Sealing the attic floor

Posted Wednesday, January 18, 2012 in Sustainable Maine

Sealing the attic floor

by Paul Kando

Most people know it is important to insulate the attic. Unfortunately, far fewer know that the attic floor must also be sealed – before it is insulated. The “houses must breathe” generation of contractors likewise overlook this detail. Yet air leaks through gaps in the attic floor can cause more problems in a house than even lack of insulation.

We all know warm air rises. If there are leaks – even tiny holes – in the top-floor ceiling (i.e. the attic floor), warm air escapes into the attic. You cannot feel this is happening because the warm air is moving upward, away from you. The warm air carries with it moisture it has absorbed from the house. Up in the cooler attic (and the more insulation on the attic floor, the cooler the attic) the air will release the moisture it can no longer hold. The moisture condenses on any cool surface it finds, for example the inner surface of the roof. The result is water damage and, since with condensation energy is released as well, the condensing water can warm the roof and cause ice dams. How much water? A lot: German researchers report that a hole as small as a millimeter across can convey as much as 1½ cups of airborne water per day. In short, a leaky attic floor causes not only large heat losses, but also moisture damage and a buildup of mold. Therefore air-sealing the attic floor should be a high priority.

If you can get access to the floor by lifting the attic insulation, your job is fairly easy. The most frequent leaks found in an attic floor include points where wiring, pipes and stacks penetrate from below; wiring boxes (over ceiling-mounted light fixtures below); the casings and ducts of exhaust fans; and cracks and failed joints in the ceiling below.

Look for dark spots on the insulation; they usually indicate an air leak underneath as the rising hot, moist air also carries dust, which deposits in the insulation. Other ways to locate probable leaks is to trace the location of wire and pipe penetrations, including measuring the location of a light fixture on the ceiling below and transferring that measurement to the attic.

Lift up the insulation above the leak and seal the leak with caulk, or if the leak is large (such as around a stack) with spray foam. In the case of electrical boxes you must caulk both around the box and the small pre-drilled holes on the top and sides of the metal box itself. Do not spray caulk or foam into the electrical box itself. Leaky exhaust fan-casings can be foam-treated or caulked the same way. Leaks in exhaust-fan ducts (often flexible and made of plastic) can be judiciously taped.

While working on exhaust fans (notorious leakers of warm air out of the house), make sure that any flexible ducting is free of kinks and that any water condensing inside the duct has a way of draining to the outdoors. No exhaust duct should ever terminate inside the attic. Also check that the outdoor end of the exhaust duct ends in a suitable outlet equipped with a flap or ball closure that seals the opening when the fan is not running. A properly installed exhaust fan/duct system should not leak air with the fan off.

Other major leak areas in the attic floor include the chimney chase, walls with open tops, recessed lights, attic hatches and folding stairs. These require special treatment and are the subjects of individual columns.

In addition to sealing all penetration-leaks, the attic floor also needs a continuous air/vapor barrier. In new construction one choice is a continuous plastic sheet taped at all joints, installed on the attic side of the sheet-rock or plaster ceiling. This is seldom feasible in a retrofit situation. An easy substitute is to turn the surface of the ceiling itself into a vapor barrier by prime-painting it with a shellac-based primer, such as BIN and Kilz. These are quick-drying primers that also offer the advantage of hiding stains, even the kind that would show through ordinary paints. Once the primer is dry, the ceiling can be coated with any finish paint of choice.

What if you cannot access the attic floor without dismantling a finished floor or moving tons of stuff? Then you can resort to a hot roof, by insulating the attic’s sloping roof and the gable ends. Hot roof will be the subject of its own column.

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