I’m fixing up an old uninsulated shed. The roof — built of metal sheeting on plywood, nailed to the 2-by-4 rafters — needs to be insulated. What is the best way to insulate the roof that avoids condensation, which might lead to mould or rot?
I can think of two options:
- Install batten insulation between the rafters, and then cover this with a vapor barrier.
- Skip the batten insulation and tack polyisocyanurate boards onto the underside of the rafters, leaving an airspace between the rafters.
Option 1 risks trapping condensation between the insulation and plywood. Option 2 doesn’t seem to offer very much insulation.
Are there other options to consider?
Best Answer
You need to prevent air from getting to the metal. If no air circulation there is no moisture will condense on the inside. Spray on insulation works well but is fairly expensive, Fiberglas works but needs to be in contact to prevent air movement. This is one area that I compress the insulation some to make sure it stays in contact. This looses some of the R value But eliminates the sweating. Those are the 2 methods I have used.