foam would also require a thermal shield, but if I can get my hands on this Thermax stuff of a descent price (1) then I might be cutting and cobbling.

    Regarding the devilish details: it would be nice to spray some foam into the bay and then squish the 2' thick pieces into it and tack them in place ensuring an air seal and contact with the underside of the floor. I have not found any spray foam kits with adhesive qualities for the application however.

    So far the plan is to put a bead of construction adhesive around the perimeter of the foam pieces, and nail them in place with a framing nailer and plywood chunks, then go back with spray foam cans and hit the cracks, or maybe I'll hit the corners with foam first then push them into place.

    That part that I am dreading the most is spray foaming the edges. I have used a couple of the "professional" hand-held guns and they never want to spray when angled up, and it will never flow through the bendable tube either. I dread having to buy a zillion cans of great stuff.

    Does anyone have product recommendations for this?

    (1)http://building.dow.com/na/en/products/insulation/thermaxsheathing.htm

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Seth Rutledge
    Seth,
    Most hardware stores sell clear vinyl tubing in a variety of diameters. Buy some tubing that will slip over the plastic dispenser tube that comes with the canned spray foam cans you intend to use. Then you can keep your cans upside down (so they dispense well) and still deliver the caulk (through the vinyl tubing) to the location you want.

    The tubing is fairly cheap, so you can dispose of the length you are using if it gets clogged.

  7. Kohta Ueno | | #7

    More Cut & Cobble Problems
    Martin--to add another data point to potential risks and failures, I looked at a cut-and-cobble roof job done by some dear friends of mine. They are located in Central MA (Zone 5A), and built a sloped unvented roof assembly using multiple layers of recycled EPS in the rafter bays (with expanding foam sealant at the perimeter), with a layer of foil-faced polyisocyanurate detailed as an air barrier under the rafters. Unfortunately, during their first winter, they started experiencing condensation dripping from the ridge. The patterns were all consistent with moisture migration up to the ridge of the attic, at the air channels formed around the foam (analogous to the "ridge rot" seen in SIPS panels from BSI-036/http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks).

    Ripping the assembly apart and redoing it with spray foam was not on the table. The half-measure that I suggested was to add a "vapor diffusion vent" (a strip of Cosella Dorken Delta-Foxx), replacing the self-adhered membrane (vapor impermable) at the ridge. I have installed a moisture content monitoring system to keep an eye on things. So far, it seems like there's a rise in moisture content in the winter, but it mostly comes down in the summer. There are variations in moisture content--the "simpler" roof bays are drier than the more "complex" ones (i.e., dormer rafter framing). This is consistent with air leakage at the more complicated framing details. I'm going to be keeping an eye on moisture contents through this winter.

    Unfortunately, this monitoring is not an ideal experiment--the "right" way to do this experiment would have been to leave a "control" bay with the failed assembly. That would have determined the effect of the diffusion vent--whether that's "saving" things. But an experiment like that is not something to do to your friends! :)

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Kohta Ueno
    Kohta,
    Thanks very much for sharing your story of a problematic cut-and-cobble cathedral ceiling. I have edited my bullet list of "disadvantages" to include your anecdote.

  9. Kohta Ueno | | #9

    Response to Seth Rutledge (Dow Insta Stik)

    Regarding the devilish details: it would be nice to spray some foam into the bay and then squish the 2' thick pieces into it and tack them in place ensuring an air seal and contact with the underside of the floor. I have not found any spray foam kits with adhesive qualities for the application however.

    I think that Martin has handled the air sealing side of the equation quite well. But if you want an option for using spray foam as an adhesive for rigid foam, I would strongly recommend the use of Dow INSTA STIK™ Quik Set Commercial Roofing Adhesive (http://building.dow.com/na/en/products/adhesives/instastik.htm). It is a single component urethane foam that you would apply to the back side of the rigid foam; it is intended for adhered commercial (flat) roofs (check out their installation videos). We have used it to adhere foam to masonry walls (also forming an air seal to the cavity between foam and substrate, if done correctly). You apply Insta-Stik to the rigid foam, press the rigid foam into place, hold it for about 30 seconds, and it "grips like cold death" (the product rep's words, but my personal experience corroborates this observation). ;)

    For examples of the masonry installation, see Building America Report 1302: Retrofit of a Multi-family Mass Masonry Building in New England (http://www.buildingscience.com/documents/bareports/ba-1302-retrofit-multifamily-mass-masonry-building-new-england/), at Figure 24: "Test installation of foam on masonry (left) and use of polyurethane adhesive (right)"

  10. Nick T - 6A (MN) | | #10

    Follow up with Martin
    Martin,

    thanks for the encouragement. On going for it. I am just north of Minneapolis, MN. Vapor barrier fire risk? Never heard that being an issue - pretty much all walkout basements in MN are insulated and vapor barriered if unfinished and left bare.

    I will be hard pressed to push the cost of reinsulating our walls - as my wife would tell me "would most people just put up sheetrock and be done with it??..." my response...."aaah, ya, i guess..doesn't make it right....."

    the hundreds if not thousands of dollars needed to insulate that wall with foam board or densepack Cel. is a energy retrofit that likely won't find traction. To replace a "perfectly good wall".... and of course we know that is a 'perfectly' wrong statement.

    So as always, my goal with any energy savings be to make a safe system that has good bang for the buck. What can be done to improve the basement (maybe 'pretty fair house' standard lol) so instead of a semi leaky code built wall...(r-20 wall) to something better?

    So far i have already IR camera'd on a cold night to help highlight weak spots (followed up with spray foam through the poly) and added cheap Fiber R30 over the existing 2" sprayfoamed rim joist.

    Most of the retrofits talked about on here are often all or nothing and higher in cost than I see doing on a regular basis unfortunately. I guess I'll keep fighting the fight! Slowly adding insulation and some XPS as the budget allows.

  11. Stacey Cordeiro | | #11

    whoa.
    Are you telling me I could insulate the cathedral ceilings in my old timber framed house with a layer of unfaced mineral wool board insulation between the rafters and in contact with the sheathing, beautifully detailed with high quality caulking in every nook and cranny, and then fill the rest of the (furred-inward) cavities with dense pack cellulose, right up to R-60 or near to it? I've been looking for an insulation solution that doesn't involve plastics. Horizontal purlins in the hip roof framing prevent a vented assembly, and the brand new roof prevents exterior rigid insulation. Is this the answer to the question that's been keeping me up at night?

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Stacey Cordeiro
    Stacey,
    Q. "Are you telling me I could insulate the cathedral ceilings with a layer of unfaced mineral wool board insulation between the rafters and in contact with the sheathing?"

    A. No. Mineral wool insulation is air-permeable, so it doesn't prevent warm, humid indoor air from contacting the cold roof sheathing. The insulation system you are suggesting would be a code violation, and it would also result in rotten roof sheathing.

    The term "cut-and-cobble" is applied to a method of installing rigid foam insulation. The term is not applied to mineral wool insulation.

    Note also that I am not recommending the cut-and-cobble approach for cathedral ceilings. That's why I warned, "Wood framing expands and contracts with changing humidity levels, raising the possibility that attempts to seal the perimeter of the rigid foam (whether with caulk, spray foam, or tape) will fail over time. Anecdotal evidence suggest that this danger is real, especially for cut-and-cobble cathedral ceilings. (A GBA reader recently posted an account of a cut-and-cobble roof insulation job gone wrong. Because of air leaks through cut-and-cobble cracks, the reader's flat roof is now raining condensation. Another report of a failed cut-and-cobble roof assembly is provided by building scientist Kohta Ueno in Comment #7.)"

    For information on code-approved ways to insulate a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.

  13. Tom Ruben | | #13

    I decided to use cut and
    I decided to use cut and cobble in a couple of small wall sections behind two minisplit units that I hung prior to completing the walls. I wanted the structure warm while I work and sleep on site. The conduit to the units does not run neatly, so I decided to create a base of foam behind the pipes and will fill around the pipes with loose fill or spray. Certainly not my preferred method of insulation but in very small doses tolerable.

  14. Seth Rutledge | | #14

    thermax adhesive
    I looked into hat foam adhesive and Dow says that it's not reccomended for Thermax panels. I guess if you have used it for that application and it worked then it might ok anyways?

    The adhesives that they recommend are here, and none of them appear to be foam :(
    http://dow-styrofoam.custhelp.com/app/answers/detail/a_id/3329/kw/foam%20adhesive%20thermax/session/L3RpbWUvMTM4NTY1MTk4Ny9zaWQva3RQNXd1R2w%3D

  15. Kohta Ueno | | #15

    Re: thermax adhesive

    I looked into hat foam adhesive and Dow says that it's not reccomended for Thermax panels. I guess if you have used it for that application and it worked then it might ok anyways?

    Sorry, Seth--didn't realize that you were exclusively thinking of Thermax... and I didn't realize Dow didn't consider them compatible. In our work, we used Insta-Stik with XPS.

  16. Roger Anthony | | #16

    Insulating walls and ceilings
    52 years ago, more or less, Dow brought Styrofoam to market, I read about people using it to insulate potato stores, deep freezers and boats, and thought this is an ideal product to insulate a home.

    10 years later I bought my first house, gutted it, lined the walls and ceilings with inch thick Styrofoam covered with water vapor proof plastic sheet; finished with drywall and had a warm dry home. To help things along I laid fibreglass between the joists and much later over the joists.

    The key thing, is facing the warm wet air inside the home with a warm surface. Water vapor always heads for the nearest cold surface to condense. Providing you only give it surfaces that are above “dew point” it behaves itself and usually condenses on the nearest cold window. Water vapor proof plastic sheet backed by Styrofoam provides a continuous warm surface, any cracks behind it in the insulation are insignificant. The usual minute holes in walls etc; allow any water vapor that does get past the plastic and insulation to escape into the colder sky, maintaining the integrity of the building.

    Since then, I have moved on, these days I add up to six inches of polystyrene sheet between the joists, sealed with canned foam; and two or three inches of polyurethane sheet on the room side.

  17. Hermann Thoene | | #17

    Continuous Rigid Insulation on Inside?
    I'm currently building a log home on Vancouver Island (north of Seattle, moist but fairly mild and moderate climate), where we plan to insulate the walls from the inside. The outside walls are "structural" with solid 2 1/2" thick manufactured T&G boards. I'm planning to install following wall assembly (from outside to inside):
    - 2 1/2" solid log wall, "failry" airtight
    - Tyvek as air barrier attached to the inside of that log wall in case the outside wall has some air leakage
    - 2 layers of 2" polyiso boards (staggered seems), fastened to the outside walls by vertical 1"x2" strapping
    - 3/4" thick interior wall panelling attached to the vertical strapping

    This assembly avoids thermal bridges and should be fairly easy to install. But the rigid insulation is on the inside, in contrast to all above suggestions (due to maintaining the outside look of the logs).

    The question I have about this: Do I run any danger of getting warm moist air causing condensation problems? I will use lag screws to attach the vertical strapping through the insulation to the outside log walls... And how well does the insulation need to be sealed in the corners and at seams?

    /Hermann

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to Hermann Thoene
    Hermann,
    As long as you pay attention to airtightness when assembling your wall, it should perform well. Of course, you still need a plan for insulating your rim joists.

  19. User avater GBA Editor
    Patrick McCombe | | #19

    My own experience with cut and cobble
    I have a lot of experience with this method of insulation. I think the pros and cons spelled out above are spot on. But if you're looking for an air-tight envelope on a rock-bottom budget, it's a viable option.
    This describes my project and my experience insulating with salvaged polyiso.
    http://www.finehomebuilding.com/item/23460/patricks-barn-insulating-with-garbage

    I used a modified siding nailer to temporarily hold the insulation in the bays while I spray foamed the perimeter.
    http://www.finehomebuilding.com/item/22229/patricks-barn-wrapping-up-the-exterior

  20. Robert Kohaus | | #20

    Cut and Cobble with damaged EPS
    I live in the midwest and we just had some severe weather (maybe you heard). Good news for me is no one was hurt. Bad news for me is that about $10k worth of ICF blocks got damaged to the point where I can't use them for my foundation walls anymore.

    So rather than throw them away, I was thinking about this "cut and cobble" concept. So this blog is very serendipitous!

    I was thinking of trying to use it in my vented attic where I had originally been planning blown cellulose to ~R-60. My idea would be to cut pieces of it to fit between the trusses against the ceiling. With extra emphasis up against the heel. I thought that I wouldn't really need to worry about air sealing the cracks/gaps between pieces because 1.) the cellulose would (hopefully) settle into all the gaps/cracks and 2.) the air barrier will be the drywall ceiling with no ductwork or lights penetrating.

    Am I missing something here?

  21. User avater GBA Editor
    Martin Holladay | | #21

    Response to Robert Kohaus
    Robert,
    You can't depend on cellulose to significantly limit air movement unless it is fairly deep (maybe 10 or more inches above the rigid foam scraps). Cellulose is cheap, and I don't see any advantages to mixing EPS scraps with your cellulose -- and there are several possible disadvantages (for example, the chunks of EPS might prevent the cellulose from reaching the nooks and crannies where you want it, and your local building inspector or fire marshal might be unhappy if any EPS was poking up).

    I would stick with 100% cellulose in the attic if I were you.

  22. Robert Kohaus | | #22

    Cut and Cobble with damaged EPS
    I'll post a new question concerning this topic. Thanks Martin.

  23. Dave Frank | | #23

    Separating from framing?
    Thanks for the great article! I'm considering this method in a cathedral ceiling.

    One of the disadvantages mentioned is that the expansion and contraction of wood framing may cause the seal to fail. Why is this any different than a "regular" closed-cell spray foam application? Does a full rafter bay of spray foam not run the same risk of separating from the framing?

  24. User avater GBA Editor
    Martin Holladay | | #24

    Response to Dave Frank
    Dave,
    To be honest, we don't yet have a full answer to your question. Suffice it to say that an increasing number of researchers are focusing on the problem of damp roof sheathing above roofs that have been insulated with spray foam. As you might imagine, these problems are more likely if the roof was insulated with open-cell foam than if the roof was insulated with closed-cell foam.

    Many cases have been reported where the cured spray foam has shrunk away from the rafters. These cases are concerning. The cases are rare, however. They are usually blamed on installer error.

    In general, I think that you are much more likely to get an airtight and problem-free installation with closed-cell spray polyurethane foam than with the cut-and-cobble method.

  25. Dave Frank | | #25

    Another option?
    In another discussion on this site, installing full sheets of rigid foam on the INTERIOR side of the rafters (not between them) was mentioned. That would seem to avoid all of the cut-n-cobble disadvantages listed above, as long as you can live with the drop in ceiling height.

    In my case, I'm looking to foam a large unfinished attic for (a) its air seal advantages and (b) to keep HVAC equipment within the envelope. But I'm put off by the expense and odor/health questions. Am I missing something or do you agree that such an application is a great solution? Is there any reason the air gap (the full volume of the rafter bays) between the insulation and sheathing would be undesirable?

    A brand new shingle roof was already installed, so exterior installation is not an option, but this seems just as good.

    Thanks!

  26. Todd Sherman | | #26

    What if there was venting
    From Kohta Ueno's comments above with an unvented assembly, what would of happened if there was a vent channel 1" or 1.5" from soffit to ridge. Wouldn't that have solved the issues and made it that the sheathing can remain dry and prevent rot?

  27. User avater GBA Editor
    Martin Holladay | | #27

    Response to Dave Frank
    Dave,
    If you want to leave your rafter bays uninsulated, and install a thick layer (or multiple layers) of uninterrupted rigid foam under your rafters, there is no reason why such an installation won't work.

    Of course, it's important to list the usual caveats:
    Pay attention to airtightness when installing the rigid foam. Use an appropriate tape at the seams.
    Multiple layers of rigid foam with staggered seams are always better than one layer.
    Make sure that the R-value of your assembly meets or exceeds minimum code requirements.
    Don't forget to install a layer of gypsum wallboard to meet fire safety requirements.

  28. User avater GBA Editor
    Martin Holladay | | #28

    Response to Todd Sherman
    Todd,
    Researchers haven't yet studied cut-and-cobble jobs, so we don't have much data on this type of roof assembly.

    If I had to speculate, I would guess that a vented cathedral ceiling with cut-and-cobble insulation would probably be less risky and an unvented one. But I'm guessing.

    Remember, a vent channel between the top of the ventilation and the underside of the sheathing isn't a miraculous solution, and it can be hard to create such a channel (especially in roofs with complicated geometry, hips, valleys, dormers, skylights, or chimneys). For more information on this issue, see All About Attic Venting.

  29. David Tontarski | | #29

    Cut & Cobble
    I'm in the planning stages for a large addition on my seasonal residence, which I plan to retire to. My original plan was to install two 1" layers of staggered seam polyiso on the exterior and to install 5.5" of cellulose in the wall cavities, but this doesn't achieve the R-40 wall target I had in mind. Time and labor are not issues, as I plan to complete all of the work myself over about a 5-year period. Would there be any drawbacks/issues with filling the wall cavities with 4-5.5" of polysio using the cut and cobble method, along with the two 1" layers of polyiso on the exterior?

  30. User avater GBA Editor
    Martin Holladay | | #30

    Response to David Tontarski
    David,
    I don't recommend that you encapsulate your OSB or plywood wall sheathing by sealing both sides with vapor-impermeable rigid foam. It's best if your wall sheathing can dry in at least one direction. I think that you should stick with cellulose between your studs.

    Your proposed wall assembly has an R-value of about R-30, which isn't bad (as long as the house is located in Climate Zone 6 or somewhere warmer; in Zones 7 and 8, you'll need a minimum of R-15 of rigid foam).

    If you really want R-40, the best way to get there is to increase the thickness of your exterior rigid foam; I suggest that you install 2 layers of 2-inch-thick foam, for a total of 4 inches of exterior rigid foam.

  31. Rick Van Handel | | #31

    Cut & cobble with cellulose as crack filler?
    I've been wondering about the viability of cutting rigid foam panels 1/2" undersized on all sides and then using dense blown cellulose as the gap filler. It would seem to mitigate the threat of framing members shrinking or moving with humidity, at the expense of some air sealing. I'm thinking of using cut and cobble for a heated woodworking shop, so I could do proper air sealing from the outside as this would be new construction.

    My own wall build is planned as follows. Please feel free to comment.

    24" o.c framing with 2"x8" studs. Ribbed metal siding fastened to 3/4" horizontal nailers, wrb over 1/2" continuous plywood sheathing, 6" of cut and cobble foam (hoping to use polyiso or xps). Interior walls to have Mooney wall strapping, so there would be 3" cellulose (1.5" from foam to stud face + 1.5" Mooney wall area. Untapped or sealed plywood for interior sheathing.

    Because foam would be left undersized
    , the gap areas would have 9" cellulose with outside air sealing only.

  32. User avater GBA Editor
    Martin Holladay | | #32

    Response to Rick Van Handel
    Rick,
    The idea behind a cut-and-cobble installation (or a flash-and-batt installation) is to prevent warm, moist interior air from contacting the cold wall sheathing. The layer of foam is an air barrier that separates the cold sheathing from the warm indoor air.

    You can't use cellulose insulation to seal the gaps around the edges of your rigid foam, because cellulose insulation is air-permeable. It isn't an air barrier.

    So, stick with canned spray foam (or, if the cracks are tiny, caulk or high-quality tape) to seal these cracks.

  33. Erich Riesenberg | | #33

    cut and cobble compared with spray foam
    I think Dave Frank makes two good points, 1) is adhesion a potential problem for both cut and cobble and spray foam and 2) covering the bays with large foam boards, not just filling between the bays.

    I have been able to fit 5 inches between bays and plan to cover the bays as well. I think expanded polystyrene works well because it can be cut close to the full width of the bay and then "shoved" into place, so that it is held by friction and also sealed with Great Stuff Pro and silicone caulk. Some has been in place for close to a full year with no adhesion problem. This is 2 layers, 2 inches each, and then a third layer of 1 inch EPS. Then at least one more full panel across the bays.

    I am glad to have a project to work on during the long winter months.

  34. Chris Thomas | | #34

    Old Home with No Sheathing
    "When a homeowner or builder wants to create a WRB in the stud bays of an older house that lacks sheathing."

    Is this the best practice when insulating with no sheathing? I was set on using vertical strips of XPS (to create a little bit of a ventilation gap), Roxul, and a 'smart' vapor retarder.

    Also, if I need a vapor retarder (climate zone 4, yes) how do you install it on a balloon frame? Past the floor, cut and tape around each floor joist? I was concerned about conditioned air from the crawl entering the wall cavity behind the drywall.

    Thanks!

  35. User avater GBA Editor
    Martin Holladay | | #35

    Response to Chris Thomas
    Chris,
    Yes, I think that the cut-and-cobble approach is often a good method of insulating stud bays in a house with no wall sheathing.

    Q. "I was set on using vertical strips of XPS (to create a little bit of a ventilation gap), Roxul, and a 'smart' vapor retarder."

    A. I don't advise your approach, because you will have no air barrier on the exterior side of your insulation. Roxul is air-permeable, and (with your approach) the performance of the insulation will be degraded by wind-washing.

    Q. "if I need a vapor retarder (climate zone 4, yes) how do you install it?"

    A. Perhaps the easiest way to install a vapor retarder is to install vapor-retarder paint or primer on the drywall layer.

    Q. "I was concerned about conditioned air from the crawl entering the wall cavity behind the drywall."

    A. The solution to this problem is air sealing, not installing a vapor barrier. (The purpose of a vapor barrier is to address vapor diffusion.) If air is entering your stud bays from the crawl space, you probably need to seal the bottom of each stud bay with spray foam before you insulate your stud bays. I suggest that you use a two-component spray polyurethane kit.

  36. Chris Thomas | | #36

    Clarification
    Great point on the wind-washing. Do you recommend setting the foam back with furring strips?

    I'm also a bit confused by your last response. My stud bay goes past the subfloor and to the sill plate. Are you saying to spray foam in between the floor joists that enter the stud bay?

    Sorry for my need for clarification. Thanks!

  37. User avater GBA Editor
    Martin Holladay | | #37

    Response to Chris Thomas
    Chris,
    Q. "Do you recommend setting the foam back with furring strips?"

    A. Yes. Aim for an airtight installation of the rigid foam, but leave a little air gap between the rigid foam and the siding.

    Q. "My stud bay goes past the subfloor and to the sill plate. Are you saying to spray foam in between the floor joists that enter the stud bay?"

    A. It's hard to know the best way to seal the air leaks in this location without looking at it, but clearly these areas need to be sealed. It may be possible for you to install block of rigid foam in each bay, and then to seal above the rigid foam with spray foam. No matter what materials you use, the goal is to prevent any crawl space air or exterior air from entering the bottom of the stud bays.

    Moreover, you also need to make sure that the rim joist is insulated on the interior with foam insulation. You can use rigid foam or spray foam -- it's your choice -- but you don't want to leave the rim joist uninsulated.

  38. Sal Lombardo | | #38

    Question re: response to Dave Frank
    Martin replied to Dave;
    "If you want to leave your rafter bays uninsulated, and install a thick layer (or multiple layers) of uninterrupted rigid foam under your rafters, there is no reason why such an installation won't work.
    Of course, it's important to list the usual caveats:
    caveats ensued

    My question Martin is, by installing a layer (or multiple) of rigid foam under the rafters, doesn't this prevent drying of the assembly to the interior? My understanding is one must always allow for drying of the assembly to the interior or exterior or both dependent on the design of the wall. Rigid foam (more so multiple layers) will likely impede drying to the interior (uncertain of the perm rating and effect of multiple layers on this variable). Would exterior drying be thwarted by tar paper and asphalt roofing shingles. Does the fact its not a heated space play significant role here? Thanks

  39. User avater GBA Editor
    Martin Holladay | | #39

    Response to Sal Lombardo
    Sal,
    Many roof assemblies can't dry in either direction -- for example, an unvented roof assembly with a vapor-impermeable type of roofing that has been insulated with closed-cell spray foam installed against the underside of the roof sheathing.

    As long as the rafters and roof sheathing are dry when these assemblies are insulated, I don't think that this type of roof assembly is particularly risky. But some builders, like you, prefer the roof assembly to be able to dry in at least one direction. I understand the logic, and you are free to design a roof assembly that dries to the interior if such an assembly seems less risky to you.

  40. Nat Hilton | | #40

    I'm currently doing this cut
    I'm currently doing this cut and cobble method. I have 3/4 inch behind studs and putting 2" bwtn stud. It says on the 3/4" that it is R4 and 2" is R10. Will this be enough in zone 3, atlanta suburbs? I looked at the zone map and R requirements, I think the minimum is R13. Is this correct?

  41. User avater GBA Editor
    Martin Holladay | | #41

    Response to Nat Hilton
    Nat,
    Although you didn't mention what type of wall you are working on, I'm guessing (from information posted in a comment on a different page) that you are working on insulating the interior of a basement wall. Is that right?

    According to the 2009 International Residential Code, the minimum R-value for basement walls in your climate is R-5 if the insulation is continuous (in other words, not interrupted by studs) or R-13 if the insulation is interrupted by studs.

    If you are installing R-4 continuous insulation -- that's the insulation between the concrete wall and the studs -- you don't quite meet minimum code requirements for continuous insulation. That forces you to try to meet the R-13 minimum requirement for insulation between studs. So R-4 continuous plus R-10 should satisfy your code inspector.

  42. Nat Hilton | | #42

    Thanks Martin,
    Yes it's me

    Thanks Martin,

    Yes it's me again. I'm just trying to post questions in the right forum. ;) none of the rigid foam are continuous b/c the stud were already up. So, R-4 is not continuous . We had to put behind studs. A lot of work, but that part is done. Now we are putting up the 2" rigid foam, tight fit, then can spray foam (gap n crack) around edges.

    Thanks again

  43. Mark Hays | | #43

    Cut-and-Cobble worked well for this major remodel
    We are nearly finished with a to-the-studs remodel of a 1950-era home on the Cape in Massachusetts. It was originally built with no insulation, so a complete upgrade was needed.

    Major work started in November, when temps were too low to safely spray foam. With many changes planned to window and door openings, we also needed an incremental solution for insulation that we could install wall by wall. Cut-and-cobble was the answer. We cut XPS panels to fit each stud bay space, and carefully sealed the edges with the 'windows' version of Great Stuff, which remains flexible. This should prevent cracks as the assembly expands and contracts. A couple of photos are attached. With two layers -- 2" and 1" -- we could nearly fill each bay for a rough R value of 15 for the interior walls.

    To boost insulation, reduce thermal bridging and block condensation on the sheathing, we also installed new housewrap over the exterior, covered by 2" of XPS foam board carefully sealed with Great Stuff. All of the panel joints and edges around window / door openings were also covered with a thick layer of flexible DAP 230 sealant. (For panel joints, empty a few tubes of the DAP sealant into a paint tray and apply it with a 3" paint roller. With pole extensions, it is easy to reach high up on your walls without a ladder.) A photo is attached.

    In short, the cut-and-cobble approach took more time, but matched the rest of our upgrades and project schedule. We ended up with ~R25 wall assemblies and a tight air seal. The next step: 8" of closed cell spray foam on the underside of the roof, which will create a 'conditioned space' attic.

    One key tip: I discovered that Great Stuff Fireblock is actually very flammable, and ignites at just 240 degrees F. We used Great Stuff Windows to seal our XPS panels -- but switched back to standard fireblocking caulk to fill and airseal holes in studs and plates. This non-flammable caulk is required for commercial buildings, and -- in my view -- is the best fireblocking solution for residential structures too.

    I hope this is helpful.

    Mark

  44. A. Bradford | | #44

    hybrid venting
    I am not sure I read anything clearly on the following method, but many posts have alluded to this:

    When insulating an attic roof or cathedral ceiling in the cut-and-cobble manner, it has been recommended to add a cold air channel between the insulation and the sheathing. This was mentioned a few times even in unvented attic situations. Is there a benefit to leaving an gap when there is no ridge vent?

    In our personal situation, we have a hip roof on a 1929 house in a zone 5b. The soffits are as good as open, although there is no technical vent there. Wood blocking was installed originally to slow airflow, but these could be pulled out with rigid foam installed. We have no ridge vent, however. If we installed blocking to the sheathing for the initial layer of rigid foam, wouldn't there be sufficient airflow the soffit to allow some kind of drying? Of course, even if this does work, it is only a backup, as the interior insulation will be air sealed.
    I am asking if a hybrid method of putting an air vent channel in an unvented assembly has any merit, as a backup in case of failure of the insulation assembly to fully block air and condensation?

    This cut-and-cobble method was recommended to me by both an insulation contractor and energy auditor. The insulation contractor also suggested this "peanut brittle" method of spraying an inch of closed cell foam over the cut rigid, to properly air seal it. I am very interested in avoiding spray, but will do whatever is required to make our situation work. We have access to both 3" and 1" polyiso at an excellent price.

  45. User avater GBA Editor
    Martin Holladay | | #45

    Response to A. Bradford
    A. Bradford,
    Assuming you have a gable roof (and not a hipped roof that looks like a pyramid), it's quite easy to install (retrofit) a ridge vent on a roof that doesn't have one. I recommend that you do so if you are planning to build a vented roof assembly.

    If you don't know how to do the work, any roofer should be able to do it for you.

  46. A. Bradford | | #46

    hybrid venting
    Martin,

    That's the problem- we have a hip roof. I would happily have a vent installed otherwise.

  47. User avater GBA Editor
    Martin Holladay | | #47

    Response to A. Bradford
    A. Bradford,
    As I wrote in my article, How to Build an Insulated Cathedral Ceiling, not all roofs are good candidates for the vented-roof option. If you have a hip roof rather than a gable roof, you need to go with an unvented roof assembly.

    In that article, I wrote, "A vented cathedral ceiling only makes sense if the geometry of your roof is simple. You need a straight shot from the soffits to the ridge. That’s relatively easy on a gable roof without any dormers or skylights, but if the geometry of your roof is complicated — with features like hips, valleys, and dormers — it’s impossible to assure air flow through all of your rafter bays."

  48. A. Bradford | | #48

    hybrid venting
    I recall reading this. I have resigned myself to the fact our roof will be unvented.
    When I read this article, I saw the following:
    "The third step is to decide whether to push the foam as far as possible toward the exterior side of the framing cavity — the method that is usually used for walls with solid sheathing or for unvented cathedral ceilings — or whether you want to leave a gap on the exterior side of the framing bay — the method that is used for walls that lack sheathing or for vented cathedral ceilings. If you prefer to have a gap on the exterior side of the foam, you’ll probably want to install 1"x1" or 1.5"x1.5" sticks at the corners of each framing bay to maintain the desired gap."
    I was curious about the function of leaving a gap between the insulation and sheathing on an unvented roof assembly, given that I am considering a risky method of cut-and-cobble. Even though there aren't proper vents, would this method allow potential condensation to escape via the soffit? Or do you only recommend a gap in fully vented assemblies? It wasn't clear in the article.
    Theoretically I will have everything sprayed over after the fact. This was recommended to me by an insulation contractor. You mentioned the 'peanut brittle' method on here but gave no opinion of it.
    An earlier post on this subject had an answer to spray 2'' inches directly on the sheathing, followed by cellulose. R values would be low but the assembly would function properly. Later I could add rigid foam to the exterior to super-insulate it.
    The trouble is, I have access to all sorts of very cheap and clean polyiso in 3" and 1", and I am quite limited in funds. I could accomplish insulating to R-46 with 2 layers of 3" polyiso and then overlay the rafters with 1", all for under $1000 in material. I just want to do a good job, if I am to take this risky method.
    I should mention the building has fairly fresh roofing shingles.

  49. User avater GBA Editor
    Martin Holladay | | #49

    Response to A. Bradford
    A. Bradord,
    Only you can make this decision.

    The right way to build a ventilated roof assembly is to provide a ridge vent at the top of every rafter bay. You can't do that, so you are breaking the rules.

    Some unvented cut-and-cobble cathedral ceiling assemblies have experienced failures, presumably due to air leaks.

    To some extent, you are entering uncharted territory. It's up to you to assess the risk and act on that assessment.

  50. JoAnn Dibeler | | #50

    rigid foam insulation
    Going on 2 years ago I gave up trying to find the leak destroying an addition, long before we bought the house and since the drywall ceiling was about to fall on me I tore the entire addition down to start over. My best friend is a guy who portrayed himself as a master builder. I believed him - 1st mistake. There is so much wrong with his carpentry and roofing that I'm now going around fixing what he got wrong, if at all possible. What we hadn't gotten to yet was the interior. This is my office/bedroom and as soon as I get the electric lines run it will finally be time for the interior paneling. My friend had me cut and cobble the RMatte PLUS-3 in between all rafters and studs. What a nasty job! I was ready to kill him by the time I was done. Last winter it went down to 13 degs here in East Texas so I scrambled to get full sheets of the R-Matte temporarily tacked over the studs on the interior. Since then I've decided to leave them up, better secured, and install the 1/4" paneling over top of them. I'm 69 yrs old and I cannot handle sheets of drywall but I'm afraid the thin paneling will warp without a solid backing This idea engendered a nearly friendship-ending email fight. He said it wouldn't work and I, having looked it up on the website for RMAXINC and they gave instructions how to put it up on top of interior studs, stood my ground. He finally said he'd do it my way and not say anything when it failed. Big surprise - I'm doing it myself, so there. . . Now I come across this article and comments and I'm feeling very deflated! Was my know-it-all friend right all along? This is a pretty big crow to try to eat! (the cut and cobble RMatte is still in the back of the studs right on the siding and caulked around)

  51. User avater GBA Editor
    Martin Holladay | | #51

    Response to JoAnn Dibeler
    JoAnn,
    You've told us a story about your cut-and-cobble job. However, I don't understand your question.

    Can you please re-state your question?

  52. Emil Mackey | | #52

    XPS or EPS?
    I live in Zone 7 (Juneau, Alaska) and this method is intriguing. I was thinking of using this cut-and-cobble method as we remodel our home over the next few years and then flash fill the remainder of the wall cavity with closed-cell spray foam. However, your article raised two questions in my mind.

    #1: Is there a disadvantage to using the cut-and-cobble method to install 4 inches of foam board (two 2" layers with staggered and sealed seams) and then filling the remaining 2" with spray-foam to ensure a seal)? It would seem to me that there would not be a disadvantage as long as I friction-fit the foam board and sealed all edges and the two-layers to eliminate air voids and penetration. Am I missing a consideration other than risk of error?

    #2: All of Alaska is seismically active. Is the added strength of XPS such that it would be superior to EPS in a cavity installation to also add strength and durability to the entire structure? Or do you think I am overthinking this? Heck, what do you prefer in this method regardless, XPS or EPS?

  53. User avater GBA Editor
    Martin Holladay | | #53

    Response to Emil Mackey
    Emil,
    Question #1: The disadvantages of this method are listed in my article. The main disadvantage for walls is that this method is very labor-intensive. The main disadvantage for unvented cathedral ceilings is the possibility that cracks will open up, leading to possible mold and rot.

    #2. I think that you are overthinking this question. I don't think that XPS has any seismic advantages. From an environmental perspective, EPS is preferable to XPS because it is manufactured with a more benign blowing agent.

  54. Brian Gray | | #54

    Cut and cobble versus partial insulation over-roof
    Martin,

    Like many others who have responded to your article, I read your cut and cobble description with a good deal of interest. We have a master BR addition that is currently framed and roofed but yet to be insulated. My wife really wants a cathedral ceiling (we have rafters versus trusses so this would just require removing some of our ceiling joists and reinforcing those that remain). At any rate I've been debating how to insulate the ceiling safely. It has a hip roof complicating venting. Personally, I'm just not comfortable with an unvented cut and cobble in a cathedral ceiling (which you also advise against). It seems like an awfully risky gamble considering there is no way to inspect for ongoing condensation and water damage.

    In another article you wrote (How to Build an Insulated Cathedral Ceiling), I get the impression that insulating the roof deck would be the better/best solution. My hesitation is the amount of work and money required to retrofit the entire roof just to address the needs of one 500 sqft room. I also hate to dump the current system as I spent several painstaking weekends two years ago crawling around our attic air-sealing. I also added R-30 batts throughout (so the attic floor is fairly air tight and well insulated). Our asphalt shingles are ~15 years old. Not new but also not yet ready to be replaced. In short, over-roofing with insulation seems like a drastic and expensive change to address a vaulted ceiling in one room.

    Here is my question. I'm wondering if there is any reason (other than aesthetics) I could not over-roof with insulation strictly over our new master - leaving the rest of the house with a traditional vented attic? We're in a ranch with a relatively simple roof line so it would definitely be noticeable that a portion of our roof was 6" higher/thicker than the rest. However, I would sacrifice aesthetics for the piece of mind knowing that our rafters were not secretly rotting. Further, my guess is the expense, while more than cut and cobble (which I'm not comfortable with), would not be much more expensive than closed cell spray foam (which I am). In short, if I'm going to bite the bullet and spend more than a cut and cobble job, it seems like over-roofing with insulation is the better approach. (the last sentence is actually a question)

    The only real drawback I can see with a partial over-roof approach is the possibility of snickering neighbors laughing at my two-tiered roof line. However, they'll get over my split level roof a lot quicker than I would get over $20K worth of water damage to our rafters. The scope of work is small enough that I could probably handle a lot of the soffit retrofitting myself (another benefit as my guess is roofers will price their bids at a premium given some of this will be a pain in the butt).

    Do you see any drawback to this partial over-roof idea?

    Thank you,
    Brian

  55. User avater GBA Editor
    Martin Holladay | | #55

    Response to Brian Gray
    Brian,
    Q. "I'm wondering if there is any reason (other than aesthetics) I could not over-roof with insulation strictly over our new master - leaving the rest of the house with a traditional vented attic?"

    A. No. Just make sure that you have considered details to make sure that the thermal envelope (the air barrier plus insulation layer) is continuous. Tricky transitions occur at the top of the wall (where the wall meets the roof) and at the kneewalls connecting the cathedral ceiling/roof with the insulation installed on the attic floor.

  56. M Z | | #56

    renovating two walls, suggestions welcome
    Hi, first, please forgive me for the wall of text, I just want to be sure I give as complete a picture as possible. I know some or all of these points have been made already, but I just wanted to be sure I completely understand what’s been said, and am applying the correct logic to my project.

    My situation:

    - In the middle of gutting a kitchen/bathroom each with an exterior wall with all plaster and existing fill removed.
    - Located just outside boston (not on the water)
    - building is multi family, work is being done on the 2nd floor with kitchen/bathroom above and below
    - building is old and very drafty. Originally had zero insulation anywhere. I had cellulose blown in 8 years ago. building is clad in two layers of wood (not sheathing, just boards), then asphalt shingles, then vinyl.
    - I don’t plan to add house wrap or any exterior insulation because the asphalt shingles supposedly have asbestos, so I don’t want to touch that.
    - I removed all the existing cellulose from the two walls (kitchen and bath) during demo.
    - stud bays vary hugely in width (of course! it’s old and badly built). All are 3.5” deep
    - it’s balloon framed, meaning I can see the cellulose stuffed into the stud bays on first and third floors
    - I was going to use the two-part “froth pak” 650 from lowes to fill all the stud bays--that’s too expensive considering I’d likely do a bad job.
    - I was thinking of hiring a contractor to spray (closed cell) foam into the cavities. but 1) my understanding is that it’s too cold in Boston in January and 2) the work is going in stages, so I’m not really able to arrange a single time for the contractor to do all the work
    - anyways, spraying that much foam… I honestly don’t like the idea of releasing that much chemical into an occupied house.

    My plan: stuffing each bay with layers of foam board and spray foaming around the edges (stack-of-pancakes):
    - first 2” XPS against the outside
    - then 1 ⅜ foil-faced polyiso (tuff-r from dow)
    - then foam around edges with great stuff window and door foam (because it will supposedly move with the house--right?).
    - ½” drywall on top. Some sections will be ½” cement backer and tile (around tub, which is up against the outside wall--unfortunate, but no choice here)

    Questions:
    - foam board choices--ok? I’m concerned that the foil face will trap moisture somehow--particularly in the bathroom, particularly behind the cement board, which will be sealed and painted with redguard, which supposedly waterproofs the shower walls. Should I use just XPS/EPS in the bays behind the cement board? In the entire bathroom? Everywhere? I’d like to get the best R-value, but let’s be honest, this house is not going to be tight, EVER, so I’ll settle for what I had before I tore out my walls: relatively warm, with no mold/persistent moisture issues.
    - the tuff-r board is foil-faced on both sides. Dow site claims there’s some difference between the facings, but for the life of me all I can see is one side has writing on it… Am I blind/stupid?
    - assuming the board choices are ok, how about that foam spray. I was planning on just cutting the boards loose in the stud bays (1” gap around edges) and spraying them in with the window/door foam because it’s flexible. Should I go smaller on the gaps, like ½”? Would the “gap filler” foam be a better choice for some reason? The gap filler seems to have some sort of fireblock chemical which I tend to avoid when I can, but let me know if I’m being silly.
    - treating the transition from the existing cellulose in the bays above/below the foam/spray I’m installing. Anything to be careful about? I was just going to stuff the boards up against the cellulose and spray around the edges

    Anything else I am overlooking? I’m new to foam and had started to buy different materials just to get a feel for it when I stumbled on your site. Obviously not the best “plan,” so any help you can provide is so greatly appreciated.

    Thanks and happy new year!

  57. User avater GBA Editor
    Martin Holladay | | #57

    Response to MZ
    MZ,
    1. It's too bad that you removed the cellulose; everything was probably just fine before you emptied the stud bays. (But perhaps you had to do some electrical wiring?)

    2. If you care about the environment, EPS is more environmentally friendly than XPS. So I would suggest that you install EPS instead of XPS.

    3. The size of the gap around the perimeter of each piece of rigid foam isn't too important, as long as the gap is wide enough for you to insert the nozzle of your canned foam dispenser.

    4. Don't worry about trapping moisture. Don't worry about which way to orient each piece of rigid foam. Just pay attention to airtightness -- that's all that matters.

    5. Don't overthink the choice of canned spray foam. Careful workmanship is the most important variable.

  58. M Z | | #58

    @Matrin
    Thanks Martin!

    1. yeeeup, sure wish I had kept that stuff. I couldn't in the bathroom due to some structural issues caused by old (bad) work, but kitchen... dang, should have just left the walls up and covered them with new sheetrock.

    2. I do care about the environment, but I also care about getting a material that works well long-run. My impression of EPS is that since it's open cell, it is prone to developing mold. But now that I think about it, I'm guessing that's only if you create vapor barriers around it, which will not be the case here. I just want to be sure I'm not creating a headache for myself.

    3.Sounds good

    4. Just to be clear: are you saying I shouldn't worry about trapping moisture because I should use EPS which won't trap moisture? Or even with the closed-cell boards I picked up already I shouldn't worry about it. Since I'm new to this, I've been kind of alarmed by all the stories of trapped moisture due to wrong material selection/bad installation. Just want to be clear why I should not worry about trapping moisture. I will follow your advice about EPS, but on the chance that for some reason I can't get EPS boards from my local center, I want to be sure that the materials I've already gotten (and that are stocked to the ceiling at my local HD) are going to work ok for my application in case I have to use them.

    5. Gotcha.

    Thanks very much, Martin! Your article and comments have been incredibly helpful to me!

  59. User avater GBA Editor
    Martin Holladay | | #59

    Response to MZ
    MZ,
    To have a moisture problem in a wall assembly, you need moisture. There are two main directions from which moisture can enter a wall: from the exterior and the interior. (It's also possible that a plumbing pipe in your exterior wall might leak, but you shouldn't have any plumbing pipes in your exterior wall.)

    The way to keep exterior moisture out of your wall is with good siding, good flashing, and a water-resistive barrier (WRB). The best wall assemblies also include a ventilated rainscreen gap between the back of the siding and the WRB.

    If there are flaws in your siding, flashing, and WRB, and your sheathing gets wet, it should be able to dry to the exterior -- unless, of course, the wall has a gross defect, in which case your wall is doomed. In any case, the rigid foam that you plan to install does not affect this drying mechanism.

    The main mechanism by which interior moisture enters a wall assembly is by piggybacking on exfiltrating air. So if you do your best to keep the drywall layer airtight -- which means limiting air leaks at electrical boxes -- you'll stop this mechanism. Again, the foam layers shouldn't cause moisture problems.

  60. M Z | | #60

    @Martin
    Got it--thanks again! I will use EPS with confidence and I'll be very meticulous about the seams.

  61. Greg Philliips | | #61

    cut and cobble wall
    Martin I gutted my 1955 main bath in a brick veneer ranch in windsor heights, iowa. There is a 1 inch space behind brick then celotex like brown board on frame (2x4). Am planning on using stack of pancake method between the studs(closedcell foam boards). Then a 3/4" layer of foam over the studs sealed well every step of the way. This outside wall is 7' long 8'high. And will have tile backer and tile 3/4of the way up. Any issues with this?

  62. User avater GBA Editor
    Martin Holladay | | #62

    Response to Greg Philliips
    Greg,
    If your workmanship is good, there is no reason why your suggested approach won't work.

  63. Andrew Toomajian | | #63

    Poly-iso cobble vs closed cell spray, w/ exterior foam in place
    Martin, this is exactly the discussion I was looking for, so thanks! I'm weighing insulation options for between rafters in a walk in attic space. We're in Western MA in a 100 year old balloon framed house, 2x4" walls (with some vermiculite in the cavities, but that's another story). This past summer we replaced the shingle roof, and in doing so we stripped to decking, and built up with an assembly of 2 layers of 2" foil faced poly-iso, with seams staggered and taped, and then furred up 1" to a new plywood deck and standing seam metal roof to install a PV array.
    I'm now looking into the options for the underside of the roof, and am considering poly-iso over closed cell spray foam, as the cure and offgassing issue is a concern for both my apartment and the rental unit in our two family home. Plus, I'm guessing there's cost benefit, as I'd outsource spray foam to pros but think I could handle the cobble job myself with the help of friends or low-hourly wage assistants. I'm familiar with the methods, already used on a rim joist last winter.
    Rafters are full dimension 2x6, but were sistered with nominal 2x8 for structural reinforcement as part of the roof build. This means I could easily get a 3 pancake stack for 6" of poly-iso inside and 4" outside (r-70?) but I wanted to get input on the impact the 3.5" of wood (from the two sistered rafters) would have on that plan.

  64. Howard Gentler | | #64

    Good discussion. We continue
    Good discussion. We continue to consider insulating a cathedral ceiling. Foam over in a few years is one option. but very expensive as someone else has pointed out. We are also considering working from the inside sooner, and there is lots of food for thought here. We have 14" rafter bays which now have 12" fiberglass batts and t&g boards beneath with no air sealing. There are soffit and ridge vents, but we don't know if there are baffles securing the venting channel. The roof has a couple of dormers, but these are not really part of the cathedral roof and are more conventionally insulated.

    We are conflicted as to how to best do this. We'd like to avoid the expense of cc foam. Any suggestions/advice? Is there are way or sense in reusing the batts in a new format? In any arrangement it seems prudent to do an inch or two of rigid foam beneath the rafters for air sealing and reduction of
    thermal bridging, as well as for increased R. We would then put t&g boards back up.

  65. User avater GBA Editor
    Martin Holladay | | #65

    Response to Howard Gentler
    Howard,
    Here is a link to an article that lays out all of your options: How to Build an Insulated Cathedral Ceiling.

  66. Howard Gentler | | #66

    Cathedral ceiling insulation retrofit...
    Martin and other advisors...I have read your excellent article on cathedral ceilings, more than once and the accompanying links. Sorry my post didn't sound like it. I think I need to ask more specific questions/advice.

    As I mentioned, we are considering the foam over for an insulation retrofit, and I've gotten good info from this site, much of it from Dana. But we are leaning more toward an interior effort now. The foam over would be very expensive (we need R-25, so 5 or 6 inches of foam, depending on type)and a few years off. There are dormers that don't need foam (not cathedral), and the details from the extra thickness, trim and otherwise would be daunting, including the poss of different levels on the roof. (I read a blog by Dr Joe of BSC about doing a foam over on his house and the challenge and expense of making the trim look good).

    If we go unvented, it sounds like a couple of inches of cc foam on roof sheathing underside would be best. We could then return the 12" batts, filling up the rafter bays. Is it then okay to do rigid foam on the underside of the rafters? Like 2" of EPS, carefully taped(this will add some R but is much more for the air sealing), furred over at the rafter undersides as nailers for the t&g boards? Are there concerns?

    If the spray foam is too expensive, my understanding is that 2" of rigid foam could be pushed against the roof sheathing and carefully sealed at the edges with tape, caulk, spray foam or all in combo. Is that correct? I should avoid foil faced poly-iso in this stickup to allow for drying to interior, correct?

    If we were to do vented (vents already there), could that stack up I mentioned above be essentially the same, -starting 2" down with rigid foam, pushed against retainers nailed to the rafters? Then the fiberglass batts, slightly compressed to 10", and then the rigid foam below for air sealing. Or is this not an appropriate stack up with vented? Do you have an opinion on whether we should do the vented or unvented? I know of your concerns with vents not working with complex roofs, but the part that is cathedral is on one end of the house and not impacted by dormers, which are in the center of the house. And, the rigid foam spaced 2" from the deck underside would improve upon the current vent channel and make it consistent.

    Thanks for any advice.

  67. User avater GBA Editor
    Martin Holladay | | #67

    Response to Howard Gentler
    Howard,
    Q. "If we go unvented, it sounds like a couple of inches of closed-cell foam on roof sheathing underside would be best. We could then return the 12-inch batts, filling up the rafter bays. Is it then okay to do rigid foam on the underside of the rafters?"

    A. No. You should re-read my article, "How to Build and Insulated Cathedral Ceiling." The approach you describe is the flash-and-batt approach. If you follow this approach, building codes require you to install a foam layer that meets the minimum R-value requirements spelled out in Table R806.5 of the IRC, and (if I understand you correctly) you need R-25 for this layer in your climate zone. That means that you need about 4 inches of spray foam, not "a couple of inches."

    Q. "My understanding is that 2 inches of rigid foam could be pushed against the roof sheathing and carefully sealed at the edges with tape, caulk, spray foam or all in combo. Is that correct?"

    A. Not quite. I don't recommend the cut-and-cobble approach for unvented cathedral ceilings; this article explains why. (There are reports of failures.) In any case, 2 inches of rigid foam isn't enough for the foam-and-batt approach.

    Q. "If we were to do vented (vents already there), could that stack up I mentioned above be essentially the same, starting 2 inches down with rigid foam, pushed against retainers nailed to the rafters?"

    A. Yes. These details are explained in my article, "How to Build an Insulated Cathedral Ceiling."

    Q. "Is this not an appropriate stack up with vented?"

    A. It will not only work -- the vented approach is better than the unvented approach.

    Q. "Do you have an opinion on whether we should do the vented or unvented?"

    A. Vented is better than unvented -- especially if your unvented approach uses cut-and-cobble insulation.

  68. Howard Gentler | | #68

    Cathedral insulation...
    Thanks Martin for noting my misunderstanding. Keeps us from making that mistake, and I see where you mentioned that in your article (R being the same as what would be needed for an above deck foam over).

    Do you advise using EPS in the two places we would be using rigid foam, - at top as vent baffle, and under rafters for air sealing, as opposed to XPS? And to avoid poly-iso?

  69. User avater GBA Editor
    Martin Holladay | | #69

    Response to Howard Gentler
    Howard,
    Q. "Do you advise using EPS in the two places we would be using rigid foam, - at top as vent baffle, and under rafters for air sealing, as opposed to XPS?"

    A. EPS would be better than XPS, because EPS is more environmentally friendly. XPS is manufactured with blowing agents that have a high global warming potential.

    Q. "And to avoid polyiso?"

    A. Polyiso is relatively benign from an environmental perspective, so you can use polyiso if you want. If you intend to use the rigid foam as a ventilation baffle, it probably makes sense to choose a material for that location that is somewhat vapor-permeable (like EPS or polyiso with a vapor-permeable facing) rather than a foam that is vapor-impermeable (like foil-faced polyiso).

  70. Roman Stankus | | #70

    1915 home insulation upgrade
    Martin, I am working on upgrading the energy performance of the upper (2nd) floor of a bungalow style wood framed wood siding home in lower zone 3 (georgia). The exterior cladding is the original heart pine lap siding without any sheathing. I will be gutting from the interior - exterior lap siding will remain in place. For walls - I am considering options that include cut and cobble with 1" - 2" of rigid EPS or XPS sealed between the studs. Does there need to be a gap between the rigid insul. board and the lap siding? If so - what dimension? The remainder of the 2x4 stud bays would be filled with mineral wool batt insulation. Alternatively, I've considered filling the stud bays with mineral wool batts and sheathing the interior of the studs with eps or xps riigid foam board and overlaying with drywall to create a two layer airtight drywall layer from the inside that would dry to the outside.

    I have a portion of the ceiling that is a normal vented attic and a portion that is a cathedral ceiling. In the normal vented attic areas (flat ceiling but very limited head room accessibility in attic) is there any advantage to sheathing the ceiling with rigid foam board below the ceiling joists infilled with batt insulation and overlaying with drywall as opposed a thicker layer of more inexpensive attic batt insulation (no rigid foam overlay). I'm considering batts because attic is so shallow and want to insulate from below.

    In the cathedral areas - I plan to ventilate the roof sheathing using baffles, then fill the rafter bays with mineral wool batts, then overlay with a layer of rigid foam insulation to get up to R30 and overlay with drywall to create an airtight assemble that is vented above. These cathedral areas vent to the main attic area and then to gable end vents.

    I also have some knee walls that will be infilled with mineral wool batts and drywall from the interior. On teh attic sideof the kneewalls - I am considering a rigid insulation board to assist in air sealing and additional R value. Attic areas on the outside of the kneewalls will be vented.

    Is there a good solution to insulation/airsealing some areas of the first floor ceilings that are painted wood beadboard ceilinsg with a vented attic space above. Currently a minimal amount of old blown in mineral wool insulation is in place. I was considering adding R30 mineral wool batts - but that doesn't address air tightness in the beadboard ceiling.

  71. User avater GBA Editor
    Martin Holladay | | #71

    Response to Roman Stankus
    Roman,
    Q. "Does there need to be a gap between the rigid insul. board and the lap siding?"

    A. Yes. This method is described in the article: "The cut-and-cobble method can be used .... when a homeowner or builder wants to create a WRB in the stud bays of an older house that lacks sheathing." The article also notes, "The third step is to decide ... whether you want to leave a gap on the exterior side of the framing bay — the method that is used for walls that lack sheathing. .... If you prefer to have a gap on the exterior side of the foam, you’ll probably want to install 1"x1" or 1.5"x1.5" sticks at the corners of each framing bay to maintain the desired gap."

    Q. "If so - what dimension?"

    A. The answer was given in the article -- either 1 inch or 1 1/2 inch. (See above.)

    Q. "Is there any advantage to sheathing the ceiling with rigid foam board below the ceiling joists infilled with batt insulation and overlaying with drywall as opposed a thicker layer of more inexpensive attic batt insulation (no rigid foam overlay)? I'm considering batts because attic is so shallow and want to insulate from below."

    A. Either way will work. Adding cellulose on top of the existing insulation is likely to be the cheapest solution, as long as you have enough room in your attic to get the R-value you are looking for.

    Q. "Is there a good solution to insulation/airsealing some areas of the first floor ceilings that are painted wood beadboard ceilings with a vented attic space above?"

    A. There are two possible solutions: (a) Remove the beadboard ceiling, install drywall, tape the drywall, and then re-install the beadboard ceiling, either re-using the old boards or installing new boards; (b) Temporarily remove the insulation above the beadboards, so that the back sides of the beadboards are exposed, and install spray polyurethane foam above the ceiling.

  72. Claython Mclaw | | #72

    Torn over spray foam
    I am building a small post and beam house (<400 ft^2) on a budget. the wall construction will be something similar to "remote system" and "wrap strap" method with recycled foam board insulation, so there an uninterrupted insulation air barrier enclosing structure. but challenge is that i really want have cathedral ceiling. are no perforations through roof or slope interruptions whatsoever--it's just going simple gable design light colored metal roof. my original plan was use true 2x12 rafters with, as you say, "cut cobbled" rigid fit in between then taped (with 2" gap above them for venting), after reading many of your articles this seems extremely risky.

    However, I have the opportunity to have closed cell spray foam sprayed for me at $0.50 a board foot, in which case I would have a few inches sprayed and then meet the required R-value with air-permeable insulation (I'm in climate zone 5). My only reservation is that spray foam seems like it could be a serious health hazard. From what I've read--assuming it is sprayed properly and allowed to off-gas for the right amount of time--it should not cause any issues in the short term, but the long term health effects are still up in the air. It only takes cursory look at history to see that it is riddled with cases of industry assuring consumers that questionable products are safe (and consumer protection agencies condoning such products without proper long term research) only for such products to later be attributed to serious health problems.

    I'm not going to ask you to allay my fears, but supposing I were to use sprayfoam, would there be a way to reliably "seal" it into the rafter bays to reduce the chances of harmful fumes entering the living space? I was thinking of some sort of heavy duty plastic stapled immediately below the rafters (and above the tongue-in-groove ceiling finish), but perhaps this would need to be so much of an air barrier itself that it would be redundant.

    Second, I know the contemporary opinion seems to be that a spray foamed (closed cell) roof does not require roof ventilation, but I wonder how much evidence there is of unvented roofs holding up over the long term. Spray foam hasn't been around for too long and it just seems that while any issue with the air barrier could cause problems, it could be a disaster if coupled with an unvented roof. Are there risks with using closed cell spray foam to insulate a roof and then venting it anyway?

    Thanks

  73. User avater GBA Editor
    Martin Holladay | | #73

    Response to Claython Mclaw
    Claython,
    1. If you have access to recycled rigid foam insulation, why not use the same type of insulation on your roof? (I'm not talking about the cut-and-cobble approach.) Install several continuous layers of rigid foam (with staggered seams) above your roof sheathing, followed by ventilation channels created by laying 2x4s on the flat, then a layer of OSB or plywood, and then your roofing.

    2. If you prefer to install spray foam from the interior, you can, but this approach results in inferior performance, because you won't be addressing thermal bridging through the rafters.

    3. I wouldn't count on any air barrier material to keep out the odors from badly installed spray foam.

    4. Jobs with stinky spray foam are very rare. Moreover, these problems show up immediately or not at all. If there are no odors after 48 hours, there is no reason to believe that there ever will be odors in the future.

    5. No matter what type of foam insulation you choose to install, it's always a good idea to have a vent channel above the insulation layer.

  74. Claython Mclaw | | #74

    Thanks for the suggestions.
    Thanks for the suggestions. I think I will go with your 1. Even assuming I can get spray foam at cost, it is cheaper. However I may see if I can find a metal roof that will fasten to horizontal strapping (overtop the vertical strapping) because it would be expensive to put down a second roof deck (especially since I keep reading that I should use plywood instead of OSB due to screws supposedly not holding well in OSB)

  75. Joe Soda | | #75

    Unvented Attic
    Am I late to the party?

    I live in 3B zone, (Southern California) in a 100 year old house. The house is unvented and with no insulation. The main house gets hot and cold, but it isn't too bad. The attic bedroom gets scorching however all the wood and framing is in excellent condition.

    A section of the unvented cathedral attic was finished into a 250sq ft bedroom that has kneewalls and ceiling done with celotex boards (put in roughly 60 years ago), however it was not insulated beyond that and gets incredibly hot. The roofing is asphalt shingles on top of planks – no plywood sheathing -- with roughly 1/2inch between each plank.

    The roof does not leak, but will need updated sheathing and maybe some insulation on top of that in the future, however new roofing is out of my budget for a few years.

    The attic is being rewired due to safety issues and the old battered celotex boards were removed. I want it to be kept as a living space to generate some rental income, so my questions are:

    1. For this climate, if I cut-and-cobble rigid between the rafters and insulate behind the kneewalls of just this room, will this help with the hot (much hotter than the downstairs) attic room issues I have, without new roofing, or insulating the entire space of the attic? (for now)

    2. The rafters only have a 4-inch depth, can I put rigid foam between and then a layer over the rafters, or should I add some depth to the rafters?

    3. When the roof is eventually redone, can insulation be placed atop the new sheathing and the attic room insulation is kept as is? Or should that work be redone.

  76. Kim Dolce | | #76

    If one was contemplating
    If one was contemplating disregarding the problems that may arise from a cut and cobble installation in a cathedral ceiling, can you tell me what needs to be addressed in terms of fire rating? What if anything, must be applied to the surface of the rigid foam on the interior before finishing with drywall or paneling?
    Thanks.Kim

  77. User avater GBA Editor
    Martin Holladay | | #77

    Response to Kim Dolce
    Kim,
    It sounds like you're planning to ignore my advice, but I'll repeat: Don't use the cut-and-cobble method for a cathedral ceiling unless there is a ventilation channel between the uppermost piece of rigid foam and the roof sheathing.

    If the rigid foam is protected on the interior side by a layer of 1/2-inch drywall, the drywall meets the code requirement for a thermal barrier. Paneling is a different matter entirely; you'd have to talk to you local building department to see if the paneling you are thinking of using would be approved as a thermal barrier.

  78. User avater GBA Editor
    Martin Holladay | | #78

    Response to Joe Soda (Comment #76)
    Joe,
    Q. "If I cut-and-cobble rigid between the rafters and insulate behind the kneewalls of just this room, will this help with the hot (much hotter than the downstairs) attic room issues I have, without new roofing, or insulating the entire space of the attic (for now)?"

    A. Probably. But it's hard to know -- you may need an air conditioner. Note that you can't cut-and-cobble between the rafters unless you first make sure that every rafter bay has soffit vents and a ridge vent. The cut-and-cobble method can't be used for unvented cathedral ceilings -- my article explains why.

    Q. "The rafters only have a 4-inch depth. Can I put rigid foam between and then a layer over the rafters, or should I add some depth to the rafters?"

    A. Either approach can work -- assuming, as I said, that you are talking about a vented assembly. You can't do that with an unvented assembly. This approach (and several more) are described in my article, How to Build an Insulated Cathedral Ceiling.

    Q. "When the roof is eventually redone, can insulation be placed atop the new sheathing and the attic room insulation is kept as is? Or should that work be redone?"

    A. If you insulate the cathedral ceiling using a vented approach, you'll need to seal the vent openings once you put rigid foam above the sheathing. It might make more sense (now) to insulate from the interior using an unvented approach. That approach (as my article makes clear) requires the use of spray polyurethane foam.

  79. User avater
    Dana Dorsett | | #79

    You realize Joe posted that over a year ago, right? @ Martin
    76.

    JAN 10, 2016 9:40 PM ET

    (2016 not 2017)

  80. User avater GBA Editor
    Martin Holladay | | #80

    Response to Dana Dorsett
    Dana,
    Occasionally, questions go unanswered on GBA, especially when things get busy. I don't like it when that happens. My parents taught me, "Better late than never."

    If you find a library book under the sofa that's been there for 12 months, you should return it to the library.

  81. Kim Dolce | | #81

    Martin, a brief flirtation
    Martin, a brief flirtation with the idea, but no, I will not be disregarding your advice.
    Kim

  82. Craig Lux | | #82

    Insulating above grade basement wall
    Martin,

    Just bought a 1910 house in Seatlle with a walk out basement. Basement needs insulation as it has none and is very drafty. I removed the lath and plaster and found no sheathing between the studs and external clap board, only what appears to be tar paper in good condition.

    Question is now what to insulate the stud bays. Cut and cobble sounds plausible but Im unsure of where to put the vapor barrier, if at all? thicknesses for rigid foam? Fill the bays with rigid foam or go with a rigid foam and fiber glass hybrid? Do I need an air gap between the rigid foam and tar paper? if so how big? etc.

    Your thoughts?

    Spray foam is not feasible due to budget and access.

    Thanks,
    Craig

  83. User avater
    Dana Dorsett | | #83

    What siding type?
    What's on the exterior side of the tar-paper, and how much roof overhang to do you have? It matters, relative to whether you absolutely need to build it with an air gap, but a minimum of 3/8" air gap would be required by code up in B.C..

    In zone 4C you don't normally need a vapor barrier or vapor retarder other than standard interior latex paint on wallboard, as long as it's reasonably air-tight. If putting low-permeance foam board on the exterior of fiber insulation you don't want anything tighter than latex on the inteiror side, but you'll need at least 15% of the total R to be the foam. eg:

    If it is framed with full-dimension 2x4s you have 4" of depth- if you slip in some half-inch polyiso that would be R3, and you'd have enough space for standard 3.5" thick R15 rock wool or fiberglass batts for a total of R18. The R3 would then be about 17% of the total, which is fine. You could put thicker foam in there and compress R13s if you want more margin.

  84. Craig Lux | | #84

    RE: Insulating above grade basement wall
    Exterior to the tar paper is the wood (cedar?) clap board that has been painted on its exterior. Roof over hang is too high up to measure so I'm guessing 24" Overhang is 2 stories up so I dont believe it offers much protection for the area I'm insulating. Is there a code/requirement for an airgap in Seattle WA?

    Some background: I'm doing all of this because basement is extremely drafty. Basement wall is wood above grade and cement above grade on 1 side of the house. Cement wall is thicker than the wood above. We plan to have the entire basement finished so what Im doing right now is temporary and will be removed or improved when basement is finished including bringing cement & wood to same thickness for a flush wall.

    That said: I will cover the interior of the insulation with drywall but dont to paint (garage finish?) Does this change any recommendations re: a vapor barrier. Or should I skip the vapor barrier and paint the drywall?

    Other notes/questions:
    bays are 3.5" deep
    I thought compressing FG wasnt advised?
    Should I seal the interior of the concrete? if so with what?
    Dana, are you with GBA or just a helpful commentor?

    Thanks,
    Craig

  85. Francois Desrochers | | #85

    Cut-and-Cobble Insulation for my assembly?
    I'm also considering Cut-and-Cobble Insulation for the 1924 - 1000 sq.ft flat-roof bungalow I bought 2 years ago. Attached is my actual structure, which has zero insulation right now (zone 5, brrrrr!). There is 2 sections over the ceiling, one that is unvented and level, and on top of it one that is vented with 2 goosenecks with no intake and has a slight pitch to drain the water in the center of the roof.

    I figured the best way to insulate this roof would probably be from the top, adding 7-8 inches of polyiso, a deck and a membrane to cap it all, but that is expensive, and since the actual tar and gravel seem to hold up well, and that I planned to do intensive interior remodeling that will require the ceiling tearing, I'd like to have a from-the-inside alternative.

    If I tear down the ceiling (blue), I guess I could have the plank deck (green) sprayed with closed cell foam from underneath for R20, but I don't really like the idea of the whole chemical thing inside the house, even if I understand that the failure rate is very low (when choosing a certified contractor).

    I then considered replacing the ceiling (blue) with a continuous 3inch of XPS under the joist, then a reflective / vapour barrier paper, then furring and gypse. The unvented sections in between each joists (red) would be filled with cellulose, leaving a 3-4 inches gap between the top of the lose-filled insulation and the plank deck (green). But I read several warnings how difficult it is to completely seal the vapor coming from inside the house, and since each independent sections would have no ventilation at all, the condensation risk on the deck (green) might be too great (?).

    Then I read about the Cut-and-Cobble technique, which seem like a good alternative to the spray closed cell foam with less active chemical involved, and I could do it myself. I would install two staggered layers of 3 inch rigid foam in between each joist (red), right under the plank deck (green). Each rigid foam board seam would be taped, each sides would be sealed with canned foam. Then the rest of each cavity (5inch left) would be filled with cellulose at the same time the new ceiling would be installed, with no vapor barrier whatsoever so the assembly can dry from the inside (that is how I understand it anyway).

    Am I on the right track here? Considering my actual situation, is this the best approach for a from-the-inside solution?

    Thxs!
    --Francois Desrochers

    .

  86. User avater GBA Editor
    Martin Holladay | | #86

    Response to Francois Desrochers
    Francois,
    The short answer to your question is that you have to follow the advice provided in this article: Insulating Low-Slope Residential Roofs.

    The longer answer would inform you that the type of ventilation shown in your sketch does not meet the requirements to ventilate a low-slope roof. You need a "doghouse" vent in the middle of the roof, and lots of air intakes at the roof perimeter.

    Finally, the cut-and-cobble technique has been associated with failures (moisture accumulation) unless there is robust ventilation above the insulation. I don't recommend cut-and-cobble for this type of roof assembly.

  87. Francois Desrochers | | #87

    Hey Martin
    Actually I thought I was following the recommendation from that article Insulating Low-Slope Residential Roofs, more precisely the section "What if you don’t want to depend on roof venting?". The poor ventilation of the top section of my roof, and the inexisting ventilation of the lower section, the one I have access from inside, had me believe I should consider the problem from an unvented perspective. I mean, even if I do improve the ventilation of the top section, it doesn’t connect to the bottom one where the insulation would be.

    If this make sense, I was looking at the last alternative for unvented roof in that article:

    "You can install a more moderate thickness of closed-cell spray polyurethane foam on the underside of the roof sheathing, supplemented by a layer of air-permeable insulation below that."

    In my case though, the closed-cell spray polyurethane foam would have been replace by the cut-and-cobble (6 inches thick) directly on the underside of the wood deck (green), and the air-permeable insulation would have been the cellulose below that.

    I like the idea of improving the ventilation of the top section of the roof with dog house, but this section would still be outside of the thermal envellop, kinda roof over the roof.

    One thing that confuse me is that most low-slope roof diagram I see only have one section, not two like mine. Here’s an example from the article you point out -- [see second illustration].

    Only one space between the drywall and the roof sheating. The unvented space I have (between blue ceiling and green deck) is almost always inexistent in the assembly I see.

    Thxs Martin.

  88. User avater GBA Editor
    Martin Holladay | | #88

    Response to Francois Desrochers
    Francois,
    My advice is unchanged. The use of the cut-and-cobble technique in unvented (or poorly vented) roof assemblies has been associated with moisture accumulation and rot. You don't want to use the cut-and-cobble technique for this type of roof.

    You have three choices:

    1. Install closed-cell spray foam on the underside of the layer you have labeled "Deck (wood planks)." You can either install all of your insulation as closed-cell spray foam, or you can install a combination of closed-cell spray foam and fluffy insulation, as long as the R-value of the spray foam layer meets the minimum R-value required for your climate zone, as explained in my article on low-slope roofs.

    2. Improve the ventilation details as explained in my article on low-slope roofs, and then insulate with fluffy insulation.

    3. Install rigid foam insulation above the roof sheathing.

  89. Francois Desrochers | | #89

    Hey Martin
    Hey Martin,
    Ok, I understand #1 and #3, but about #2:

    since the channels between red joist, below the wood plank deck (green) are not communicating with the above section, which is ventilated (poorly but still), do I need to improve the top section ventilation (doghouses) AND create some kind of openings in the wood plank deck (green) to help ventilate the below section, or the whole assembly will dry just as well?

    I get that the deck (green) might not be totally air sealed in the first place, but would it be enough to dry any condensation happening on the underside of the cold deck (green) when moisture would actually hit it?

    Do I need to create some openings between top and bottom section to help the air flow of the bottom section?

    Thxs

  90. User avater GBA Editor
    Martin Holladay | | #90

    Response to Francois Desrochers
    Francois,
    As I explained in my article on low-slope roofs, you have two choices: you can create an unvented assembly (an approach that requires either rigid foam above the roof sheathing, or closed-cell spray foam below the roof sheathing), or you can create a vented assembly (an approach that requires a central "doghouse").

    In my Comment #89, #1 and #3 referred to unvented approaches, while #2 referred to a vented approach.

    There is never any reason to create holes in the green layer in your assembly. Airtightness is good.

    If you want to create a vented assembly, you need (a) a vented doghouse, and (b) adequate openings near the perimeter of your "attic" to allow exterior air to enter the attic.

  91. Francois Desrochers | | #91

    Response to Martin Holladay
    Ok I see.

    When you say:

    "If you want to create a vented assembly, you need (a) a vented doghouse, and (b) adequate openings near the perimeter of your "attic" to allow exterior air to enter the attic."

    I assume those perimeter openings would be at the top section level, not at the below section level where the insulation would be installed from inside, correct?

    So in your expertise, the wood plank deck (green) separation would not prevent the below section with insulation to dry properly if the top section is properly vented, am I understand this correctly?

    .

  92. User avater GBA Editor
    Martin Holladay | | #92

    Response to Francois Desrochers
    Francois,
    Q. "I assume those perimeter openings would be at the top section level, not at the below section level where the insulation would be installed from inside, correct?"

    A. Correct.

    Q. "So in your expertise, the wood plank deck (green) separation would not prevent the below section with insulation to dry properly if the top section is properly vented. Am I understand this correctly?"

    A. You never want to encourage water vapor to flow from your warm, humid interior to your cold, dry attic. The point of the insulation layer and air barrier is to create a barrier that separates the warm, humid interior from the cold dry exterior. Barriers are good. This barrier should be airtight and should have a high R-value.

    The purpose of the ventilation in your attic is to keep your attic dry, not to remove moisture from your house. For more information on these concepts, see All About Attic Venting.

  93. Francois Desrochers | | #93

    Follow-up questions
    Hey Martin,

    so after much discussion with people around me, I have a some few follow-up questions if you don’t mind.

    Concerning method #1, when you say:

    1. Install closed-cell spray foam on the underside of the layer you have labeled "Deck (wood planks)." You can either install all of your insulation as closed-cell spray foam, or you can install a combination of closed-cell spray foam and fluffy insulation, as long as the R-value of the spray foam layer meets the minimum R-value required for your climate zone, as explained in my article on low-slope roofs.

    Would the peanut-brittle insulation method you mentioned could be used instead? Or does it absolutely have to be closed-cell spray foam for the whole thickness corresponding to the required R value (for me, R-20)?

    If yes, when applying the peanut-brittle insulation method, let’s say for a 2’ x 8’ rigid foam board section, is the whole surface needs to be covered by the 1” inch thick closed-cell spray foam, or is covering only edges and seams sufficient (with a generous overlap)?

    Concerning method #2:

    2. Improve the ventilation details as explained in my article on low-slope roofs, and then insulate with fluffy insulation.
    My understanding is that I should concentrate my air / vapor sealing on the ceiling layer (blue), underneath the cellulose, to minimize the air / moisture transfer from inside the house into the below section of my attic. Possibly using a class one vapor barrier?

    I should not do any vapor sealing between the below section of the attic and the top vented section of the attic, so whatever moisture that manage to go from inside the house to the below section, should eventually dry to the exterior / top section which ventilation has been improved with doghouses and openings on the perimeter.

    All this is correct ?

    .

  94. User avater GBA Editor
    Martin Holladay | | #94

    Response to Francois Desrochers
    Francois,
    Q. "Would the peanut-brittle insulation method you mentioned could be used instead? Or does it absolutely have to be closed-cell spray foam for the whole thickness corresponding to the required R value (for me, R-20)?"

    A. I don't recommend the cut-and-cobble approach or the peanut-brittle approach for unvented roof assemblies. This approach is risky.

    Q. "My understanding is that I should concentrate my air / vapor sealing on the ceiling layer (blue), underneath the cellulose, to minimize the air / moisture transfer from inside the house into the below section of my attic. Possibly using a class one vapor barrier? I should not do any vapor sealing between the below section of the attic and the top vented section of the attic, so whatever moisture that manage to go from inside the house to the below section, should eventually dry to the exterior / top section which ventilation has been improved with doghouses and openings on the perimeter.
    All this is correct?"

    A. There is no requirement for an interior Class 1 vapor barrier -- only a vapor retarder (a less stringent layer). This code requirement can be met with vapor-retarder paint. That said, you can install polyethylene on the interior if you want.

    What is far more important is that you install an air barrier at the ceiling. This requirement is usually met by installing taped drywall. It's also essential to address air leakage through electrical boxes, electrical cable penetrations, plumbing vent penetrations, and access hatches.

    You are correct that you never want to install a vapor barrier on the top side of your attic insulation.

  95. Francois Desrochers | | #95

    More peanut brittle questions
    I’d like to understand a bit more about the peanut-brittle and the associated risk.

    The way I understand it, peanut-brittle method is replacing some of the volume of spray foam with rigid panel to save on cost, and seal the whole thing with an inch of closed-cell spay foam at the end.

    Is it risky because of the shrinking and movement of the peanut-brittle layer underneath? The 1 inch of closed-cell spay foam isn’t enough to keep the whole thing properly sealed?

    How risky it really is? Like risky-it-WILL-fail, or risky-it-most-likely-will-work but better options out there makes it a wrong choice?

    **********************

    Honestly, I’m getting more and more confused. None of the 3 methods you mentioned appears very attractive to me:

    1. Install closed-cell spray foam on the underside of the layer you have labeled "Deck (wood planks)." You can either install all of your insulation as closed-cell spray foam, or you can install a combination of closed-cell spray foam and fluffy insulation, as long as the R-value of the spray foam layer meets the minimum R-value required for your climate zone, as explained in my article on low-slope roofs.

    Honestly, we are very uncomfortable with the idea of chemical sprayed inside the house. We understand the vast majority of spraying done by competent contractor are going well, but there is an element of uncertainty that we would like to avoid.

    2. Improve the ventilation details as explained in my article on low-slope roofs, and then insulate with fluffy insulation.

    Reading on the subject, it might prove to be difficult to achieve a good ventilation across our roof because of the poor access to the sides of the building. There is no soffit communicating, and the external walls are all brick.

    It is also difficult to find a local contractor that doesn’t look at you like you are an alien when you mention Doghouses to them. Most will just say to install 2 - 4 Maximum and be done with it, regardless of the missing intake.

    3. Install rigid foam insulation above the roof sheathing.

    This would require a new roof membrane (the actual roof is still in good shape), so it would be very costly, but also we aren’t sure how we would insulate the perimeter of the house under the roof sheating. All sides of building are brick wall in good shape. We could access the below-deck from inside, but the upper-deck would require the complete removal of the roof sheating from outside or cutting through the deck from inside. A hell of a job.

    So the lack of a clear choice for me is why I’m pushing to find alternative methods, but those seem to just add to my confusion.

    Thxs for your time.

  96. User avater GBA Editor
    Martin Holladay | | #96

    Response to Francois Desrochers
    Francois,
    Ultimately, you are going to have to step up to the plate and make your own decision -- especially if you are resistant to my advice. That's fine. I can provide advice, but you have to make your own decision.

    Q. "Is the peanut-brittle method risky because of the shrinking and movement of the peanut-brittle layer underneath? The 1 inch of closed-cell spay foam isn’t enough to keep the whole thing properly sealed? How risky it really is?"

    A. I don't know how risky it is. I don't have a number. I have heard reports from people I trust of cut-and-cobble failures when the method is used for unvented roof assemblies.

    I'm conservative when it comes to unvented roof assemblies, because there are lots of failures. Since I'm giving advice over the Internet, I take my responsibility seriously. I don't want to provide risky advice.

    The safest way to proceed (if you want to insulate an unvented roof assembly from the interior) is with closed-cell spray foam.

    I have also listed other options. What you do next is up to you.

  97. Jennifer Gruenke | | #97

    Cut and cobble in 1933 cape with no sheathing
    Thanks for this article. I am working on remodeling an old house, and because I have lots of time but a tight budget, this looks like the best method for me. I am in zone 3a (west TN). The existing walls have 2x4 studs with tongue-and-groove wood on both the inside and outside of the studs. The inside then has paneling or wallpaper over the T&G, and the outside has wood siding over the T&G, covered by 1/2" XPS coated with plastic film, which is then covered by ugly vinyl siding. My plan is to eventually remove the vinyl and foam and refinish the original wood siding.

    I have a few questions:

    1. I understand that I will need an air gap between the foam board and the exterior T&G cladding as described in the "insulating walls with no sheathing" article. But that article recommends an air gap of .75" -1", which would mean I can only put 2.5 to 2.75" of foam in the walls. I would prefer to use a 0.5" or even 0.25" air gap so that I could put in more foam. Is there any reason that the air gap could not be thinner than 0.75"?

    2. Would it create problems to put foam board inside the walls before I rip off the vinyl siding and foam board (which presumably creates a vapor barrier)? I would like to finish at least some of the inside before I tackle the outside so that I can live there during the renovation. But it will create a temporary (perhaps up to a year) vapor barrier sandwich with wood in the middle. Is that a terrible risk?

    3. The house is a cape cod style with two dormer windows. The ceiling of the second floor is T&G wood. Loose-fill cellulose has been blown into the what is essentially a cathedral ceiling. The roof appears to be unvented -- there is no visible ridge vent. The roof is brand new. It sounds like the safest bet is to not try to further insulate the ceiling, but to leave the loose cellulose and to consider putting foam under the next roof. Is that correct? And it would be okay / desirable to put air-sealed drywall on the obviously air-permeable T&G ceiling, correct?

    4. When installing foam board in the walls, is there any advantage to using construction tape like 3M all-weather tape either instead of or in addition to spray foam? Might it resist deformation over time more than does canned foam? It seems like it would made the job less messy if the first layer had tape so that the foam could not squish to the outside. But it might also create undesirable air gaps around that first layer. Any thoughts?

    Thanks in advance!

    Jennifer

  98. User avater GBA Editor
    Martin Holladay | | #98

    Response to Jennifer Gruenke
    Jennifer,
    First of all, the advice in the article you refer to ("Insulating Walls in an Old House With No Sheathing") is irrelevant, because your house has sheathing. In fact, your house has high quality sheathing -- namely, tongue-and-groove boards. This type of sheathing was used for at least a hundred years before the invention of plywood, and many builders still install board sheathing.

    For more information on board sheathing, see this article: "Wall Sheathing Options."

    Q. "Would it create problems to put foam board inside the walls before I rip off the vinyl siding and foam board (which presumably creates a vapor barrier)?"

    A. Probably not. Needless to say, if you see any signs of moisture entry during your renovation work, that would be a red flag that should cause you to reconsider your approach.

    Q. "The house is a Cape Cod style with two dormer windows. The ceiling of the second floor is T&G wood. Loose-fill cellulose has been blown into the what is essentially a cathedral ceiling. The roof appears to be unvented -- there is no visible ridge vent. The roof is brand new. It sounds like the safest bet is to not try to further insulate the ceiling, but to leave the loose cellulose and to consider putting foam under the next roof. Is that correct? And it would be okay / desirable to put air-sealed drywall on the obviously air-permeable T&G ceiling, correct?"

    A. You're on the right track. Task #1 is to install taped drywall on the interior side of the ceiling. Task #2 is to install an adequately thick layer of rigid foam on the exterior side of your roof sheathing.

    Q. "When installing foam board in the walls, is there any advantage to using construction tape like 3M All-Weather tape either instead of or in addition to spray foam? Might it resist deformation over time more than does canned foam? It seems like it would made the job less messy if the first layer had tape so that the foam could not squish to the outside. But it might also create undesirable air gaps around that first layer. Any thoughts?"

    A. It's hard to get a tight fit with rigid foam, so the usual method is to adopt a loose fit and to seal the perimeter with canned spray foam. That said, a high-quality tape makes sense for any seams that appear tight (in other words, seams without obvious voids). Use common sense.

  99. Jennifer Gruenke | | #99

    Response to Martin Holladay
    Thanks for the prompt reply. Are you saying that I can fill the whole wall cavity with foam, skipping the air gap? I don't think that there is a vapor barrier over the T&G sheathing, but I suppose there is no air gap with siding over housewrap in modern construction.

  100. User avater GBA Editor
    Martin Holladay | | #100

    Response to Jennifer Gruenke
    Jennifer,
    You wrote, "I don't think that there is a vapor barrier over the T&G sheathing."

    That's good. A house should never have a vapor barrier on the exterior side of the sheathing.

    You are probably thinking of housewrap or ashalt felt. Neither of these is a vapor barrier. Housewrap is vapor-permeable -- the idea is to allow the sheathing to dry to the exterior if it ever gets wet.

    Older homes often had asphalt felt or rosin paper between the sheathing and the siding. In some cases, there was no such layer, and the siding was nailed directly to the sheathing. That doesn't meet modern standards, but it is what it is.

    Unless you note signs of water entry, don't worry. There's not much you can do about the problem until it's time to replace your siding. When you replace the siding, it certainly makes sense to install a water-resistive barrier (WRB) like housewrap or asphalt felt.

  101. Jennifer Gruenke | | #101

    Response to Martin Holladay
    Okay. That makes sense.

  102. Nymiah | | #102

    Unvented Tiny House roof assembly
    Dear Martin,
    I hope you still check this post!
    I have read the relevant articles and all of the posts on this article.
    I know you advise that if you cannot put rigid foam on the outside your only option is cc spray foam.
    Here's my situation:
    I am building a tiny house on wheels. It is 20ft long and 8 1/2ft wide. I have 10ft of unvented cathedral roof at a 38° angle and 10ft of roof that is a long former at a low slope of 7 1/2°. The roof has 1/2" plywood sheeting covered by Grace Ice and Water Shield and directly on top of that is a 1" standing seam metal roof.
    The rafters are nominal kiln dried 2x4's. My original plan was to have someone spray cc spray foam. Unfortunately we only have one contractor that does this in my area. He says he sprays a thick layer that ends up 2 - 2 1/2" thick. Everything I've read about spray foam emphasizes the absolute importance of proper application which, as far as I've read means thin layers, one at a time. Otherwise it may not dry properly and possibly offgas for years. I am already somewhat chemically sensitive so this terrifies me!
    In the article about how to insulate cathedral roof assemblies, a couple of times, in parentheses, it says... "or arguably rigid foam" instead of spray foam. Again, I understand that you don't recommend it, but this is what I am thinking... Someone in the earlier post mentioned using an adhesive on the side of the rigid foam against the interior side of the roof shearing. I was thinking this sounds good as it would make a solid roof assembly where no moisture could get behind the foam because of the adhesive... So... I would use 3 - 3 1/4" of rigid foam with adhesive on the underside of the sheathing, friction fit and sealed air tight with "one of those European tapes." The bays would have solid pieces of foam, no cobbling...
    So what do you think? Can this work? I know it's not as good as spray foam, but if I am meticulous, can it work?
    Thanks so much for your time!
    Nymiah

  103. User avater GBA Editor
    Martin Holladay | | #103

    Response to Nymiah
    Nymiah,
    If you want to avoid spray foam, the best approach is to install rigid foam above the roof sheathing, not below the roof sheathing.

    If you ignore my advice and install cut-and-cobble rigid foam under the roof sheathing, you are choosing a risky approach. It's possible that you will get moisture accumulation or sheathing rot. The colder the climate, the higher the risk -- so one way to lower the risk is to park your tiny house in San Diego.

  104. Nymiah | | #104

    S " of hen
    PS i live on the northcoast of California in Eureka. We are in the 4c marine climate zone. This calls for an r10 value for above roof rigid foam which you also say is the minimum for spray/rigid foam beneath the roof sheeting. I gueas if 3" of rigid is too much rigid, I was thinking 2" of rigid foam for the r20, adhered and taped for excellent air tightness etc. And then air permeable insulation for the remaining 1 1/2", and then maybe to finish, we're going to cover all with 1/4" finished plywood painted. Drywall too brittle for moving houses. ☺ And there we Are! What say you Martin of the building arts?

  105. User avater GBA Editor
    Martin Holladay | | #105

    Response to Nymiah
    Nymiah,
    My advice is unchanged. Your approach is risky (although not as risky as it would be in a colder climate like Minnesota).

    With changes in temperature and humidity, your 2x4s and roof sheathing will expand and contract. These expansion and contraction cycles eventually stress the caulk, foam, or tape used for air sealing, putting the sheathing at risk for moisture accumulation. That's why I advised you to install the rigid foam as a continuous layer above the roof sheathing rather than cutting narrow rectangles and installing the rigid foam between the rafters.

    That said, it's your house. You get to evaluate the risk, and you get to decide your own appetite for risk.

  106. Nymiah | | #106

    Just saw that you wrote more,
    Just saw that you wrote more, sorry.

  107. Nymiah | | #107

    But I would still like to
    But I would still like to know what you think of the adhesive idea... trying to find small things that will add up to less risky.

  108. Nymiah | | #108

    Also: i already Installed the
    Also: i already Installed the expensive metal roof and cannot afford to take it apart and redo it so as to be able to put rigid foam above the roof sheathing...

  109. Nymiah | | #109

    Martin, your article says...
    Martin, your article says... "In spite of these disadvantages, cut-and-cobble sometimes makes sense. It can be used:"... and here I can point to 2 issues on your list that lead me to this method. So I am confused now when you say in these posts that it shouldn't ever be done, while your article lists "cut and cobble sometimes makes sense..." When... which is what I'm dealing with. SO... Will a good construction adhesive applied on the rigid foam and into the corners and then pushed into the sheathing and held tight while it has a quick dry so the sheathing at the back is completely sealed, which would hopefully make it unlikely moisture could get into the sheathing to condensate... So my question stands... Could this work?

  110. User avater GBA Editor
    Martin Holladay | | #110

    Response to Nymiah
    Nymiah,
    Q. "So I am confused now when you say in these posts that it shouldn't ever be done, while your article lists 'cut and cobble sometimes makes sense.' When?"

    A. Cut-and-cobble sometimes makes sense for walls or vented roof assemblies. As my article makes clear, it doesn't make sense for unvented roof assemblies.

    Q. "So my question stands... Could this work?"

    A. I've answered your question a couple of times. My answer isn't really a yes-or-no answer. My answer is, "It's risky." Here at GBA, we regularly receive reports of cathedral ceilings with damp roof sheathing. These reports have made me conservative in my recommendations.

    Q. "But I would still like to know what you think of the adhesive idea."

    A. Adhesive and caulk are similar (and in many cases identical). Either method of air sealing (adhesive or caulk) is better than nothing, but your suggested approach can still fail.

    Q. "I already Installed the expensive metal roof and cannot afford to take it apart and redo it so as to be able to put rigid foam above the roof sheathing."

    A. It's really important to finalize your insulation plan before you begin building. In your case, it's too late to take that advice. But other GBA readers might benefit from that advice. Here is a link to a relevant article: Plan Ahead For Insulation.

  111. Nymiah | | #111

    Here's the fancy European
    Here's the fancy European tape...
    TESCON VANA Specifications (PDF – English)
    https://foursevenfive.com/wp-content/uploads/2016/06/TesconVanaSpec.pdf

  112. Nymiah | | #112

    Thank yo for answering so
    Thank yo for answering so quickly Martin!

  113. Nymiah | | #113

    I am wrong to feel hopeful
    I am wrong to feel hopeful regarding this part of your article about code? "In the 2012 version of the IRC, the language was corrected to include cathedral ceilings. The relevant section of the code (Section R806.5 of the 2012 IRC) reads, “Unvented attic assemblies (spaces between the top-story ceiling joists and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters [otherwise known as cathedral ceilings]) shall be permitted if all the following conditions are met…” The required minimum R-values for the foam layer haven’t changed."

  114. Nymiah | | #114

    Martin,
    Regarding your

    Martin,
    Regarding your comment about how I should have planned better... As I said in my initial contact I Had planned for spray foam, but I began to hear horror stories and have come around to wanting something else... too late. I am 58, and with minimal carpentry experience (4 years of stage carpentry in college and home projects over the years) I am finally building my own home! It has been a steep learning curve, and so I did not have all the information I needed at the time I put the roof on...

  115. Phil Jamison | | #115

    Insulating Thin Rafters
    I'm restoring a small 150-year-old stone house with a hip roof in France. It has a slate roof and no sheathing. The rafters are narrow (10cm X 10cm; about 3-7/8"), and the ceiling is low above our low second floor. I was considering this "cut & cobble" method of insulating covered with T&G pine for a ceiling. I'm thinking the rafters are not tall enough for batt insulation. Any ideas?

  116. User avater GBA Editor
    Martin Holladay | | #116

    Response to Phil Jamison
    Phil,
    First of all, I envy you. I imagine it's a beautiful building.

    Second, it's important for you (a) to comply with all local regulations, of which I am ignorant, and (b) to get local advice. You don't want to mess up a historic building.

    Here are some basic principles:

    1. From a moisture perspective, a slate roof is forgiving. It dries readily to the exterior after it gets wet. That said, it benefits from having an air space between the underside of the slates and the air barrier or insulation barrier beneath the slates.

    2. Every insulated roof assembly needs an air barrier. You need to plan for an airtight ceiling. Remember that tongue-and-groove boards leak like a sieve. They are not an air barrier.

    3. When rafters are too shallow to provide enough room for insulation, it's common to add extra framing members to increase the available depth. Some of these techniques are mentioned in my article, How to Build an Insulated Cathedral Ceiling.

    4. I don't recommend the cut-and-cobble method. To reduce thermal bridging through the rafters, and to improve your chances of obtaining an airtight ceiling, you might consider installing multiple layers of continuous rigid foam on the interior side of the rafters, with the seams in each layer carefully taped with high quality tape.

    Good luck. Below is a cheat sheet with French technical vocabulary.

    L'amélioration thermique et énergétique des batiments résidentiels = Thermal and energy upgrades of residential buildings

    La rénovation basée sur l’efficacité énergétique = energy retrofit work

    l'étanchéité à l'air = airtightness

    l'étanchéisation à l'air de l'enveloppe = reducing envelope air leakage (air tightening work)

    pare-air = air barrier

    pare-pluie = water-resistive barrier

    pare-vapeur = vapor barrier

    l’isolation thermique = insulation

    la mousse plastique de polystyrène = polystyrene foam

    la mousse plastique de polystyrène expansé = Expanded polystyrene foam

    la mousse plastique de polystyrène extrudé = Extruded polystyrene foam

    l’isolant-mousse en panneaux rigides = rigid foam insulation

    Le le fibre de verre - fiberglass

    La laine minérale - mineral wool

    Isolants pulvérisés sur place = spray foam insulation

    les ponts thermiques à travers les matériaux d’ossature = thermal bridging through framing members

  117. Phil Jamison | | #117

    Thin Rafters
    Thanks for your reply and the French terms, Martin. Any insulation would improve this space. When the sun shines, it's hot. When the wind blows, cold. I describe it as a Tiny House (a term the French use, though they should say Mini Maison), heated with a wood stove. The very low ceiling in this attic (which I hope to use as a sleeping space) precludes lowering the rafters much. I'll let you know what I do.

    Phil

  118. User avater
    Peter Engle | | #118

    Mini Maison
    Phil,

    Show us more pictures. Everyone is jealous. The photo you should looks like a tile floor, and roofing that comes almost down to floor level. Is this the attic? It looks almost like ancient barn construction. I'd lover to see more.

    You didn't mention what part of France, and what climate zone the house is located in. That makes a big difference.

    An inch or two of cut & cobble insulation in the rafters would help, if you also include an inch or two of rigid foam insulation fastened to the interior of the rafters and taped as Martin mentions.

    This would be relatively labor intensive with rough sawn (or hewn) rafters, but certainly a big improvement over nothing at all.

  119. User avater GBA Editor
    Martin Holladay | | #119

    Response to Peter Engle
    Peter,
    The rafters don't appear to be hewn, although they may be rough-sawn (see enlarged photo below). I'm guessing that the roof framing and slates are newer than the stone house. The rafters, skip sheathing, and slates look in excellent shape.

    .

  120. Phil Jamison | | #120

    French Maison
    Our house is in a small village in the Morvan in central France. I had the roof replaced about 15 years ago. The original was slate with under-size split tree limb rafters which were sagging under the weight. Before that, the roofs in this area were thatch. The contractor replicated the original look. Attached is a photo taken last year when we had the chimney restored. The second floor has enough height for me to walk along its center, and tapers down to about 16 inches at the eaves. When I bought the house (in 1991), access to the attic was by a crude exterior ladder. I added a small interior stairway. A long-time project!

    Phil

  121. User avater GBA Editor
    Martin Holladay | | #121

    Toute petite! Et si mignonne!
    Thanks for the photo and details.

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