When warm moist air from inside the conditioned living area of a home makes its way into the attic, we need to vent that air out in a quick and efficient manner. This is especially true during the very cold Minnesota winter months.
In the picture above, we see frost accumulating on the underside of the roof deck in the attic. The warm moist air that has made its way up from the living area below is not being vented out efficiently. The air contacts the cold roof deck and releases some of its moisture in the form of condensation. This condensation then freezes, causing the frost.
Eventually the frost will melt, allowing moisture to drip onto the insulation and wood components below. This can get bad enough at times that it may show up as a water stain on the finished ceiling below. The resulting build-up of moisture in the attic is a major cause of building material deterioration and usually results in mold growth.
There are multiple angles to attack this issue from. The first is properly sealing the conditioned living area of the home from the cold attic. This is done using plastic sheathing behind the drywall and by properly sealing any penetrations through the ceiling, such as recessed lights and the attic access hatch. Keeping the interior conditioned air out of the attic is the primary line of defense.
We also want to make sure any HVAC ducts that are routed through the attic are well-sealed. This includes bathroom exhaust fan ducting, as the air in those ducts is usually very damp. Finally, we want to make sure the attic ventilation system is designed, installed, and operating properly. This helps ensure any warm, moist air that does get into the attic, will be quickly transferred to the outdoors.