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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
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  3. www.diychatroom.com
  4. Erich Riesenberg

51.
Dec 16, 2014 2:17 PM ET

Response to JoAnn Dibeler
by Martin Holladay

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.
Dec 18, 2014 7:00 PM ET

Edited Dec 18, 2014 7:01 PM ET.

XPS or EPS?
by Emil Mackey

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.
Dec 19, 2014 7:08 AM ET

Response to Emil Mackey
by Martin Holladay

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.
Dec 28, 2014 12:38 PM ET

Cut and cobble versus partial insulation over-roof
by Brian Gray

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.
Dec 28, 2014 5:26 PM ET

Edited Dec 28, 2014 5:27 PM ET.

Response to Brian Gray
by Martin Holladay

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.
Jan 4, 2015 10:34 AM ET

renovating two walls, suggestions welcome
by M Z

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.
Jan 5, 2015 7:14 AM ET

Response to MZ
by Martin Holladay

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.
Jan 5, 2015 8:53 AM ET

@Matrin
by M Z

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.
Jan 5, 2015 10:05 AM ET

Response to MZ
by Martin Holladay

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.
Jan 5, 2015 5:27 PM ET

@Martin
by M Z

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


61.
Jan 7, 2015 12:15 PM ET

cut and cobble wall
by Greg Philliips

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.
Jan 7, 2015 12:26 PM ET

Response to Greg Philliips
by Martin Holladay

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


63.
Jan 11, 2015 10:53 PM ET

Edited Jan 11, 2015 10:55 PM ET.

Poly-iso cobble vs closed cell spray, w/ exterior foam in place
by Andrew Toomajian

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.
Jan 20, 2015 5:12 PM ET

Good discussion. We continue
by Howard Gentler

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.
Jan 20, 2015 5:45 PM ET

Response to Howard Gentler
by Martin Holladay

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


66.
Jan 21, 2015 8:55 PM ET

Cathedral ceiling insulation retrofit...
by Howard Gentler

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.
Jan 22, 2015 6:05 AM ET

Edited Jan 22, 2015 6:14 AM ET.

Response to Howard Gentler
by Martin Holladay

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.
Jan 22, 2015 10:45 AM ET

Cathedral insulation...
by Howard Gentler

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.
Jan 22, 2015 11:04 AM ET

Response to Howard Gentler
by Martin Holladay

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.
Feb 13, 2015 9:33 AM ET

1915 home insulation upgrade
by Roman Stankus

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.
Feb 13, 2015 10:14 AM ET

Edited Feb 13, 2015 10:31 AM ET.

Response to Roman Stankus
by Martin Holladay

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.
Mar 19, 2015 3:54 PM ET

Torn over spray foam
by claython mclaw

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


74.
Mar 19, 2015 4:03 PM ET

Edited Mar 19, 2015 4:13 PM ET.

Response to Claython Mclaw
by Martin Holladay

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.


75.
Mar 23, 2015 9:24 AM ET

Thanks for the suggestions.
by claython mclaw

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)


76.
Jan 10, 2016 10:40 PM ET

Unvented Attic
by Joe Soda

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.


77.
Jan 13, 2017 2:55 PM ET

If one was contemplating
by kim dolce

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


78.
Jan 13, 2017 3:39 PM ET

Response to Kim Dolce
by Martin Holladay

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.


79.
Jan 13, 2017 3:48 PM ET

Response to Joe Soda (Comment #76)
by Martin Holladay

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.


80.
Jan 13, 2017 4:17 PM ET

You realize Joe posted that over a year ago, right? @ Martin
by Dana Dorsett

76.

JAN 10, 2016 9:40 PM ET

(2016 not 2017)


81.
Jan 13, 2017 4:23 PM ET

Response to Dana Dorsett
by Martin Holladay

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.


82.
Jan 14, 2017 11:57 AM ET

Martin, a brief flirtation
by kim dolce

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


83.
Jan 18, 2017 4:00 PM ET

Edited Jan 18, 2017 4:01 PM ET.

Insulating above grade basement wall
by Craig Lux

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


84.
Jan 18, 2017 4:27 PM ET

What siding type?
by Dana Dorsett

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.


85.
Jan 19, 2017 3:15 PM ET

Edited Jan 19, 2017 3:18 PM ET.

RE: Insulating above grade basement wall
by Craig Lux

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


86.
Feb 1, 2017 1:05 PM ET

Cut-and-Cobble Insulation for my assembly?
by Francois Desrochers

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

.

Desrochers image 1.jpg


87.
Feb 1, 2017 2:09 PM ET

Edited Feb 2, 2017 4:29 AM ET.

Response to Francois Desrochers
by Martin Holladay

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.


88.
Feb 2, 2017 4:34 AM ET

Hey Martin
by Francois Desrochers

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.

Desrochers image 2.jpg Doghouse vent.jpg


89.
Feb 2, 2017 4:43 AM ET

Response to Francois Desrochers
by Martin Holladay

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.


90.
Feb 2, 2017 8:44 AM ET

Hey Martin
by Francois Desrochers

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


91.
Feb 2, 2017 8:51 AM ET

Response to Francois Desrochers
by Martin Holladay

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.


92.
Feb 3, 2017 7:18 AM ET

Response to Martin Holladay
by Francois Desrochers

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?

.

Desrochers image 3.jpg


93.
Feb 3, 2017 7:24 AM ET

Response to Francois Desrochers
by Martin Holladay

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.


94.
Feb 14, 2017 7:34 AM ET

Follow-up questions
by Francois Desrochers

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 ?

.

Desrochers image 4.jpg


95.
Feb 14, 2017 7:42 AM ET

Response to Francois Desrochers
by Martin Holladay

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.


96.
Feb 14, 2017 4:34 PM ET

More peanut brittle questions
by Francois Desrochers

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.


97.
Feb 14, 2017 4:40 PM ET

Response to Francois Desrochers
by Martin Holladay

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.


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