Attic Insulation Retrofit and Moisture
My Massachusetts house was built in 1901 and renovated in 1998. As part of the renovation we added R30 fiberglass batt insulation to the vented attic. The second (top) floor has forced air heat from an airhandler in the attic. Ducts have insulated jackets. We have about a dozen recessed canister lights in the second floor ceiling. Most of the top floor has the original beadboard ceiling, and the fiberglass was just laid down in the attic joist bays over the beadboard.
Of course this attic leaks like a seive. Because air sealing the beadboard would be impractical, and because I didn’t want to move or remove the fiberglass, I added R20 of Icynene spray foam to the rafters and sealed the vents. Now I haved a sealed attic with fiberglass on the floor and foam in the rafters. But I also have a stone foundation and high humidity. I have just installed a dehumidifier and I think I should be able to keep the basement at no more than 50% relative humidity.
Do I need to worry about mold in the attic this winter? From a bit of reserch it seems like mold/mildew need RH of 60 or more, and temperatures of 55F or so. If I keep the house at 68F and 50% RH, the dew point should be 49F. As long as the attic temperature at the foam is above 50F am I OK?
thanks in advance!
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Replies
Bil,
In your climate, Icynene installed in rafter bays needs a vapor retarder on the side facing the attic. The usual way to accomplish this is to spray vapor-retarding paint on the underside of the cured foam.
If you don't do that, interior moisture can diffuse through the open-cell foam and accumulate in your cold roof sheathing.
Thanks Martin. I have been considering putting a perforated radiant barrier product on top of the fiberglass in the attic floor. Do you think that would do the trick as a vapor retarder, while also giving me a little bit more insulation performance?
Bil,
No, a perforated radiant barrier is not a vapor retarder. Nor is it a dependable way to improve the performance of your attic insulation. (As soon as a radiant barrier gets dusty, its performance begins to deteriorate.)
Attic air can be humid enough during the winter to lead to moisture accumulation in your roof sheathing unless you spray your cured Icynene with vapor-retarder paint. I urge you to do that.