© 2013 ASHRAE ABSTRACT 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. A parametric study was carried 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. INTRODUCTION Conventional attic construction involves providing insu- lation on the floor of the attic and ventilation of the attic space (vented attic) from the outside. When an air-distribution system is installed in the vented attic, any duct or air-handler leakage creates an energy penalty in addition to conduction and radiation losses. An alternative to conventional vented attics is an unvented attic (UVA) (also known as a sealed attic). Insulating the attic roof deck and blocking ventilation of the attic space to the outside moves the thermal boundary to the sheathing roof line. The air-distribution system is now located within a semiconditioned space, which one would expect increases its overall efficiency, durability, and maintainability. The sealed attic can be built as an UVA with insulation in direct contact with the roof deck or with a ventilated roof deck with an airtight interior layer in the attic side of the ventilation gap. The ventilated roof deck is not common in UVAs, but is an option for a cathedral ceiling. The cathedral ceiling is an over- conditioned living space, whereas the sealed attic is only indi- rectly conditioned (no ventilation or intentionally heating/ cooling supplied to the attic). The conventional vented attics and the sealed attics both have clearly defined pros and cons. A house with a vented attic typically has lower conduc- tion heat loss and heat gain through the ceiling due to thicker insulation deployed on less heat transfer area than a house with an UVA approach, but the HVAC systems in the attic can cause an energy penalty. In a house with an UVA the HVAC system in the attic is inside the thermal boundary. The typical construction practice to build UVAs deploys spray foam sprayed directly under the roof sheathing. The UVA has its thermal boundary at the roof deck, whereas the vented attic has the thermal boundary at the ceiling level. The application to install the insulation in direct contact with the roof deck (UVA) results in a higher temperature gradient across the insulation Moisture Performance of Sealed Attics in Climate Zones 1 to 4 Mikael Salonvaara Achilles Karagiozis, PhD Andre Desjarlais Member ASHRAE Mikael Salonvaara is a research associate at Owens Corning, Jefferson, GA. Achilles Karagiozis is Global Building Science Director at Owens Corning, Granville, OH. Andre Desjarlais is group leader of the Building Envelope Group,Oak Ridge National Laboratory, Oak Ridge,TN.