How would you insulate a metal Quonset hut?
I have a client interested in building a home inside a metal Quonset hut. I am concerned about how to insulate the building. Applying a closed-cell foam directly to the interior of the structure seems like a plausible option that would be like creating an invented roof assembly. However, I am concerned about the following: (1) might there be a negative chemical interaction between the foam and metal that could cause corrosion? (2) Are there moisture risks that could lead to problems? I can see moisture getting in between the foam and metal two ways – via cracks that develop over time or are caused by less than perfect application of the foam (which would allow moisture to pass through from the interior of the house); or if the foam should separate from the metal over time as a result of the differential expansion and contraction of the metal and foam. If this should happen, could moisture from the ambient air get between the metal and foam, condense at times, and lead to corrosion?
Do you know of analogous applications where people apply spray foam to the underside of sheet metal roofs? Can you refer me to examples of people who have successfully built energy efficient homes inside quonset huts?
Thank you very much!
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Replies
Andy,
As far as I know, closed-cell spray polyurethane foam will work in this application, and is in fact probably the best insulation option. Closed-cell spray foam is used to insulate the exterior side of giant stainless-steel wine tanks (huge outdoor tanks), and it seems to work for that application.
I wouldn't.
Goofy ideas... are goofy ideas. Good luck.
Closed cell polyurethane bonds well to metal, and even if it separated from the metal it is sufficiently rigid & structural to form a self-supporting free standing monocoque if formed in an arch.
But put it on the exterior- use 2" of 3lb foam (about R14) with an UV protective spray on coating or cementicious stucco-like coating to keep the sun from degrading the foam (an approach used for flat roofs) plus 3-4" of continuous rock wool or cellulose on the interior. (R11-R17), which would be a sufficient R-ratio for dew point control at the metal surface from interior moisture drives without interior vapor retarders other than latex paint (unless you're in central AK or something), and would protect the exterior from direct rain-wetting. The steel skin would also protect the occupants from potential outgassing of the foam, and would not need a carefully implemented thermal barrier on the interior for fire protection.
Insulating on the outside makes more sense as Dana is suggesting, but I don't see how you're going to attach and cover the rock wool or cellulose on the inside.If you have to cover both sides of the metal, it seems like the cost and hassle of the project would exceed what the homeowner is trying to do, i.e., build a cheap house. In that case, A.J.'s advise is the one to go with.
As you're suggesting, you could spray foam the interior. You don't need much, however. Just spray a thin shell of the closed-cell stuff (say, 1") all along the interior to encapsulate the metal and serve as an air barrier, since moist interior air would love to condense on that cold metal in winter. After that, you could build framed walls inside the quonset hut, perhaps with a cathedral ceiling, drywall it, and then fill the cavity between the drywall and the spray-foamed metal exterior with cellulose. That would give a "conventional" appearance inside with straight flat walls too. If your client lives in the desert, it's probably safe to omit the spray foam from this assembly entirely, provided the drywall is done with an eye towards airtightness.
How cost-effective this approach would be is another question entirely, of course. There's also the fact that you'd still need to deal with one or more of the ends that are often open or have a garage door type thing and that this place would have no windows on the long sides unless you cut them yourself, possibly compromising the structure and substantially complicating the detailing. This might actually be one of those circumstances where a couple of solar tubes could be useful for daylighting if the ends don't gather enough light.
Nathaniel's description of how to finish the interior illiterates the fundamental problem with trying to insulate quonset huts: their chief appeal is their shape and materials. If you insulate the exterior you end up with, as Dana suggests, some protective coating. Move inside, and rather than being able exploit their soaring curves as part of the living spaces, you end up trying to square things up, leaving strange voids.
Converting quonsets was quite popular in the 70's. The trend died away without leaving any really successful examples.
I know of two insulated quonsets in my area. Both were insulated with ccsf approx 20 years ago and are holding up fine.