Expert: Nazim Nice - 9/19/2009

Question
I have a cathedral roof built with I-joist and insulated with r-38 faced insulation. I had a professional insulator come in and spray foam insulation at the top of the wall filling the cavity so that no air could pass through the soffit into the roof. I didn't put a ridge vent on, also for the same reason. Is there any reason that this roof system I have built will fail due to condensation problems? I have been questioned regarding this. I would be interested in your thoughts on this matter.

Thank you for your time.  

Answer
Hi Evin,

It sounds to me like it may be a problem, although I'm not 100% positive that I understand the question.  When you say r-38 faced insulation, I presume you mean fiberglass insulation with a backing on one side? If that's the case, yes, you need 1" of air space with ventilation at the roof eave and at the ridge for this system to work well.  Fiberglass insulation won't prevent moisture from traveling through the roof assembly, so when it gets past the insulation it needs to be vented out through the air space, otherwise it will have no where to go and will condense on the underside of the roof sheathing, and rot it out. There are some foam products that may be able to be used without a vented cavity, but typically the local building inspector will have to buy off on these as well since the building code typically does not allow these.  

Nazim Nice
Principal / Architect
http://www.motion-space.com
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