Home Repair Expert: Zolton Cohen

Kalamazoo Gazette

Q: I had my roof sheathing replaced a year ago; it was plywood and had delaminated. The contractor thought it might have been due to excess moisture in the attic. This winter, I went up into the attic through the pull-down stairway to put away Christmas ornaments and was surprised to find frost on the underside of the new particle board sheathing. The contractor had added roof vents, so this was not supposed to happen. The interesting part is that the rest of the house seems very dry to us during the winter even though we run the furnace’s humidifier and also a portable one upstairs. Why is our attic wet and our downstairs dry?

A: Your attic is wet because much of the moisture you’re putting into the air in your downstairs is winding up in the attic. Up there, it condenses on the cold framing materials. If this situation is not corrected soon, you might find mildew or mold growing up there.

This sounds as though it may be a two part problem. One is that you are likely putting too much moisture into the house air. The other is that this humid air is finding easy routes into the attic. You might be able to correct both problems by finding out where the air is leaking into the attic. After sealing the leaks you might find that enough moisture stays in the downstairs that it is comfortable without having to run auxiliary humidification devices.
How is the moisture getting into the attic? One suspect is that convenient pulldown stairway. Unless you’ve gone to great pains to seal up the perimeter of the stairway door, air is likely rushing up through the gaps and fissures along its edges.

Incidentally, air in the upper floors of a house is pressurized; its heat makes it buoyant and that forces it upward, as it does inside a hot air balloon. So its leakage through holes in the ceiling is not passive and leisurely. It is driven. And that air contains a lot of the moisture that you’re adding to it with the humidifiers.
Creating a good seal for a pulldown stair with compressible foam weatherstripping tape is difficult. The door tends to sag away from the tape unless held in place with a mechanical latch. However, there are kits available commercially that enable you to construct a box that fits over the stairs inside the attic (check out the Battic Door Attic Stair Cover system, at https://www.batticdoor.com, or Battic© Door Energy Conservation Products – P.O. Box 15 – Mansfield, MA 02048-0015).
Other sources of air leakage into attics are older style recessed canister ceiling lights, each of which can allow up to a third of a gallon of water a day (in the form of water vapor) to pass from the house into the attic, and ordinary hanging ceiling lights with bases that seal poorly.

Inside the attic, chases or wiring holes, unsealed drywall along interior and exterior wall top plates, and openings alongside chimneys can all contribute to air bypass. These can all be successfully sealed with expanding foam or caulk, or sheet metal in the case of the chimney gaps, though the work is messy and uncomfortable. The result, however, will yield less moisture reaching the cold attic (and more moisture retention in the house below), less heat escaping from the house, and lower heating and cooling bills


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