Exterior Rigid Foam for Thermal Break
Greenhouze
| Posted in General Questions on
Im building a 2×6 house with 7/16th zip. Looking to add rigid foam on exterior to create a thermal break. Im leaning towards open cell spray foam on interior.
Id like to add exterior rigid foam as a thermal break.
What thickness of rigid foam will be a sufficient thermal break?
Will adding exterior insulation to the zip which is the WRB create a issue with condensation?
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Replies
You should consider using Zip-R, which has polyiso bonded to it already so it’s a one-step process to install. This way your exterior rigid foam and sheathing go up together in the same step. If you’re already planning to use Zip sheathing, Zip-R is a logical step to get your exterior continuous insulation. You can also put your own rigid foam up the old fashioned way, and it won’t increase condensation risk as long as you use enough for your wall thickness and climate zone (there is an article on GBA that explains how to do this).
I would skip open cell spray foam and use mineral wool instead. You could also use high density fiberglass batts, which get you about the same R value as mineral wool at less cost, but with a slightly more difficult installation process. Do a good job sealing your exterior sheathing (which is easy with Zip or Zip-R) and you’ll have a good air barrier. As long as you have a good air barrier, spray foam adds nothing to your wall — it isn’t really much better than R value compared with the two batt products I mentioned earlier, and you don’t need it for air sealing if your exterior sheathing is handling that. Save money and use batts instead of spray foam in your walls. Use the savings to add more exterior rigid foam, which will ultimately give you a better performing wall for the money.
Bill
What climate zone are you in? You only need about 1/2" to 1" of foam to create a thermal break, but depending on what materials you use and what climate zone you are in, you need different amounts of exterior insulation to protect against condensation within the walls.
I am in zone 6. If I don’t have any exterior insulation does condensation inevitably end up in the wall? Seems that most houses here don’t have exterior rigid foam.
Without exterior insulation, walls can usually dry toward the exterior, unless something (such as steel siding) prevents it. EPS foam insulation is relatively vapor-open and would probably be safe to use, but foil-faced polyiso would not allow drying. XPS would allow drying when used in a thin layer like that but conscientious builders don't use XPS because the blowing agents are potent greenhouse gasses. You might hear about a new formulation of XPS that has reduced environmental impact but it's still worse than any of the alternatives.
Would Rockwool ComfortBoard 80 attached with furring strips be an option? If so, is the proper thickness for colder climate zones as critical since it still allows drying to the outside?
Jassy