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DavidYon
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Posts:53
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| 10/14/2008 9:38 PM |
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Over in the Geo forum I have a short description of my attic problem, but here's the longer version:
- 1200 sq ft of ceiling (approx), 10/12 pitch on the roof
- 2x6 joists
- Original insulation was blown-in fiberglass, allegedly to R30
- No vapor barrier
- Electric vent fan in roof with soffet vents
I have since done the following damage:
- Added light fixtures in upper-floor rooms
- Put a subfloor down over most the area for light storage
- Added drop-down stair for easy access
This has compressed the insulation over most of the area. I'm guessing I'm down to R12 or so. The fixtures and stair have probably created lots of air infiltration problems (blower-door test last year showed .6 ACH). In the winter if I run too much humidification I get condensation under the subfloor---interim fix was to drill lots of 1/2" holes in the subfloor to give the vapor an escape path.
So now there is a big subfloor up there with lots of stuff on it. If I was starting fresh I'd have spray foam or cellulose put in, which would give good air sealing and a higher R factor at 6". But I don't want the expense and hassle of ripping everything out so that I can evacuate the fiberglass and put down the foam.
My original thought was to pull up only the center of the subfloor area so that perhaps enough access could be given for vacuums to pull the fiberglass and foam to be sprayed in. That would air seal and give me high R-20s. If the spirit moved me I was considering a DIY addition of putting R10 rigid EPS boards over the roof trusses (also 6") and cross-beams, enclosing the space. That would extend the thermal envelope and enclose the storage, moderating the temperature. There would still be enough spacea for the vent fan to pull air since the foam board would mounted to the trusses rather than between them.
I spoke with a spray foam installer over the phone, and he was pretty skeptical. Didn't think the pull-up-the-center idea was enough access for closed-cell insulation, and would have to resort to open-cell injection foam. Claimed that I would still be left with an air infiltration problem on open-cell. He also thought that pulling out the fiberglass would be problematic without complete access. His opinion may change once he sees the space on Thursday, I don't know.
His initial recommendation was to just close up the attic fan and spray the roof directly, sealing off air at that level. It's similar to my idea but puts the air sealing at the roof rather than the ceiling. It also converts to a hot-roof non-ventilated space. That would give me high-20's R factor at the roof, plus whatever the fiberglass gives, the total probably around R40.
But this has me nervous on a couple of fronts:
- The common wisdom I've heard repeatedly over the years is to "let the roof breathe", so always leave an air space and ventilate well.
- Since I've been having condensation under the subfloor, seems like I still run that risk given the vapor barrier will be in the roof not the ceiling.
I can see where if you foam-in the roof properly and get a good vapor seal, that you address the first option by blocking moisture and getting the same "top-side is same temp as bottom-side" result as with venting.
I can also see that since the attic will be in the middle of the thermal envelope rather than the outside, that condensation could potentially be a non-issue.
Unless the first issue is a really high risk (and I suspect it isn't), I suppose I can always try foaming the roof as a first step. If moisture travel truly becomes a problem then I could just bite the bullet and foam the ceiling, moving the air barrier down.
Thoughts?
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