Moisture performance of sealed attics in climate zones 1 to 4

Sealed (also called unvented) attics are insulated at the roof deck while the ceiling plane is uninsulated. Sealed attics are usually not directly conditioned, but instead the conditions float between indoor and outdoor temperature conditions. The moisture performance of sealed attics has been investigated using a whole house hygrothermal simulation model to understand the risks for high moisture content in the roof sheathing and for high humidity in the attic. The results have also been validated against field test data. The analyzed shingled roof decks have been insulated either with open-cell spray foam or air-permeable insulation (fibrous insulation). The concept of the air-permeable insulation to perform as the condensation-control layer is analyzed. Aparametric studywascarried out varying attic air leakage both to the indoors and outdoors, water leakage to roof sheathing, vapor permeance of the insulation, as well as the indoor air-moisture loads and exterior climate. The results show that the sealed attic can experience elevated humidity at warm temperatures which can create favorable conditions for mold growth unless the attic is intentionally conditioned or via duct air leaks. The vapor permeance of the insulation layers (and if a vapor-retarding coating is present) was a key factor that controlled the moisture content of the roof sheathing together with the overall airtightness. The air impermeable but vapor permeable insulation was not sufficient alone to prevent condensation in the roof deck.