Insulating open rafters in my shop

[David Hostetler]

The thought of open rafters, Washington state, and insulation all scream MOLD to me...

["Patrick G Rainsberger"]

James, I was thinking of using a spray insulation, but this would seal the sheathing between two water proof layers. If by chance I had a slight leak in the waterproof underlayment, this could eventually rot the sheathing. The insulation should buffer a rapid temperature change that could cause condensation to form. The unfaced fiberglass insulation should allow some breathablility to expel any moisture. I'm trying to rationalize here, I have no expertise.

[james glenn]

James, I was thinking of using a spray insulation, but this would seal the sheathing between two water proof layers. If by chance I had a slight leak in the waterproof underlayment, this could eventually rot the sheathing. The insulation should buffer a rapid temperature change that could cause condensation to form. The unfaced fiberglass insulation should allow some breathablility to expel any moisture. I'm trying to rationalize here, I have no expertise.

Patrick, I am not sure of the climate there so that makes it a little hard. But here in upstate NY, the temps don't get above 32F too often in the winter, so insulation directly against the sheathing will cause it to condensate. Because the insulation does allow some heat to pass at least over time, and the sheathing is always cold thus causing the condensation and the decay of the sheathing.

I get what you are saying about the potential for leak, but generally if you have a slight leak, it will rot the sheathing before you can find it even with fiberglass insul.
We have used the spray foam in the past and so far so good.

Just another idea for you

[Bob Winkler]

I'm in CT and it gets cold here. I sealed the rafter bays with spray-in foam. I'm not too worried about moisture rotting the roof decking because 1. the foam seals the rafter bays tight preventing room moisture from reaching the decking and 2. my workshop has fewer sources of moisture than a house (no shower, appliances, etc). There are several good building science articles about vented vs unvented roofs, and the debate still rages on.


I'm not worried about roof leaks, and given your use of water & ice shield, I wouldn't either if I were you.

[Steve Ryan]

I had Icynene foam sprayed directly on my roof sheathing, 10", and in the 2x6 stud cavities. The theory is that the surface of the foam will never get cold enough to condense moisture, and the foam prevents moisture from getting to the rafters and sheathing. That was two years ago and the addition is close to being finished, but the HW baseboards are not in yet. Residual heat from the main house and sunlight keep the addition about 55 on the coldest 0 degree winter days, and the addition is bigger than the house. Icynene is 3 times the price of fiberglass, but the payback can be as little as 5 years.

[Kevin W Johnson]

Is there a reason to keep the trusses open? Other than "I prefer it that way"? With them open, it will cost more to heat your shop, when you do heat it. It will also extend the amount of time it takes to heat your shop to your comfort level.