Attic: Hot roof? Cold roof? Closed? Open?

marcoloFebruary 12, 2011

I live in Boston (Zone 5) in a 1924 colonial, and had some serious ice dam problems this winter. We've also had significantly below-normal temperatures, and my heating bill is astounding (despite new HVAC, wall insulation, new windows). The new HVAC includes a furnace/AC in the attic, with insulated flexible vents. So there are a lot of penetrations of the attic floor, including vents, potlights, electrical, etc.

I had an insulation contractor come by today looking to spray open-cell foam on my attic roof; I also requested a quote for closed cell.

Insulating an attic roof is against everything I have ever heard in my life, but I have been trying to read up on Building Science articles regarding hot roof systems. I am not really obsessing about shingle life; I am far more worried about water vapor, condensation, roof rot and the like. (We have a basement that is quite damp.)

So: Open cell, or closed? If open, with vapor barrier paint, or no? Does fire paint count as vapor barrier paint?

Does anybody know of any clear discussions or recommendations? I am a homeowner, and do not have time to become a professor of attic insulation. Clearly the building sciences are on a fence about this issue; they recommend both vented and unvented, with lots of caveats that are conveniently unavailable in real life.


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I think with mechanicals and ducts in the attic unvented is the way to go. throw in a lot of recessed lights..even more reason to go unvented.

What I'd want to know is if it is closed cell or open cell for your area. Instead of contacting foam mfg's check with your state's department of energy for their recomendation.
I'd bet they can give you unbiased info..and those goverment types like to talk.

When you decide, ask to visit some installs.
For me, I always look to see that all the top plates are seald. with existing homes if there is insulation on the attic floor, it should be moved back 18" to 2' from eaves so that foam can seal these areas.

Here we are open cell, but have to really push to get the full rafter depth install. I like to see all faces of framing members covered and a good 6" in a 2x6 rafter.
closed cell will be less foam, but rafters should be flash coated at the very least.

best of luck

    Bookmark   February 12, 2011 at 3:59PM
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Good ideas.

YOWZA. I just got the estimates--closed cell is twice the cost of open.

    Bookmark   February 14, 2011 at 12:27PM
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