Question from the field… performance after cutting spray foam
hull187
| Posted in General Questions on
Hey all,
I’m a contractor interested in building performance. It’s common practice that after installing windows and doors, you spray foam to seal the rough opening. This then gets cut back to accept drywall/trim/siding.
I’ve always wondered if cutting the spray foam affects the spray foam’s performance? Before cutting, the foam develops a nice hard skin and it seems to me that doing away with this might be a bad thing. Is there any data on this?
Thanks,
Steve
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Replies
Steve,
I've heard builders express the opinion that you just shared, but I haven't seen any research or data to back up the theory.
Here's what I do know, however: even if you have a very careful trigger finger, and manage to install just the right amount of canned spray foam around a window so that it doesn't expand beyond the plane of your framing, so that no trimming is required, the spray foam still isn't airtight. You'll still get a little air leakage -- either through pores in the foam, or due to imperfect adhesion with the framing lumber and window frame.
That's why builders obsessed with airtightness use a belt-and-suspenders approach (canned spray foam plus European tape on the interior).
Steve,
Don't ignore chinking to air seal the window frame. The new formulations really are amazing materials:
http://www.sansin.com/product/timber-tec-chinking/
I like this product, a lot.
had the same question? any updated in 10 years?
One big change I've learned in recent years is that filling the gap with insulation is far less important than providing good air seals at the interior and exterior. Leaving the gaps unfilled is fine as long as you use sealant and backer rod, or a high-quality tape, at the interior. Flashing tape should seal the exterior.
I agree Mike, the Rvalue potential from the small amount of insulation that could be sprayed or stuffed around a window is minimal at best. While it certainly doesn’t hurt to insulate this gap, I think time is better spent on sealant or taping.
sommerbros,
And if it is empty it gives you a drainage plane down onto the sill-pan if there is ever a leak.
Christine Williamson suggests insulating the inner third of the gap, and leaving the rest open.