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Cut-and-Cobble Insulation

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

Posted on Nov 22 2013 by Martin Holladay

Here at GBAGreenBuildingAdvisor.com, 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 polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. . 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . That way, the foam will interrupt thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. 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 XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., 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 (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.) 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.


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Image Credits:

  1. Michael Brahmey
  2. DC
  3. www.diychatroom.com
  4. Erich Riesenberg

1.
Nov 22, 2013 8:45 AM ET

Cut'n Cobble
by Greg Labbe

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.
Nov 22, 2013 11:16 AM ET

Options
by Armando Cobo

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.
Nov 22, 2013 5:09 PM ET

Edited Nov 22, 2013 5:10 PM ET.

alternatives interior?
by Nick T - 6A (MN)

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.
Nov 23, 2013 7:38 AM ET

Response to Nick T
by Martin Holladay

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.
Nov 24, 2013 12:25 AM ET

option for floor joist retrofit
by Seth Rutledge

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.
Nov 24, 2013 6:39 AM ET

Response to Seth Rutledge
by Martin Holladay

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.
Nov 25, 2013 9:51 AM ET

More Cut & Cobble Problems
by Kohta Ueno

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! :)

P1100221 (Small).JPG P1100449 Resize (Small).JPG


8.
Nov 25, 2013 10:02 AM ET

Edited Nov 25, 2013 10:04 AM ET.

Response to Seth Rutledge (Dow Insta Stik)
by Kohta Ueno

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-mult...), at Figure 24: "Test installation of foam on masonry (left) and use of polyurethane adhesive (right)"


9.
Nov 25, 2013 10:04 AM ET

Edited Nov 25, 2013 10:05 AM ET.

Response to Kohta Ueno
by Martin Holladay

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.


10.
Nov 25, 2013 12:18 PM ET

Edited Nov 25, 2013 12:19 PM ET.

Follow up with Martin
by Nick T - 6A (MN)

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.
Nov 27, 2013 7:00 PM ET

whoa.
by Stacey Cordeiro

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.
Nov 28, 2013 7:42 AM ET

Response to Stacey Cordeiro
by Martin Holladay

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.
Nov 28, 2013 10:04 AM ET

I decided to use cut and
by tom ruben

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.
Nov 28, 2013 11:27 AM ET

thermax adhesive
by Seth Rutledge

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%2...


15.
Nov 28, 2013 12:06 PM ET

Re: thermax adhesive
by Kohta Ueno

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.
Nov 28, 2013 1:48 PM ET

Insulating walls and ceilings
by Roger Anthony

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.
Nov 28, 2013 6:36 PM ET

Continuous Rigid Insulation on Inside?
by Hermann Thoene

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.
Nov 29, 2013 8:38 AM ET

Response to Hermann Thoene
by Martin Holladay

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.
Dec 2, 2013 2:31 PM ET

Edited Dec 2, 2013 2:32 PM ET.

My own experience with cut and cobble
by Patrick McCombe

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...

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...


20.
Dec 3, 2013 12:30 AM ET

Cut and Cobble with damaged EPS
by Robert Kohaus

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.
Dec 3, 2013 5:44 AM ET

Edited Dec 3, 2013 5:59 AM ET.

Response to Robert Kohaus
by Martin Holladay

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.
Dec 3, 2013 2:48 PM ET

Cut and Cobble with damaged EPS
by Robert Kohaus

I'll post a new question concerning this topic. Thanks Martin.


23.
Dec 4, 2013 7:29 PM ET

Edited Dec 4, 2013 7:30 PM ET.

Separating from framing?
by Dave Frank

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.
Dec 5, 2013 6:40 AM ET

Response to Dave Frank
by Martin Holladay

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.
Dec 5, 2013 10:03 PM ET

Another option?
by Dave Frank

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.
Dec 6, 2013 8:52 AM ET

What if there was venting
by Todd Sherman

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.
Dec 6, 2013 9:07 AM ET

Response to Dave Frank
by Martin Holladay

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.
Dec 6, 2013 9:17 AM ET

Response to Todd Sherman
by Martin Holladay

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.
Dec 9, 2013 9:21 AM ET

Cut & Cobble
by David Tontarski

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.
Dec 9, 2013 9:38 AM ET

Edited Dec 9, 2013 9:40 AM ET.

Response to David Tontarski
by Martin Holladay

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.
Dec 15, 2013 9:14 AM ET

Cut & cobble with cellulose as crack filler?
by Rick Van Handel

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.
Dec 16, 2013 10:31 AM ET

Response to Rick Van Handel
by Martin Holladay

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.
Jan 9, 2014 10:41 PM ET

cut and cobble compared with spray foam
by Erich Riesenberg

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.
Jan 13, 2014 3:48 PM ET

Old Home with No Sheathing
by Chris Thomas

"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.
Jan 13, 2014 5:43 PM ET

Response to Chris Thomas
by Martin Holladay

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.
Jan 13, 2014 7:15 PM ET

Edited Jan 13, 2014 7:16 PM ET.

Clarification
by Chris Thomas

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.
Jan 14, 2014 5:45 AM ET

Edited Jan 14, 2014 5:47 AM ET.

Response to Chris Thomas
by Martin Holladay

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.
Apr 3, 2014 11:02 AM ET

Question re: response to Dave Frank
by Sal Lombardo

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.
Apr 3, 2014 11:44 AM ET

Edited Apr 3, 2014 11:45 AM ET.

Response to Sal Lombardo
by Martin Holladay

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.
Apr 8, 2014 9:12 PM ET

I'm currently doing this cut
by Nat Hilton

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.
Apr 9, 2014 5:18 AM ET

Response to Nat Hilton
by Martin Holladay

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.
Apr 9, 2014 7:45 AM ET

Thanks Martin, Yes it's me
by Nat Hilton

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.
Jun 17, 2014 7:50 PM ET

Cut-and-Cobble worked well for this major remodel
by Mark Hays

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

Insulation - living room.jpg Insulation - kitchen.jpg Exterior with foam board and trim - for HD.jpg


44.
Oct 7, 2014 3:32 PM ET

hybrid venting
by A. Bradford

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.
Oct 7, 2014 4:23 PM ET

Response to A. Bradford
by Martin Holladay

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.
Oct 8, 2014 8:29 AM ET

hybrid venting
by A. Bradford

Martin,

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


47.
Oct 8, 2014 8:42 AM ET

Response to A. Bradford
by Martin Holladay

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.
Oct 8, 2014 10:08 AM ET

Edited Oct 8, 2014 10:34 AM ET.

hybrid venting
by A. Bradford

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.
Oct 8, 2014 10:39 AM ET

Response to A. Bradford
by Martin Holladay

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.
Dec 16, 2014 1:54 PM ET

Edited Dec 16, 2014 2:02 PM ET.

rigid foam insulation
by JoAnn Dibeler

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)


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