Open-cell SPF in an attic with no vapor barrier
Hi,
I live in Pittsburgh and am doing a renovation of an old home. I’m doing a lot of the work so far myself, with the help of my father. At the same time I’ve been working with an architect to figure out the overall project. And there is disagreement between my two spheres on how to insulate the attic.
The (hopefully) key points: it’s currently a vented attic, I’m going to be putting HVAC and ductwork in that space.
My father is recommending I keep the attic as vented, put down batting on the attic floor, and do various things to ensure the HVAC unit is leak free and insulated
The architects recommend spraying open cell foam between the rafters and make the attic an unvented insulated space.
The biggest concern about the open cell SPF (other than it’s a lot more expensive) I have at the moment is the fact that’s it’s not a vapor barrier. But, the architects seem to think this isn’t a problem. How could moisture not condense in the foam? If it does, why doesn’t that matter?
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Replies
Will,
If you intend to install HVAC equipment and ducts in your attic, I agree with you that it makes sense to insulate the sloped roof so that the attic becomes an unvented conditioned space.
It's perfectly possible to use open-cell spray foam for this purpose. In a cold climate like yours, it's best to include an interior vapor retarder to avoid moisture accumulation in the roof sheathing during the winter. The usual way to do this is to spray the cured foam with vapor-retarder paint. That's perfectly adequate.
In many jurisdictions, you may also need to install a thermal barrier like gypsum drywall so that the spray foam insulation is not exposed. You should consult your local building department to determine what is expected in your jurisdiction.
For more information, see Creating a Conditioned Attic.
Thanks for the answer! I've been relying on the architects on all of the complicated and messy code issues.
My concern is that everyone I've talked to that is "pro-foam" uses fairly wishy washy language when it comes to vapor retarders/barriers (you "could" do this or that), making it unclear to me what is required vs. optional.
I found this from the SPFA:
http://www.sprayfoam.org/downloads/pdf/AY%20118.pdf
The key line:
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If night or winter temperatures are cool enough, the water vapor normally diffusing harmlessly through the foam may condense. It is important to provide a vapor transmitting covering system (high perm rating) to the exterior surface of the polyurethane foam to prevent the build up of humidity within the foam and, thus, avoiding condensation.
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So it sounds to me like a high perm rating covering is non-optional, if the spray foam industry says it's important.
I'm not sure if this matters, but the plan was to completely fill my 2x8 rafters and even cover the ends. I believe the claim (from the first bidder I talked to) was that at an 8" inch thickness, the open cell foam itself counts as a high perm rating barrier. But, from my understanding, all that would mean is that "not a lot" of water is condensing in the foam. And, from what I've been reading, water and open cell foams aren't very friendly.
But overall what I'm hearing is: given the HVAC/ducts as a requirement, my best option is open cell foam with a layer of vapor-retarder paint on the interior.
Thanks again for the feedback!
Bill,
You are in climate zone 5. The rule of thumb is that open cell foam must be protected with a vapor retarder in zones 5 and higher. Regardless of thickness, open cell foam is very vapor open. That said, if you keep your home dry in the winter, you would probably be okay without it - but why risk it? My advice is to coat the open cell or better yet, go with a denser foam with a sufficiently high perm rating. The trouble with paint is that it adds a step (and hence cost) to your project. Additionally, open cell foam is tough to paint properly because of the uneven surface. We work in Cleveland, which obviously has climate close to yours, and over the years we have installed many open cell roofs without any problems. We switched to higher density foam a couple of years ago, in part because of the issue we are discussing here, and a very happy with our choice. Feel free to contact me if you would like to discuss this over the phone. I'll be happy to give you my two cents worth.