Wall drying ability when using foil-faced polyiso
I have a new construction project in coastal Delaware (zone 4A), with a nominal wall design as follows:
Hardie clapboards
1x furring strips for ventilated rain screen
15# felt paper primary WRB
3/4 inch Rmax foil-faced polyiso panel
Structural sheathing
2×6 stud wall with dense pack blown fiberglass
Recently I was having a critical design review with my architect and general contractor. When reviewing these details, I made the comment that I understood the Rmax panels would be taped at the seams. My general contractor said that he did not plan on taping the panels. He and the architect justified that by saying the sheathing needed a pathway to dry if the sheathing ever got wet. The architect pulled out the dreaded, “The wall needs to breath” statement. My thought was that by taping the Rmax I would have the felt paper as the primary WRB and the taped foil facing on the Rmax as a backup. In the unlikely event that water got behind those layers I’d have to hope that it would eventually dry to the inside when the weather warmed up. They obviously feel differently, right or wrong.
Since I’ve never built a house before, I want to use a detail that the general contractor can be comfortable with also. It seems to me that by not taping the Rmax panel seams, I’m accepting a bit more risk that water will find its way to the sheathing—how significant I don’t know. I’m thinking the foil facing on the Rmax poses a more significant impediment to the wall ever being able to dry to the outside than taping the seams does.
Questions:
- If the GC’s concerns about the sheathing being able to dry to the outside are reasonable, isn’t the better answer to not use the foil-faced Rmax and use EPS or GPS panels instead? Those panels would be permeable enough to eliminate the risk that the assembly would not be able to dry to the outside, correct?
- Am I making a mountain out of a mole hill here? Perhaps the risk of my sheathing getting too wet is actually very low (let’s assume the flashing and WRB details are done correctly) and I ought to focus on bigger problems.
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Replies
1. Correct, eps or gps would allow more drying potential.
2. I don't think so, no. Good bulk water management and redundancies serving that end are priority #1 for the enclosure. I would tape the seams on foil faced and use a bit thicker foam given the choice. R7 would be decent for zone 4a.
It's common practice to tape the seams. The wall doesn't need to "breathe". You'll have drying to the inside with your dense pack fiberglass (which is very vapor open), and presumably a drywall interior wall surface. You only need the wall to be able to dry in ONE direction. That doesn't mean that allowing it to dry in BOTH directions isn't an advantage, but it's not necassary.
Your rain screen also greatly reduces the chances of a mositure issue with the sheathing. The rain screen will help to ensure the exterior sheathing is never subjected to long periods of extreme moisture to begin with, which will reduce the potential need for drying to begine with.
I would argue that NOT taping the seams actually INCREASES the chances of moisture issues since it allows for more potential bulk water entry issues. You're also correct that the far greater area of the panel itself is much more of an impediment to drying than the very much smaller area of each taped seam.
You're a little above the minimum require R value of rigid foam for your wall in your climate zone. I'd bump up to 1" (R6 if is using polyiso) for a bit of extra safety margin if you can. The more exterior continuous insulation you use, the warmer, and thus dryer, the wall sheathing will be.
Bill
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Thanks for the comments Jason and Bill