Here at GBA, readers regularly ask about the best way to install rigid foam insulation between studs or rafters. A typical question might go like this: “I’d like to insulate between my studs with strips of 2-inch-thick polyiso. I plan to cut the rigid foam pieces a little bit loose, and seal the edges of the polyiso with canned spray foam. Will this work?”
Here’s my standard answer: “If you want to insulate your walls with rigid foam, you shouldn’t cut the foam into thin strips. Instead, you should keep the sheets of foam intact and install the foam as a continuous layer on the exterior side of your wall sheathing. That way, the foam will interrupt thermal bridging through the studs.”
Sometimes, I also point out: “Although the method you suggest, informally known as ‘cut-and-cobble,’ is often used by homeowners, it is such fussy, time-consuming work that it is never used by insulation contractors.” (The first person I heard use the term “cut-and-cobble” was Dana Dorsett, a regular contributor to the Q&A column on GBA. Dorsett first used the phrase in a web forum post in April 2012. “I’m not entirely sure if I was the first to use that term in a rigid insulation context, but I might be,” Dorsett told me.)
Time for a confession
If I had to summarize the theme underlying my cut-and-cobble advice, it would probably be, “Don’t do it.” But this advice leaves me feeling somewhat guilty. It’s time to come clean, and, like a newcomer at a 12-step meeting, announce: “My name is Martin, and I have cut and cobbled.”
Yes, I’ve done it — for the same reasons that lots of other people have done it. Sometimes, cut-and-cobble makes sense.
Cut-and-cobble is cheaper than spray foam
Cut-and-cobble is a well-established method of insulating rim joists. When used for insulating above-grade walls, the cut-and-cobble method is usually chosen by homeowners or owner/builders who are leery of fiberglass batt insulation. Although fiberglass insulation is inexpensive, it is air-permeable, hard to install well, and attractive to mice. Homeowners who prefer to install foam insulation have three choices:
Cut-and-cobble can be used in a way similar to flash-and-batt (by installing a layer of rigid foam against the wall sheathing, and filling the rest of the stud bay with fluffy insulation); or cut-and-cobble can be used to fill the entire stud bay with rigid foam (an approach that I call the “stack of pancakes” method).
Cut-and-cobble has some disadvantages:
- To address thermal bridging, it’s always better to put the rigid foam on the exterior side of the sheathing. However, in an existing house, homeowners are rarely willing to demolish their siding just to install a layer of rigid foam.
- This insulation method is very time-consuming. If you want to do a good job, it will take longer than any other insulation method.
- Wood framing expands and contracts with changing humidity levels, raising the possibility that attempts to seal the perimeter of the rigid foam (whether with caulk, spray foam, or tape) will fail over time. Anecdotal evidence suggest that this danger is real, especially for cut-and-cobble cathedral ceilings. (A GBA reader recently posted an account of a cut-and-cobble roof insulation job gone wrong. Because of air leaks through cut-and-cobble cracks, the reader’s flat roof is now raining condensation. Another report of a failed cut-and-cobble roof assembly is provided by building scientist Kohta Ueno in Comment #7, posted below.)
In spite of these disadvantages, cut-and-cobble sometimes makes sense. It can be used:
- When a homeowner can’t afford the cost of a spray foam contractor or a two-component spray foam kit. (As Richard Briede posted on the Q&A page here at GBA, “Please help with a poor man’s insulation technique. I know closed-cell spray foam would be the best for this application, but I simply can’t afford it.”)
- When a homeowner has access to leftover or recycled pieces of inexpensive rigid foam.
- When a homeowner is leery of the “lingering odor” risk associated with spray foam.
- When a homeowner or builder wants to create a WRB in the stud bays of an older house that lacks sheathing.
- When a homeowner wants to tackle a large job in phases, and can’t manage to have everything ready for a spray foam crew all at once.
- When sheathing is too cold to be used as a substrate for spray foam.
- When a homeowner wants to insulate an unvented cathedral ceiling without using spray foam or adding exterior rigid foam. (Note that this approach only applies to homeowners who are willing to disregard reports of problems that arise when the cut-and-cobble method is used for cathedral ceilings.)
Cut-and-cobble, step by step
If you want to use the cut-and-cobble method to install rigid foam in your walls or ceilings, what are the necessary steps?
- The first step is to calculate the minimum necessary thickness of your rigid foam. Here is a link to an article that tells you how to figure this out: Calculating the Minimum Thickness of Rigid Foam Sheathing. Note that minimum foam R-values are generally higher for cathedral ceilings than for walls. (You can skip this first step if you don’t plan to install any fiberglass insulation, but intend instead to use the stack-of-pancakes method.)
- The second step is to choose which type of rigid foam to use. Polyisocyanurate is considered the most environmentally friendly foam. If you plan to use the cut-and-cobble method to insulate an unvented cathedral ceiling, some experts say that EPS or fiber-faced polyiso are preferable to foil-faced polyiso or XPS, because EPS and fiber-faced polyiso will allow the roof sheathing to dry (very slowly) to the interior if necessary.
- The third step is to decide whether to push the foam as far as possible toward the exterior side of the framing cavity — the method that is usually used for walls with solid sheathing or for unvented cathedral ceilings — or whether you want to leave a gap on the exterior side of the framing bay — the method that is used for walls that lack sheathing or for vented cathedral ceilings. If you prefer to have a gap on the exterior side of the foam, you’ll probably want to install 1″x1″ or 1.5″x1.5″ sticks at the corners of each framing bay to maintain the desired gap.
- Most cut-and-cobble veterans suggest that it makes more sense to cut the rigid foam for a loose and sloppy fit rather than for a tight, perfect fit — especially if you plan to seal the perimeter of the rigid foam with canned spray foam. After all, the spray foam nozzle needs a gap of at least 1/8 inch for proper insertion. Others, however — especially those who don’t want to use canned spray foam — advise cutting rigid foam on a table saw and aiming for a tight fit.
- The last step is to seal the perimeter of each piece of foam to prevent air leaks. Most cut-and-cobble veterans prefer canned spray foam for this step, but if your cracks are small, caulk will work. There is some evidence that canned spray foam does not create an airtight seal; if you are a fanatic for air sealing, you may prefer to invest in a European tape to seal these cracks.
A variation on cut-and-cobble
What if you want to fill your stud bays all the way full with spray foam, using a two-component spray foam kit? (This kits are available at building supply outlets for $300 to $600.) If you do a little math, you’ll realize that the price of these kits is so high that they are really too expensive to fill stud bays; that’s why they are usually used for air sealing, not insulating.
Some homeowners have figured out, however, that if they can fill most of the volume of their stud bays with rigid foam scraps, the amount of spray foam required to fill the remainder of the stud bays is quite small. If you tack up the scraps in the stud bays like jigsaw puzzle pieces, all you have to do is seal the edges of the rigid foam and then install a skim coat of foam on top of everything. I hereby dub this method the “peanut brittle” method.
The peanut-brittle insulation method is similar to the old hippie trick of throwing some big stones into foundation forms while the Ready-Mix truck is discharging its load of concrete. It’s a way of stretching expensive store-bought material with something cheap.
What does the building code say about cut-and-cobble for cathedral ceilings?
The 2009 International Residential Code (IRC) provided rules for creating an unvented insulated sloped roof assembly. Unfortunately, the language was flawed.
This type of insulated sloped roof assembly occurs in at least two locations: cathedral ceilings and unvented conditioned attics. The language used in section R806.4 of the 2009 IRC clearly applied to unvented attics, but never addressed cathedral ceilings.
The section of the 2009 IRC that can be interpreted as supporting the use of cut-and-cobble is section R806.4.5.3: “Air-impermeable and air-permeable insulation. The air-impermeable insulation [e.g., rigid foam] shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation [e.g., fiberglass batts] shall be installed directly under the air-impermeable insulation.” The table calls for a minimum of R-5 foam for Climate Zones 1-3, R-10 for Climate Zone 4C, R-15 for Climate Zones 4A and 4B, R-20 for Climate Zone 5, R-25 for Climate Zone 6, R-30 for Climate Zone 7, and R-35 for Climate Zone 8.
In the 2012 version of the IRC, the language was corrected to include cathedral ceilings. The relevant section of the code (Section R806.5 of the 2012 IRC) reads, “Unvented attic assemblies (spaces between the top-story ceiling joists and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters [otherwise known as cathedral ceilings]) shall be permitted if all the following conditions are met…” The required minimum R-values for the foam layer haven’t changed.
Thermal bridging still matters
Before I end this article, I’d like to come full circle, and return to the advice I gave at the beginning. Now that we’ve explored all of the reasons why someone might use the cut-and-cobble method, it’s important to emphasize that thermal bridging still matters.
So if you possibly can, try to install a continuous layer of rigid insulation on the exterior side of your wall sheathing. Exterior insulation is better than cut-and-cobble in all respects.
Martin Holladay’s previous blog: “All About Embodied Energy.”