Standing Seam over Rigid Foam – Asked a thousand times but still need clarification
Good afternoon everyone. I know that this question has been asked a thousand times and answered more than that but I haven’t found an exact answer that seems to fit our use case. I’m building a farm office/workshop for myself but this is not a regularly occupied structure nor does it have any plumbing. It’s a 2-story structure with the “office” above and a small farm workshop below (with an overhead coiling door). I’m experimenting with various details gained over years of practice and thought I had this all figured out. As I was talking to a friend who builds a lot of smaller but well-performing houses, I mentioned my detail and he said “Oh, you’re done it backwards”. Here’s what I’ve done and why he said that.
The roof structure is 2×10 rafters with 3/4″ plywood sheathing. On top of that is a layer of self-adhered Henry Blueskin PE200HT. My plan has been to install 2″ of rigid XPS foam over that with vertical and horizontal 1x furring (see attached roof detail) to allow for snap-lock standing seam clips to be installed at 16″ O.C. both horizontally (16″ 24 ga. panels) and vertically running up a 4:12 pitch roof. My intention was to install Comfortbatt (or similar mineral wool) R-23 in the roof cavity inside to effect a net R-34 for the roof. I was planning on this being an unvented roof assembly with an ERV and mini-split for environmental control. We’re in Nashville, TN.
He suggested that with all of the trees in our area he was worried that the metal roofing could easily be penetrated by a falling branch or similar. He offered that I should have installed the insulation directly against the sheathing and then used a second layer of sheathing on top of the XPS and then apply the underlayment on that layer of sheathing and then the clips could affix directly to (or through) the upper layer of sheathing. In all honesty I’m not sure how many exterior insulation projects he’s done as he is a very strong proponent for I.C.F.
Here’s my dilemma. The structure is built. The (rather expensive) vapor-permeable PE200HT is already installed so it’s not coming back up. I own the XPS so I’d like to use it and not have wasted the $$ on that.
Any thoughts on the best way to proceed here or any I stuck installing an upper layer of (thinner) sheathing as a cover board of sorts with a roll of underlayment on that? My two primary concerns are:
1. Will the original detail work as designed with the understanding that I’m relying on the thickness of 24gauge steel to prevent damage?
2. Will the XPS defeat all of the work I’ve done to try and let vapor escape (and let the sheathing dry)? I thought that with the correct interior finish applied (zero V.O.C. latex) and the R-23 Comfortbatts, the sheathing can still dry to the interior.
3. If the second layer of sheathing is suggested on top of the furring (or is the furring needed at that point?) would the underlayment on that layer need to be vapor open?
I’m aware that this amount of roof R-value is not enough for code here but this is technically an agricultural structure and is really more of a test-bed for details than it is a structure for the ages. If convicted hard enough, I could be convinced to install a second layer of 2″ over the XPS to result in a net R-45.
Thanks for any comments or help you can offer.
-Paul Marshall
GBA Detail Library
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Replies
Paul,
1. Metal panels on strapping are more susceptible to damage than if they have a solid substrate under them. How much that matters is very site dependent. A solid substrate is also more resilient in high wind areas, where you can detail that upper-sheathing as a secondary weatherproof surface. https://fortifiedhome.org/roof/
That said, your detail is a very common one and in most circumstances performs well.
2. You have it right. The layers below the foam are expected to dry to the inside. The complication is that the assembly relies on having sufficiently thick foam to keep the inside surface warm enough to prevent condensation. Depending on where you are 2" may be insufficient. https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
3. It's a trade-off between between the increased resistance to water leaks the impermeable underlayment gives and the small amount of drying the permeable would yield. No clear winner.
Malcolm, thank you for your reply. This is the first structure I've built from scratch for myself in over 20 years as an architect. It's one thing to research and draw details for a client but another thing to experience the analysis paralysis of being the owner (ha).
I'll do my own risk analysis and see which direction I decide to go. I've got about 12 more sheets of 2" foam (about 1/3 of the roof surface) that i can apply to get 4" of XPS. Maybe I'll bite the bullet to do it right. I'll update this post with install photos. Thanks again
Paul,
You are probably well ahead of where I was when I built my first house. I had never owned a framing hammer, or build anything larger than a 6 ft x 6ft porch. I found the work addictive and moved into construction after that.
One other concern is that if you don't have a solid substrate under the metal, in certain conditions moisture will condense on the underside, so be sure there is a good WRB over the sheathing and that it leads to open air, not into a wall.
Either option would work. If you go with another roof deck, you don't need peel and stick on there, it can (and usually should be) regular roofing underlayment. I prefer to use a permeable underlayment here but either would work fine.
If you are going with strapping, you can use regular lumber. Since it is not exposed to the elements, PT is not needed there. You can also save a bit of labor by only installing horizontal strapping, this is what I have done in the past. If you get an underlayment with markings and install it with some care to keep it straight, it will be much easier to line up your screws.
You can also reach out to your roofing metal supplier as some clip type can be installed directly over the rigid using bearing plates like this:
https://www.mbci.com/products/bearing-plates/