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Spray Foam in finished attic - Open Cell

I am building a new house, on an old foundation in CT, and looking to use spary foam, probably icynene in the finished attic. I was told by my building inspector that I would need to fill the entire rafter bay and then sheetrock.

I was suprised that I would have to go to the expense of filling the entire 10 inch rafter. I would appreciate any expreience in my climate with this, and tips on venting or not venting the attic.

Also trying to decide how to inulate the crawlspace.


Re: Spray Foam in finished attic - Open Cell

If you are using open cell it has about the same r-value as fiberglass. Generally in the North East you want an r-38 so filling the entire rafter bay would be required since open cell is about r-3.6 per inch. It would not quite be r-38 but because it seals everything better than fiberglass it should be about equivalent. When I had mine done I had closed cell put in which is r-7 per inch since my rafters were old 2x6's (actually 2" x 6") so I needed the extra r-value. With closed cell you do not vent the attic but I'm not sure about the open cell. I actually had the ridge vents and soffit vents in place before I finally decided to go with the spray foam. The guy who came to estimate the job said that it wasn't necessary and in fact inspectors are requiring roofs that can not be vented to be spray foamed. Hope this helps you out.


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.


Also trying to decide how to insulate the crawlspace.

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.

Hope this helps.:)

Re: Spray Foam in finished attic - Open Cell

i spray foam and 6in or more would do fine and as far as craw space leave it because heat from your house will hold the heat in your attic:cool:

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