Moisture-safe attic design in extremely cold climate: Hygrothermal simulations

Attic ventilation is a commonly used method for the removal of moisture build-up in attics through air leakage from indoors, which is generally applied to cold climates. Un-ventilated attic is a fully sealed construction, which can prevent wind-driven rain penetration or snow accumulation. However, it does not provide an effective path for moisture removal, which may cause moisture-related issues. Northern Canada suffers from extremely cold conditions and very fine snow particles caused by extremely low temperature have high chances of penetrating into attics through vents or un-intentional openings during periods of high winds. To provide adequate attic design recommendations for this climate, hygmthermal models of ventilated and un-ventilated attics were firstly validated by comparing with field measurements and then used for a parametric study. The attic ventilation rate, ceiling air leakage rate and un-intentional air infiltration rate are set as variables for the parametric study to evaluate their effect on the hygrothermal performance of attics. Mold growth index (MGI) on sheathing is used as the performance indicator. For the ventilated attic, there is no mold growth risk, while for the un-ventilated attic, the MGI increases with the increase of ceiling air leakage rates and the decrease of un-intentional air infiltration rates. North-facing roof sheathing has higher MGI compared to south-facing sheathing. The parametric study results show that attic ventilation is required for the extremely cold climate and an un-ventilated attic has greater risks of moisture problems unless the ceiling air leakage from indoors is well controlled.