More Q&A Spotlight
The gap between a window frame and the rough opening was, until fairly recently, just that—a gap. It wasn’t without purpose. Windows in the Victorian era employed the gap to house the weights that helped raise and lower the sashes. Today, it’s a space that allows window installers to use shims to make sure the window is plumb, level, and square, to allow for drainage below the window and to center it in the opening. These functions aside, once the window was installed the gap was simply left as open space. What occupants experience as drafty windows was often a drafty gap around the window.
Sometime around the 1960s, it became standard practice for builders to stuff the gaps with fiberglass insulation, on the theory that it would stop the air from leaking around the windows. (Hey, it was a start.) More recently, canned spray polyurethane foam has been used to fill the gap, with mixed results—it may create an air seal, but it may not allow any water to drain from the sill. Lately, installers have been using various combinations of foam backer rods, caulk, spray foam, mineral wool, and/or tape to effectively insulate and air-seal this gap. There doesn’t seem to be one proven method, as the conditions can vary from house to house.
In a recent Q&A post, homeowner “tjones1014,” whom we’ll just call T Jones, is helping to remodel his house. He writes that he’s having a hard time sealing the gap with spray foam, without completely filling the gap. “We want to leave a small path for water to exit, as our sills are sloped toward the exterior,” he says. “Is putting backer rod in the gap before spray foaming to control the depth a decent solution to this?””
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8 Comments
This article would be much improved by including building science field testing of the different systems being offered. The suggested solutions are hypotheses in need of verification. Well designed testing could include air leakage, moisture movement, aging of materials, movement of structure, ease / difficulty of construction and probably a few more factors. And, add in cost of installation.
Henry, for the "Spotlight" series, Chuck uses forum questions and has someone knowledgeable add their thoughts to create an article. The elements you mention are exactly the things that the commenters and the guest expert consider when sharing their thoughts. Few of us care only about performance; we all operate in the real world and have to stand behind our work so we take costs, longevity and interactions with different materials into consideration.
A full-blown, well-designed study of these options would be great but it would take a lot of time and money to complete. If you have several tens of thousands of dollars and weeks of time to do the testing, we would all love to see it. I'm confident that the results would show that what was suggested are all good options.
Henry,
This isn’t an article about insulating around windows. It’s a closer look at one of the recent questions in the Q&A - and the format of this regular feature is to have an expert comment of the advice the OP was given. While maybe desirable, I don’t think it’s realistic to expect GBA to commission field testing of every solution offered as advice.
I am surprised that this hasn't been mentioned: standard 'canned' foam (e.g. Great Stuff) is brittle after it cures and can crack / separate from the wood. So EVERYONE should use the 'WINDOW and DOOR' version, which remains flexible after it cures. This flexibility is essential around windows and doors, which shift as temperatures change. For example see:
https://www.homedepot.com/p/GREAT-STUFF-12-oz-Window-and-Door-Insulating-Spray-Foam-Sealant-230612/202893747
If you are working on a larger project and need to start/stop the foam application, you should definitely use a Great Stuff 'Pro' gun and 'Pro' cans. The straws used in the retail cans often freeze up between applications.
I hope this is helpful.
BvilleBound
I think we all had the window-and-door type of canned foam in mind when providing our experiences and opinions. It holds water like a sponge and it's not a reliable air-seal in tight spaces.
Someone claimed that Great Stuff canned foam will absorb water and won't maintain an air seal. This is in correct, on both counts. See:
https://www.greatstuff.dupont.com/faq.html
bvillebound,
The consensus in this blog and elsewhere seems to be that relying on any foam as the air-barrier isn't a good idea. I'm with Michael Maines: I haven't used foam around windows and doors for years.
Many DIY and contractors have used standard canned foam, or worked on a remodel where someone previously used standard canned foam - around windows, doors, etc. - and found that the foam cracked or separated from the wood. This should not be a surprise. Standard Great Stuff and similar one-component canned foam expands aggressively and becomes hard and brittle when it cures. As framing naturally expands, contracts and shifts, the foam cracks. Hence the "consensus" you noted.
This is why Dow and other companies created versions for windows and doors, which remains flexible after it cures. These versions also do not expand aggressively, which can distort the trim / frame around windows and doors. So use the right version, and it will work great. Use the wrong version and it won't. This is true for many types of products.
https://www.greatstuff.dupont.com/window-door-foam.html
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