Am presently building a cottage with cathedral ceilings throughout. We plan on using spray in foam insulation. The rafters are 2X10. The contractor says that approx. 7″ of foam will provide the necessary R value.In homes bulit using SIP’s as the roof sheeting, it’s my understanding that there is no requirement for ventilation air. If the entire cavity is filled with foam, does this not create a similar situation, and if so, does it negate the need to provide ventlation air at the soffits and roof ridge?
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Replies
If you use spray foam in the rafter cavities there is no space, or need, for roof ventilation via soffit and ridge vent. The purpose of a roof vent system is to remove moisture which passes through the ceiling, vapor barrier, and insulation. With spray in you should not have a moisture migration problem and thus no need for venting.
My question for you is why does your contractor want to give you only the "required" R-value? You are paying large just to get the foam men on site and scaffolding to the roof, why not spend a just a few more bucks and fill the 9.25" space? The extra R-value will only lower you future heating bills. The only possible downside of filling the spaces is that you probably will incur the additional cost of trimming the excess down to the bottom of the rafters. The foam guys should have a hot wire trimmer that does this real quick. As always time is money.
A last thought, trimmed foam is cool looking, untrimmed foam is ugly. Your scrapbook of construction pictures will look much better with trimmed foam.