Detailing wall with exterior rigid and hollow girts with fiberglass cavity insulation
These questions may seem very nit-picky, but I want to finish this project right.
I have a relatively large steel building (zone 4) that will partially be used for “living” space. I am attempting to do a good job insulating it to make it as affordably as possible to do this. The building has steel girts, and I have the wall stackup as shown in my attached picture.
In a previous question (https://www.greenbuildingadvisor.com/question/vapor-barrier-on-interior-with-rigid-foam-exterior) it was advised to have NO dead space on the exterior side of my cavity insulation. However, I have a couple specific questions:
1. The girts have a C shape that includes a small “leg” that protrudes inward about 1/2″ (see picture). I was able to secure a good quantity of ACTUAL 24″ wide R-19 unfaced insulation which I wanted so I could fill the dead area inside the girt as well as the cavity. However, as you see in the picture, this will undoubtedly create a couple small voids as the insulation curves around the girt “legs”. Is this OK or a recipe for some kind of problem?
2. The exterior rigid foam (1″ XPS) is glued (with Great Stuff foam) and taped (Owens Corning HOMESEALR tape). However, from the inside, there are small gaps (Typically 1/8″ or less) where the foam boards didn’t meet up perfectly and/or the foam adhesive didn’t ooze all the way through. Is it worth trying to fill each of these small gaps with foam? Should I tape these seams from the inside as well? These processes are a nontrivial amount of work, but wanted to see if they are beneficial enough to consider doing. It should be noted, any larger gaps (greater than 1/8″) I will certainly at least fill with foam.
Thanks in advance.
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Replies
Your ratio of CI to cavity insulation is correct, you shouldn't have much of a problem with condensation forming on the interior side of the XPS. The small gaps in the insulation shouldn't be a major concern if the wall is airtight (metal studs are common in commercial construction, I have yet to see an insulation contractor try to fill the small voids left by the C-channel), but if you are worried about them, you could rip each batt so that the C channel is filled separately with fiberglass, then add the cavity insulation to finish the installation. That's going to take a lot of time and effort, you'd want to make some sort of jig for cutting the insulation.
As far as the gaps in the XPS seams, yes, fill the larger ones with canned foam. The smaller ones, I would probably use some sort of sealant instead of the foam. Even though you're taped from the exterior, this extra sealing is easy to do and adds a belt and suspenders to the air sealing/water management system.
The very small gaps aroung the steel studs aren't really a problem. Commercial buildings are build and insulated like this all the time, it's pretty standard. You won't have any significant convection airflow in such small gaps, and that airflow is the primary concern with gaps in insulation.
The gaps between XPS panels should be either filled with foam or taped. I often use the Dow "pro" foam gun with the small plastic tips to let me inject canned foam into gaps like this. Taping is probably going to be an issue if you can't access things from the exterior, so foam is probably the way to go here.
Bill
1. If I am reading correctly, it is OK to fill the cavity like I have in my picture and just leave the small voids caused by the legs. I understand what you are talking about Randy, but I would rather not deal with the 2-piece way of filling the channel of the girt if I can avoid it.
2. To clarify, I DO have the exterior of all seams taped, we are only talking about the inside at this point. Sounds like hitting the small gaps with foam is still preferable?