Flash and Batt
jambo13
| Posted in GBA Pro Help on
Hello, I have been reading so much about flash and batt lately I am second guessing my original plan to insulate a home we are building. We had anticipated dense packed cellulose walls (9 1/4″ cavity) and 24″ of blown in cellulose in the attic. Are there significant reasons to flash both or either of these areas with foam first? If so should I just go to an open cell foam for everything? Seems like much of what I am reading is contradictory, what is the right choice here?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Is the purpose of the foam air sealing? I think you can do that cheaper, and maybe better, w/ tapes and caulks and/or the airtight drywall approach. I am by no means an expert on this, though.
James,
John K is right. You can achieve an airtight wall assembly without foam. Foam is expensive. Tapes, gaskets, sealants are much more cost effective. You've just got to get the details right (this website has plenty of air sealing details).
By the way, how are you achieving a 9 1/4" cavity? If you are framing with 2x10 studs, reevaluate your strategy. I would use a double wall or Larsen Truss instead.
Thank you. I will look into these alternative air sealing options. We did go with 2 X 10's on this project as we have found they are more cost effective for us to build than the double wall. With exterior foam on the house I felt this was OK.
Where are you at in the project? Have you started framing yet? I have a hard time believing 2x10's are more cost effective than double 2x4, either in materials or labor. Better off making the frame wall assembly thicker and leaving out the foam.
James,
With a super insulated wall behind exterior foam, The thickness of the exterior foam becomes extremely important. The insulation value's that you are intending without enough exterior foam in a heating climate may cause condensation on your exterior sheeting behind the foam. I'm guessing that your aware of this through your flash and batt research, but if not, do familiarize yourself with what exterior foam values you will need for your location and wall section.
There are plenty of posts on the subject here on GBA and http://www.buildingscience.com/index_html