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Attic insulation - how to fix missing/inadequate air barrier

Attic insulation - how to fix missing/inadequate air barrier


Old 02-16-10, 06:56 AM
Thread Starter
Join Date: Jan 2010
Location: North Carolina
Posts: 55
Attic insulation - how to fix missing/inadequate air barrier

Poking around in the attic, I found that there is no special air/vapor barrier under the blown-in fiberglass insulation other than the gypsum wallboard and the paint on the other side of it. In some places, particularly around the attic hatch and into the adjacent attic space over the bonus room, there are faced fiberglass batts. Some of these are grey/black where they touch the joists, indicating that air is coming through. Indeed, I can actually feel the air, so there are certainly gaps in the wallboard. I haven't mapped the walls below the attic, but I wouldn't be surprised if the air is coming in through the wall/ceiling junctions.

I was planning on increasing the existing insulation by putting R-30 unfaced batts on top, at a cost of approx. $0.30/sq.ft. I know that fiberglass doesn't stop air flow, so how do I best seal everything first? Would it be sufficient to go around and spray Great Stuff into every wallboard/joist junction, or is there a better solution? I could also put down 1/2" of rigid foam board, edges sealed with Great Stuff, but that would obviously increase the bill. Would it be better to move the blow-in fiberglass aside, put blow-in cellulose down and then cover that up again with the saved blow-in fiberglass? That would be a lot of work. Or simply put cellulose on top of the fiberglass (approx. $0.40/sq.ft. for R-30)?

Thanks! MM
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Old 02-16-10, 09:23 AM
Join Date: Feb 2010
Location: usa
Posts: 38
I am going to jump on-board in this thread with the same question:
I have 50 year old rockwool/fiberglass insulation with paper backing in the attic. The paper is black and brittle . Because the house has knob and tube wiring (which is in good shape and has no need for replacement) there are 1" air gaps along the celing joists wherever the knobs are poking in on the insulation. There are also air gaps wherever there is stuff under the insulation (such as the wooden supports for closet doors.)

I am going to go back through the attic and try to seal the air leaks around the wires/cans/boxes/etc with expanding foam and caulk.

However, my question is the same as xvimbi's: what should I do about the inadequate vapor barrier? I want to either blow in cellulose over top or (preferable) install rolled fiberglass insulation (this is preferable only because I can roll it up to get access for running wires, etc.)
Old 02-16-10, 12:24 PM
Join Date: Oct 2008
Location: New England
Posts: 9,995
Two different homes will be a bit confusing.
For both questions, air barriers and vapor barriers can be one-in-the-same, but don't have to be. Sheetrock makes a fine air barrier, as long as all of the bypass leaks are sealed. Those bypass leaks can be almost anywhere as the sheetrock is normally not glued to the framing. So air can enter under walls, through electrical boxes and light fixtures, under sink plumbing, and hundreds of holes for wiring and pipes through the house. Once air is behind a wall, any wall, it can seep between framing and drywall via an adventurous path into the attic.

Here is a link to get both of you going on air sealing. http://www.efficiencyvermont.com/ste...ide_062507.pdf

jm, dealing with K&T is risky and not something I can advise on. Insurance companies, inspectors, and prospective buyers don't like to see it, so the sooner you remove it the better. I know you mentioned future wiring, but from my position, hundreds or thousands of miles away, I can provide only that advice. Others may chime in with different thoughts or you can ask over in the electrical forum about your options. They are very knowledgeable over there.


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