EPS vs. polyiso as exterior insulation
I’m in Marine zone 4C. I really would like to do an exterior wrap of insulation in an upcoming build.
First choice was Rockwool ComfortBoard but it too expensive for my budget. I really liked the idea of a non-foam product that at least appears extremely durable.
If one is considering cost, your pretty much left with foam. But what about the “vapor-closeness” of foam. EPS almost seems like a better choice than foil PolyIso because it has the capacity to “breath” a little better.
Is it not a concern to wrap a house in foil? I understand if you get the layers right you don’t really worry about moisture but drying potential in all directions just seems like good insurance.
thanks
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Replies
EPS is more vapor permeable than foil-faced polyiso in reasonable thicknesses, but if you put a thick enough layer EPS will act as a vapor barrier too.
You really only need your wall to dry in one direction. If you sheath the house with rigid foam, you’ll generally want your walls to dry inward. That means no interior poly or vapor barriers (a great time to use unfaced mineral wool batts). A rain screen on the exterior helps to keep things from retaining too much moisture on that side. There is no need for drying in both directions.
Bill
Breathing to the exterior is advantageous but use the recommended %R and you will be fine either way.
I have never used this product so I'm not giving it my endorsement and don't know its availability in your area. I haven't even seen a price per sheet in my area, but it looks interesting as it claims to be vapor open up to 2" thick and supposedly cost-effective, although I'm sure still more than standard pink and white stuff.
https://buildwithhalo.com/#head
Anyone had experience with this Halo GPS?
Ben
Joe,
Design your wall according to the usual design guidelines, and don't worry about the vapor permeance of the foam. More information here: "Calculating the Minimum Thickness of Rigid Foam Sheathing."
Joe,
Always good to get drying wherever you can but the rule of thumb is: get good drying potential in ONE direction, which with a low vapor permeable outer layer means drying to the interior.
Way back in 1993, I actually got Heinz Trechsel (one of the two editors of the book, Moisture Control in Buildings published in 1994)) on the phone (I was doing a research project at the NAHB Research Center on spray foam) and asked him about which directional drying is better, and he said (I am paraphrasing): vapor open assemblies that can dry in both directions are best, but one direction--either one--is good.
Peter