Re: Spray Foam in finished attic - Open Cell
dpw .... sounds to me because you are building a new structure the inspector is holding you to current building codes. I'm guessing the rafters will be on the other side of a pitched ceiling along with knee walls.
He's considering the rafter bays as being attic space which probably has to be a minimum of R40 in your area.
On one hand this is in your favor .... the space will be well insulated .... it will be warm in the winter and cooler in the summer.
However .... the requirement is a R value of 40 which doesn't mean you have to solely use the foam insulation to achieve this..... theoretically you could use a combination of foam and other insulation materials. In other words .... apply about 2 inches of foam to be cavities to seal for air infiltration and apply a more economic material to fill the rest.
There wouldn't be a need to vent this space since it becomes indirect conditioned space which can be a good thing in maintaining a consistent insulation R value.
As sabo4545 mentioned ..... the advantage of the foam is sealing the majority of points for air infiltration which is important regardless of which insulation is used.
You mention using icynene which is a low density ( usually around 0.5 lb ) open cell foam which has a good air barrier permeable rating .... but .... does allow vapor transmission and should have a vapor barrier installed before the drywall goes on.
If you insulate the the rafter bays completely and apply a layer of foam to the back side of the drywall to prevent air from the living space entering ,,,, this again will be indirect conditioned space and shouldn't need ventilating.
Also trying to decide how to insulate the crawlspace.
Hope this helps.
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