Open Cell vs Closed Cell

fperkinsMarch 26, 2011

I live in Connecticut and recently had a home energy audit and the main issue they found is that my attic is not insulated enough. My attic floor is insulated [not enough] and covered in plywood as we use it for storage. It also contains an air blower for forced hot air [hot water comes from boiler in basement] and AC.

I have contacted three different spray foam contractors and they all recommend spray foaming my rafters and foaming over my soffit and ridge vents creating a conditioned space. Two of the vendors will do close cell foam although I had to specifically request closed cell as their first reco was open cell. The third only does open cell and is strongly recommending open cell. He says you want open cell so the system can breathe and "not" trap moisture.

None of these three contractors recommended a vapor barrier if were to use open cell.

I checking my building codes: and found this: "An air-impermeable insulation is applied in direct contact to the underside/interior of the structural roof deck. "Air-impermeable" shall be defined by ASTM E 283-04. "

I also found this article here: "Open-cell foam is relatively vapor-permeable. Three inches of open-cell foam have a permeance of 16 perms. When installed in a cold climate, the interior face of open-cell SPF should be covered with a vapor retarder (for example, vapor-retarding paint"

I'm distressed that I can't find a contractor that will offer me a solution that meets everything I read online. I'm starting to second guess myself, but this is what I think NEEDS to be done as not to prevent a moisture problem:

- CLOSED cell foam against the roof rafters

- Ensure all gable walls are spray foamed

- Spray soffit and ridge vents to create a conditioned space


- OPEN cell foam against the roof rafters

- Ensure all gable walls are spray foamed

- Spray soffit and ridge vents to create a conditioned space

- AND apply some sort of vapor barrier in the interior

Is all my research correct? Can anyone recommend a spray foam contractor in CT that will do this work?

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Either approach you outline is acceptable.

See Building Science Corp. here.

The main debate amongst contractors is whether or not oc is preferable to cc because you can find potential leaks easier.

I prefer cc as no separate barrier is needed and I have had some poor experiences with oc--though that may have well been the particular installer.

Be sure you specify the correct R Value.

Here is a link that might be useful: Building Science Corp. on Unvented Roofs

    Bookmark   March 28, 2011 at 8:26AM
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with foam the install is crucial.
inexperienced untrained installers do not
get the air sealing that makes for an air tight seal.

we use open cell here, full rafter depth and
faces of 2x rafters covered. this is a hot
humid climate.

ask to see installs of jobs they have done for others
or to visit sites where jobs are ongoing.
insulation should not dip in and out in rafter bays
but be mostly even. 6" of foam is 6".. not 3" to 6".

open cell has less R-value than closed cell.
and all should be installed to specific R-value spec'

best of luck

    Bookmark   March 30, 2011 at 6:02PM
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