Spray foam, moisture, air circulation
I’ve been planning to add insulation to our attic. Although our climate is relatively mild (SE Pennsylvania) I live on a high hilltop and get air, which is what the neighbors call our high winds. The home is pretty tight and ordinarily not too difficult to heat. But when it’s 5 degrees and 20-25 mph winds gusting 30 to 50? You wear a heavy sweater and listen to the heater run a lot.
The attic is mostly unfloored with fg batts between the floor joists. There is a working replacement window at one end and, at the other, the window has been replaced by a fan with louvers. We seal the fan up with plastic sheeting in winter and use it to ventilate the attic on hot summer days.
Our first thought was to add chopped fg to double the depth of the batts and cover the tops of the joists. Now, I’m interested in spray foam, but have misgivings.
Chiefly, I dislike the idea of the foam directly on the underside of the roof deck. I worry about trapped moisture from any future leak, and about the temperature of the roof on hot sunny days. I have seen here, and on our last home, how much more quickly the shingles on the Southern face of a roof degrade than those on the North. And no one is ready to swear that it’s all UV, not temperature.
I’m thinking to apply some sort of material (OSB?) to the bottoms of the roof rafters and to spray foam on that material. That way I preserve airflow from the soffit vents to the ridge vents and trap no moisture against the roof deck. This would make the attic airtight (Depending on what I do about that fan.) and leaves me wondering about moisture migrating from the level below. The bathroom on that level is now vented with a Fantech fan to the exterior, but surely there is other moisture.
Your opinions/thoughts/comments are welcome, desired even.
Replies
I'm kind of on the same path as you right now, except it's new construction.
I haven't made up my mind yet, but I'll tell you what I've heard -
I was told that shingles may last longer because the attic doesn't remain hot at night, and they claimed that shingle damage was from them getting "cooked" from the bottom (hot attic).
Also am told that closed cell foam doesn't absorb moisture at all, so leaks dry out to the outer surface (don't know about that one).
Also, you need to close off any ventilation to the attic, make it conditioned area in a sense.
I don't think I would close off the bottom of the rafters - that may cause some moisture problems. -?
I think you opened a can of worms - you'll get some responses. Thanks for posting it.
Bump...insulation pros?...........
buildingscience.comAndy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Once and Future King
And http://www.greenbuildingadvisor.com
And search the archives here for "spray foam." Acres of reading.
Thanks guys...