A case study in condensation

Evaluating and preventing attic condensation in a residential building

Condensation on the undersides of roof decks in attics can be a common problem for different residential roof systems. Condensation in attics can occur more frequently in the colder climates of the upper Midwest and Northeast and typically on roofs that have inadequate ventilation and insulation, as well as attic bypasses. When dryer vents, bathroom fans or furnace exhaust ducts vent directly into an attic space, the problem is intensified.

Although building codes recommend and require adequate attic ventilation above roof insulation to remove heated air and water vapor that has entered an attic from the surrounding air or a structure's occupied portion, the codes are not strictly adhered to during the design or construction phases. In addition, attic bypasses and openings in dryer vents and furnace ducts must be sealed to minimize moisture infiltration into an attic. This would prevent troublesome condensation within the attic space and on the roof deck's underside. Unfortunately, joints in vents and furnace ducts sometimes are not installed in an airtight manner.

The primary signs of severe condensation in an attic space (as viewed from the living spaces) are water stains on ceilings, windows and walls. Such stains generally are the result of melting frost and/or icicles on a roof deck's underside. When staining because of melting frost and/or icicles occurs to a building's interior and damages interior finishes, prompt action is required to limit water damage. Water also can saturate insulation, reducing its effectiveness, and contribute to conditions that would be favorable for the development of stains that appear to be mold, as well as long-term deterioration of sheathing and framing.

A case study