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I have a small attic (19 * 12) with an 8/12 pitch that I use for storage. The attic is not vented and has been that way since the 1890’s. I do not see any moisture problems (i.e. ice crystals, black spots, mold, etc.) I am located in Wisconsin and have been in the attic when its -25 F outside. What I want to do is attach 1″ 4 * 8 blueboard to the rafters and foam/tape seams and edges. I would leave a 3″ gap at the top to blow in cellulose to fill each rafter bay then seal the 3″ gap with blueboard. I would like to eliminate the ice damming that occurs and also maybe keep the attic cooler in summer. Am I on the right track?
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I don't see how relatively low value R-14 insulation is going to reduce ice dams to any great extent. Your attic may become cooler in summer with less heat radiating through roof but I hazard to think of the increase in temp experienced by your roofing material with no circulation below - some shingle mfg. say no problem but given the quality of some of today's shingles I'd venture to say you may get some bubbling of shingles - I've been seeing that on roofs with newer 2nd layer of shingles. You are creating a modified "hot roof" (seen in some vaulted ceilings but sheathing too often suffering heat and moisture damage) but insulation value is low so still will have significant heat loss and, as in hot roofs, there is no venting thus moisture control, dew points, and condensing surfaces must be closely coordinated. You mentioned that moisture was not problem in the past but you are introducing a significant new factor to the formula and there will be a reaction somewhere.
*Fred Lugano does what you want to do for a living...He says you can definitely improve your situation.http://www.google.com/search?hl=en&q=Fred+Lugano&btnG=Google+Searchnear the stream,aj
*Pyro,If your house is circa 1890's it is probably balloon framed and has major air bypasses into the attic from the main living area. That would be the first thing I would address. But I think your insulation plan is sound. My one thought is that I don't think exposed foam is up to firecode. Sheetrock or OSB over it would solve that.Randy,He's going to do R-14 plus the cells which will probably put it closer to R30 depending on how deep the rafters are. That ought to significantly reduce or eliminate the ice dams. As far as moisture goes, the densepacked cells along with the foam ought to eliminate just about all air movement through the roof cavities, which is the source of about 95 percent of all moisture transference. Shingle life is an endless argument. The numbers I've seen in various articles say you lose maybe 5% of the shingle life. Seems a small price to pay in order to not have to pay to heat the great outdoors.Steve
*see another thread cuurently running: Bob Walker "Attic Condensation" 1/12/02 7:57am
*Thanks for the responses. I believe the framing is baloon, not 100% sure how to tell. I tore out most of the plaster / wood lathe and used unfaced fiberglass bats (R19) with a plastic VB throughout most of the first and second floor. Dumb didn't know any better in retro should have used cells. The attic rafters are real 2 * 6 (not dim) as far as I can guess the R factor I'm sugesting would be around 28 (6 for the foam plus 22 for the cells). The shingles are 2 year old GP 30 year dimensional over 1/2 OSB over 1" fir. I believe I have substantially reduced warm air into the attic. The attic faces the West the East attic has been vented.Q1 Do you think shingle temp will raise enough to stantially ruduce there longevity? Ice dams can't be good for them either.Q2 What other alternatives do I have - I really do not want to vent this attic. I thought about having the rafter bays sprayed with Icynene but do not know what advantages or costs that would be involved.Thanks again, Pyro
*Pyro,Since you are planning to insulate at the rafters rather than at the attic floor, air leaks into the attic become inconsequential, since the attic is still within the thermal envelope...another good reason to proceed as you plan. The walls themselves are not as critical as the air seal at the top of the house, so I wouldn't kick myself too much over what's in place already.I've seen it in print, though I don't recall where, that an unvented roof deck will cut approximately 5% off of the shingle life. That's 1 1/2 years on a thirty year roof. Not anything I would worry about. What color is your roof? Darker roofs are more prone to heat degradation, whether they are vented or not.Sprayed on foam would be the best performing solution, both in terms of moisture control and air sealing. It's quite a bit more expensive, but if you have the budget for it, it's bulletproff (IMHO). Also, you would have less trouble convincing your inspector that it's a code-acceptable solution.Steve