Spray Foam for Exterior Wall Insulation in Houston
R_A
| Posted in Energy Efficiency and Durability on
Hello, my wife and I live in Houston, TX and have decided to build our new home out of steel. From our understanding, the exterior wall assembly is made up of 24 ga metal cladding, steel columns, 3.5″ of open cell spray foam and 1/2″ drywall. However, based upon our research one cannot use open cell in an exertior wall assembly if one needs a vapor barrier. If this is true, should we require a closed cell spray foam to be used and if so, how thick in a 6″ cavity? I think the R value per inch of closed cell is R-7. This is a big concern of ours and just wanted to reach out for more information on this subject. Thank you,
Roger
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I'm not very familiar with steel buildings or with the Texas climate, but I'm going to tell you what little I know, partly in hopes of bumping this to the front page for more expert input.
In the summer, open cell would be fine. In the winter, you might get moisture accumulation in the foam. I don't know whether your winters are cold enough for that to be a concern.
If you go with closed cell, specify "HFO" foam to avoid the biggest climate impact of spray foam, and get better performance as well.
If the steel columns or steel studs bridge the insulation, you will lose a lot of the benefit if using high-performance insulation. Is there a plan to address that?
Are you required to have vapor barrier in the wall? If not, then you'll have some other options. It's usually better to use a smart vapor retarder, true vapor barriers often result in mositure problems, especially if you're in a climate with heating AND cooling needs.
Spray foam in walls is a waste due to the thermal bridging of the studs. Basically there are other ways to spend that extra money to get a better performing wall. What I would do is to put up exterior rigid foam, which will help a lot to cut down on thermal bridging, then just use batts in the stud bays. I like mineral wool myself, but fiberglass works too. You'll end up with a better performing wall this way without the need for spray foam, and the rigid foam will give you a vapor barrier if you use thick enough EPS or XPS (note that we're not fond of XPS on this forum), and especially if you use foil-faced polyiso, which would also give you the best R value per unit thickness.
Bill
Thank you for your quick replies Charlie and Bill. I've discussed it with my wife and we have decided to spend the extra money to go with what the steel manufacturer recommended, which is the Energy Saver FP package from Silvercoat (See attachment). We had already spec'd this out for the main residential building, but are not thinking this is the best bet all the way around and to use it on the back building also. This solves a lot of problems by giving us the vapor retarder, multiple layers of mineral wool with a vinyl backing towards the exterior, and it does not cause any thermal bridging since it is a multiple layer system.
Sorry, the Energy Saver info pack was too large to upload, but you can find it on their site.
It looks like a good system--the additional layer of insulation over the studs will do more than what's between them, but the stuff between the studs will still help some. Other nice features include fire resistance and lower climate impact.
As long as there is a layer of insulation over one of the exposed faces of the studs, then you'll have much reduced thermal bridging. If the layers are all BETWEEN the studs, then the thermal bridging issue remains. That's the only thing you should check on, and my guess is the manufacturer has a layer on the exterior over the studs to address this.
Bill
In warmer climate getting an air tight assembly is more important than insulation. You do loose a lot of R value because of the steel columns (assembly is ~50% of cavity R value) but that is good enough when your design temperature is 35F. Not great but will work. If you can get some of the insulation over the studs it will make a big difference though.
A leaky assembly however will be an energy hog all year around, so make sure you deal with this. Air tight drywall can work but it is hard to get trades to detail it properly. Air tight sheathing is much easier to get right and more robust in the long run. So even though you are building a metal building and can skip the sheathing, make sure there is an air barrier there. This can be one of the membrane products or even a well detailed heavy duty house wrap but OSB/CDX is much better.
Also make sure you take care of the wall/ceiling intersection. This is an intersection that is often overlooked and the source of large air leaks.
If you're in a flood zone I would look at closed-cell spray foam for the ground floor level.
I'm curious what your thinking is here. I wouldn't expect spray foam alone to do much to act as a swimming pool liner, keeping your first floor dry in a flood with multiple feet of standing water outside, so are you thinking it would dry out quicker, protect the structure as a sealant better, or otherwise be more resistant to being submerged for a while? I would think mineral wool would offer similar quick-drying ability, as would EPS or XPS rigid foams. Polyiso could potentially be a problem if left submerged for a while.
Bill