Vapor barrier for a steel building used as a residence?
We’re going to build an engineered steel building (44x70x12′) for a residence. It will have four main steel supports on the long end, each a bit more than 1′ deep. Purlins will sit inside the webbing, and are 8″.
I wanted to put rigid foam on the exterior of the purlins, between the purlins and the exterior steel paneling. Unfortunately, the building company wouldn’t validate that design.
My new plan is to use spray foam on the inside. So, I’ll build up the steel beams, then put on the exterior steel panels for the walls and roof. On the inside, I’ll put about 2″ of spray foam insulation. Then I’m going to build 2×6 stud walls on the inside of that, with the plan of putting either spray cellulose or spray fiberglass in the 2×6 wall cavity. The 2″ spray foam gives me a thermal break between the steel and these interior walls.
Looks like this:
Outside — Steel wall panel — purlins/steel beams — 2″ spray foam — 2×6 stud wall with cavity insulation — drywall — inside of house
Q1: Do I want open cell or closed cell? From things I’ve read, it seems like I want open cell so that it will breathe a bit.
Q2: Do I need a vapor wrap anywhere here? We’re in El Paso, TX, so it’s fairly dry here (Zone 3). It will be a residence, so heated in the winter and cooled in the summer.
Thanks!
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Not an expert, just a lurker. I just finished(minus trim and overhead doors) a 40x40 clear span building in NE OK. I chose r-10 blanket insulation with a vr-r facing. The insulation isn't r-10 where it is sandwiched between purlins and the sheeting but it will provide something. Industry standard seems to allow up to 6" sandwiched between purlin and sheet and maintain what they call diaphragm bracing. Which is a real thing, that skeleton is wobbly as all get out before skinning.
More insulation can be put below between purlins and the vapor barrier applied at the bottom of that, but I am unaware of how that insulation is affected by thermal bridging. I am not sure of the validity of the claim but I found somewhere that r-10 would be r-6 total.
As to the foam, you would want closed cell on the skin and you would want/need to encapsulate the framing or thermal bridging would defeat any r value. That and cost is why I went with the blanket. I would also consider a vapor barrier under the concrete, no matter what your concrete guy says.
Is the whole buliding going to be a residence or only part of it? The built sq ft price might not be competitive if you are going to have to build an entire 2x6 frame inside a metal building. Are you having it contracted or diy?
Steve,
I agree with Ken: If you have to build a wood-framed building inside your steel building, why build with steel? It might make more sense to just build a wood-framed building.
Will you be spraying the foam against the interior side of the steel cladding? If so, that's going to complicate future replacement of rusting steel panels. (Look around at older steel buildings -- you'll see a lot of rust at the bottom of the wall panels due to splashback.)
If you go ahead with your plan, you need closed-cell spray foam. Make sure that you cover all exposed steel with spray foam before you build your wood-framed wall and wood-framed ceiling. If you proceed as planned, you won't need or want an interior vapor barrier.
Ken, thanks for the input. We intended on putting the foam on the outside, so we wouldn't have to completely frame out the inside, but that isn't an option anymore. This is all DIY, and the wood really isn't that expensive. Plus all of the engineering is concentrated in the shell; no need for roof trusses or load bearing walls, etc (single story).
Rough estimate of our costs for exterior, steel and wood framing VS to do stick framing and stucco (common out here), costs would be almost similar (excluding roofing), so oddly enough it isn't that expensive.
We're definitely doing vapor barrier under the slab, and will insulate the perimeter too. At one point we were going to do hydronic in-slab radiant heating (and thus insulation under the slab), but the redundancy in adding forced-air cooling made it ridiculous (I kept trying to find some single-room hydronic fan coils, but no one seems to make them anymore aside from industrial/commercial sizes).
And my plan is to spray EVERYTHING that's steel. Except maybe the vertical faces of the purlins, I think I might cut rigid foam to fit instead so I can butt the 2x6 wall right up against it and get some structural support. OR will the spray foam be just as rigid as XPS or polyiso sheets?
Martin, by splashback, you meain rain right? We have 5' overhangs fully wrapping around (wife wanted a wrap around porch, and we need the overhangs for our massive number of windows--we have a spectacular view) so I think that will minimize that kind of problem. Also, even if we do need to replace a panel, I don't see it being a huge problem considering the 2x6 wall won't be right up against the exterior panels, they'll be about 10" (8" purlin + 2" foam, then the 2x6 wall). I think that will give me the room to respray if needed.
Why do I want closed cell foam? I'm trying to find the reference, but I ran across something a few months ago saying NOT to put closed cell foam up against the steel panels. If I can find it again, I'll post it.
Thanks for the help!
I am not sure what ccspf goes for in your area, but not accounting for roof pitch or the sides of the purlins and rigid frame, 2" of foam is about 11,400 board feet. That's a chunk of change for achieved r value. I also don't know how they are structurally accomplishing the 5' overhangs but that might have a thermal bridge as well.
I am pretty sure closed cell(ccspf) is needed in contact with the sheeting and frame or moisture could travel through open and promote rusting.
I'm not sure that we are on the same page in regards to the interior wood structure. Is there going to be an "attic space" above a single story flat structure or is the wood going to follow the pitch of the roof?
Also, I would imagine humidity is as important as temperature in El Paso, but I've never been there. In any case, pay special attention to sizing the a/c as bigger is not better.
Martin, I meant to credit you on the metal in wood stucture as i parroted it from your various posts on the subject but forgot to cite my"wisdom".
Steve,
Q. "Why do I want closed-cell foam?"
A. As Ken correctly noted, open-cell spray foam is vapor-permeable (allowing interior moisture to diffuse through the foam). In winter, when the steel panels are cold, you don't want interior moisture to diffuse through the foam and condense against the cold steel.
If you do a metal building with wood framing make sure to look at the metal building design criteria. Metal buildings are usually designed with large deflection allowances. Metal can flex more than wood! The metal building can then move and push on your wood framing and crack your drywall. Make sure they design the Steel frame to be compatable with your wood framing!