Space Between Windows and Framing
Not completely filling spaces around windows due to pressure:
I remember reading something, somewhere (perhaps here) that one should not fill up the entire space between the windows and framing due to pressure. It was/is something I do not understand…but it stuck with me.
I’m using backer rod followed by caulking to insulate and seal the space. I only needed 250′ of the backer rod but….found that I could get 1500′ for only $5 more. So now, I have 1500′!
With plenty of backer rod, I thought “Why not do two layers in the voids for the extra insulation?”. Then, that thought lead to the question “Heck, why not use three layers?”. I have the material, after all…and the time.
I could press in three layers of backer rod around my windows and still have some empty air space….which, from what I remember reading, was desirable….for some pressure related reason.
Could someone explain this to me and advise on my idea of three layers of backer rod?
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Replies
I don't know the answer to the pressure question, but in general, air spaces of about 3/8" or less are too small to generate internal convection currents, so as long as they are airtight, they insulate at least as well as insulation. All insulation depends on trapped air for its insulating ability (other than newly installed closed-cell foam, where the gaseous blowing agent in the cells does the insulating until it is displaced by air). I don't see a downside to using additional backer rods, but no real advantage either.
Thanks, Michael. Most of the spaces in my rough openings are 1/2".
I searched for the pressure piece I had stumbled across to no avail. I'm almost certain it was on these forums.
Can anyone explain it?
I've certainly seen caution urged against filling the void with high expansion foam, because the pressure exerted by the foam expanding can bow the window jamb inward.
Is that the sort of thing you might be remembering?
Patrick,
Christine Williamson champions leaving a gap, especially with rain-screens, to equalize the pressure around those spaces with the outside to stop water forcing its way in past the exterior water c0ntrol layer. The idea comes from larger (and taller) commercial buildings where pressure differentials between the wall and exterior came be quite high.
What I don't know is if moving this approach to houses has been studied and is effective, or was is just suggested because it works on those bigger projects. Maybe someone else has looked into this and can comment.
Leaving a gap of an inch or so behind the nailing flange does provide a drainage path for any bulk water that gets through the window frame at the jambs.
Patrick,
I've seen that caution many times...but wasn't what I was remembering. Malcolm described it in his post. Thanks, Malcolm!
Dean
The most recent windows I installed said that the use of any spray foam would void the warranty. I don’t know how many other manufacturers say this but another reason to forgo foaming windows.
While we’re at it, because I’m about to fill in a bunch of windows, reason you’re/anyone use backerrod and caulk over cans of window spray foam? My guess is some carbon/ global warming potential but I have never read the reason so thought I’d ask.
brooksbend,
The caulk/backer rod is primarily for air-sealing, the foam for insulation. Some people worry the foam won't retain a good air-seal. I'm not entirely sure insulation in that narrow, short gap makes much difference in many climates.
Brooksbend,
I decided against spray foam due to a few reports of people finding that it had disintegrated after ~10-15 years.