Foam in attic
rus54
| Posted in Energy Efficiency and Durability on
My geothermal HVAC contractor also does foam insulation. He says rather than encapsulating the attic by spraying under the roof deck, he uses deck shield on the roof then sprays foam on the attic side of the ceiling Sheetrock. He said makes a more efficient home as well as being more comfortable andit takes less foam therefore some less expensive. Any yeas or nays?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Rus,
As long as you don't have any ducts or HVAC equipment in your attic, there is no need to create an unvented conditioned attic. It's less expensive and more logical to create a vented unconditioned attic, as your insulation contractor suggests.
However, I wouldn't use spray foam on an attic floor. Cellulose is much more affordable. Just make sure that you have completed your air sealing work before the cellulose contractor begins work.
At 3-4 cents per R per square foot cellulose is by far the cheaper option.
With closed cell foam you're looking at 17-18cents/R-foot, and with open cell 12-13 cents.
Either way it's at least 3-4x as expensive to go with spray foam than open-blown cellulose.
Dense packed cellulose is different story...
Back in Holland, people usually insulate the roof deck, though traditionally with batts rather than foam. The houses are small and the roof pitches are fairly steep, so people like using their attics for storage or extra rooms.
I don't know where you live, but in many places in the US, attics are really not built to be usable space. The construction isn't rated for it, you often have framing criss-crossing the space, and the roof pitches tend to be the least they can get away with given the snow load, so unless you live in Minnesota you may barely be able to stand up in there. So people just treat the attic as outdoor space and insulate the attic floor, which is much easier and cheaper. And it works fine, unless you have ducts in the attic. You can seal and insulate the ducts all you want, but it's always best to run them through conditioned space.
Attic floor insulation is cheap because they can use pretty much any insulation material at all, and blow it in rather than painstakingly attach it to the right spots. Since the insulation doesn't need to stick to the underside of something, there's no real reason to use expensive foam or labor-intensive batts. I suppose foam does eliminate the need for a separate air sealing step (where they crawl around with the contractor-grade equivalent of a can of Great Stuff), which is labor intensive but absolutely crucial if you do blown-in. I suppose in some cases the savings in labor may outweigh the cost in materials, but I doubt those cases are common or we'd be seeing more attic floor spray foam.