Exterior Foam
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| Posted in Green Building Techniques on
We are buiding a conventional stick frame house. We have a number of sheer walls, so the entire house already has a layer of OSB. I would like to add exterior foam. Is this possible since I already have OSB on the house? Would 1″ suffice? Since I live in Sacramento (Zone 3C) I am not going to worry about a vapor barrier or moisture issues. I’m trying to get my contractor to buy into this, and he has asked me to bring him a sample of the foam. Is there a type you recommend for my climate/situation? Thanks.
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Replies
Laura,
Take it from someone who has designed tons of houses in the SW (CZ3-CZ4). You need to install a weather resistant barrier (WRB) in your house (not a vapor barrier), including in Sacramento.
Rigid foams are closed cell and impermeable, and some brands, if taped and seal, can be your moisture barrier, like Dow Styrofoam, and ½” is enough to mitigate thermal bridging in CZ3, however, 1” is better.
If your cladding is stucco, you need two WRB (by code). If you are using siding or brick, I WRB and a ventilated screen would be sufficient.
You need to be careful to not install closed cell foam in the stud cavities, as it can trap moisture on the OSB with little chance to dry. Best insulation to use would be cellulose.
Thank you for your guidance. So, from the inside out I should have 5/8 drywall, 2x6, cellulose, OSB, 1" foam, WRB, rain screen and fiber cement siding. What type of foam? Thank you. (I know for some of you these are pretty basic questions. But, reading all the different options/opinions can be quite confusing, so I appreciate any and all responses.)
Laura,
You can use any type of rigid foam you want -- EPS, XPS, or polyiso. Of these three types of foam, polyiso is the most environmentally friendly. It also has the highest R-value per inch.
Laura,
"So, from the inside out I should have 5/8 drywall, 2x6, cellulose, OSB, 1" foam, WRB, rain screen and fiber cement siding."
This may be splitting hairs but it may be easier to (from inside out) do OSB, WRB, 1" polyiso. Putting the WRB up against the sheathing from my perspective is easier than putting it on top of teh foam. You still have to tape the foam to keep water from getting in behind. But putting the WRB under the foam may mean that you put fewer holes in and do less damage to the foam.
Just my perspective! It'd be interesting to know what others think.
Oh, and there is good reason to go with cellulose in the stud cavity when you have foam on the outside of the sheathing. Don't let anyone suggest that you put some other insulation in the cavity. There is a BSC article out there explaining why.
Andrew