Closed-cell foam on attic floor?
Jake Huff| Posted inEnergy Efficiency and Durabilityon
Does it make sense to use a couple of inches of closed cell/ or open cell foam to seal my attic floor before adding blown fiberglass? The home would have a vented attic and located in Zone 5. Just wondering if it is overkill. Any thoughts would be appreciated.
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It doesn't take a full covering of foam to air-seal the floor, but it SHOULD be air-sealed at all penetrations prior to blowing insulation- it's essential in fact. But you'd likely be able to air-seal the attic floor with less than 20 board-feet of foam and a few tubes of acoustic sealant caulk, with maybe some sheet-metal & fire rated mastic to air seal around flue penetrations. What it takes depends on what you really have, but sealing with a full cover of foam only makes sense for sealing planked surfaces. It's extreme overkill for air-sealing sheet gypsum/plywood/OSB, even if you had a hundred electrical & plumbing penetrations to deal with.
Blown fiberglass significantly under-performs cellulose in open blown applications, at the wintertime temperature extremes you'd see in a climate zone 5 location, unless a top-side air-barrier is added (which isn't always practical). Low density cellulose is an order of magnitude more air-retardent than low-density fiberglass, and effectively blocks air convection between the cold attic and the entrained air in the insulation layer, at least from a thermal performance point of view (but not from a moisture-transport point of view), whereas without top side air barriers fiberglass loses performance with increased temperature difference in a cold-side-up configuration.
Thanks Dana!
As tangential question to OP's:
I live in zone 5 as well. About 3/4 of the ranch level of the home has a 13-ft ceiling with the rest 1/4 9-ft. So, there's a knee-wall in the attic where the ceiling height drops down. Would Open-cell Spray-foam be the best way to insulate it? I am considering a local Icynene contractor for the work. I am not considering closed-cell foam because of the high GWP. Or are there better ways to insulate the knee-wall? Thanks in advance.
Jake,
Dana gave you good advice. Here is a link to an article that describes the necessary air-sealing work that needs to happen before the cellulose is installed:Air Sealing an Attic.
Venkat,
Q. "Are there better ways to insulate the kneewall?"
答:什么类型的insulatio并不重要n you use, as long as the work is done with close attention to air sealing. If a fluffy insulation is chosen, there must be an attic-side air barrier. For more information on reducing air leaks through kneewalls and insulating kneewalls, see:
Two Ways to Insulate Attic Kneewalls
Solving Comfort Problems Caused by Attic Kneewalls
Video: How to Air-Seal an Attic Kneewall