Expert: Daniel Humphrey - 11/14/2008

Question
What is the best way to insulate a shed type roof with a 1x12 pitch in a cold climate?  Right now we have 16 in BCI's with 12 in of insulation and it is condensating.  I can tell it is condensation as it only happens when the outside temperature is in the 30's and it is dripping down my bathroom fan.  Do we need more or less ventilation?

Answer
Dear April,

Since the water is dripping from the bath fan, you need to determine where the air is exhausted to and whether the condensation is happening inside or outside of the air duct. If it is inside, running the fan more will warm up the duct and stop the condensation until the moisture is expelled. If it is coming from the outside surface of the duct, it might mean you have air leaking from the room into the insulated space. A third possibility is that the water is leaking into the duct from outside.

If you can block moist air from passing beyond the sheetrock, the condensation within the roof space will be much less. Usually, caulking around vents and light fixtures and any other penetration or crack does the trick. Another good coat of paint over the sheetrock helps too.

Ideally, all air from the interior is blocked and cold outside air moves freely from the eave to the peak. In my area, I always make soffit vents and ridge vents, keeping the insulation back so that the air can pass over. The cold, dry air removes moisture that comes up from the room.

Daniel