Will faced rigid foam insulation trap moisture?
alanferg
| Posted in Energy Efficiency and Durability on
Will a faced insulation product such as Halo Exterra installed over a continuous air/vapor/water barrier trap moisture?
My home will be sheathed in ZIP with taped seams or AdvanTech with Henry Blueskin. Two layers of staggered 2″ rigid foam will be installed outside the sheathing. I’d like to use Halo Exterra GPS (made with BASF Neopor with a laminated, punctured facing). My concern is with foam panels staggered in two layers, moisture will be trapped between the foam facing and air/vapor/water barrier.
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Replies
I am looking at a similar assemble but using the Brinc neopor gps foam. That has a higher perm rating.
Faced insulation will definitely act as a vapor barrier. If you want to use rigid board, either consider EPS or make 100% sure your assembly can dry to the interior.
Consider using taped plywood, DrainWrap then foam. Better removal of any bulk water and better drying to the interior.
In all cases, a rainscreen gap adds moisture resiliency.
If the rigid foam you're planning on using has a perforated facer on BOTH sides, then it's facer isn't a vapor barrier -- that's what the perforations are for. With 4 inches of foam though, it doesn't make a whole lot of difference since with that much foam the foam itself is essentially a vapor barrier.
If your wall is designed to be able to dry towards the interior then you should be OK. If you're worried about the possibility for a little bit of moisture accumulation at the interface between the rigid foam and the sheathing itself, then use wrinkled housewrap or one of the other variations that allows for some drainage. If you detail your WRB correctly and put it in the right place (GBA has an article called "Where does the housewrap go?" that you should read), then you don't really have anything to worry about.
Bill
William, thanks for the recommendation. I looked at the BRINC website. Are you using ThermalTight? It too has a WRB laminated to the foam, though BRINC says the WRB is permeable. My all-in-one air/vapor/water barrier will be adhered to the sheathing, so everything outside that continuous layer must dry to the outside. I'm hesitant to add another layer that could possibly trap moisture.
Bill, thanks for your input, I'm probably overthinking my assembly. I read the article you referred, "Where does the housewrap go?" (https://www.greenbuildingadvisor.com/article/where-does-the-housewrap-go). Due to installing innie-windows, the article confirmed my decision to put housewrap between foam and sheathing. I also read "Wrinkled Housewrap Behind Exterior Rigid Foam" (https://www.greenbuildingadvisor.com/article/wrinkled-housewrap-behind-exterior-rigid-foam). It seems my worry of trapping moisture is negligible as I'm following the recommended wall assembly. There won't be an interior vapor barrier, there will be enough insulation to keep the sheathing and interior side of the foam above dew point, and there will be a rain screen gap.
I am still in the design stage-and looking at all the options. ZIP R-12 is my default assemble on the walls - but also looking at Plywood with the BRINC ThermalTight as an alternative. Right now, I am working through the design details on a vented roof assembly and using the the Pen-Test/Big-Red test on the continuity. From a materials perspective, I like the Neopor option as it appears to be very permeable if you omit a facing on one-side of the material.
I like the Logix brand and it is in my design specification for the ICF foundation and under-slab construction. There is another Neopor option that is becoming available for roof applications. Still under investigation.
Hey guys, FYI I am building a house right now with a similar wall assembly. 2-1/8 inch EPS foam with NEOPOR .That thinkness gives me R-10. What I wanted to let you know is you can go directly to a EPS foam manufacturer and save quite a bit of money. There are plants all over the country and alot of them can add the NEOPOR to the foam. I ordered 350 4x8 sheets and with shipping and sales tax ended up at $21 07 per sheet. Might be worth looking into.
I'm looking doing the same now. What EPS manufacture did you go with?
Hey Alan, Halo Exterra GPS is designed to be your WRB so I would recommend using taped plywood or the Advantech sheathing versus Zip with that product. The company nearest me that produces Halo also produces a product called Chrome GPS, which is an unfaced GPS insulation, that also has a termite treatment called Preventol. It is what I am planning to use in conjunction with a rain screen under my siding to allow some drying potential.
Ron, I've previously ordered EPS directly from a manufacturer for my slab foundation. Even with freight, the cost was significantly less than ordering XPS locally. I wasn't aware I could do the same for GPS. Thanks!
Mark, I'll look into the Halo Chrome GPS... good to know I can find an unfaced GPS insulation. My air/vapor/water barrier will be on exterior side of sheathing so Halo Exterra is no go for me.