Exterior insulation with steel studs and foil-faced drywall
RustySaddle
| Posted in General Questions on
Hi, I’m looking to add exterior insulation to my 1960s ranch. (Climate Zone 5)
My 2×4 exterior wall assembly:
– Fiberboard
– Steel stud
– Cavity insulation (fiberglass)
– Foil-faced drywall
– Latex paint
I’ve read that foil-faced drywall is similar to poly, and is considered vapor impermeable.
Would moisture problems be likely if I installed 1-2 inches of foam/poly iso in this scenario?
Additionally, does anyone have experience fastening siding to steel studs? I was considering self-drilling screws, but perhaps wood strapping screwed to the studs, and then siding nailed to the strapping would work better.
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Replies
Foil faced drywall would be a vapor barrier unless the foil facer is perforated, which is the case with some projects (I'm not specifically sure how common that is with drywall, since I've never worked with foil faced drywall before). You could use EPS on the exterior, which is the most vapor open of the commonly available rigid foam materials, but it's still not going to allow much drying. Rigid mineral wool is very much more vapor open than any rigid foam material, but it's a lot more expensive too.
If you screw furring strips to the metal studs, you could hang your siding in the usual way with a nailer. Without the furring strips, you have to stick with screws for the siding, which may complicate things in some cases. Adding furring strips would also allow you to create a rain screen, and that would be a big plus here where you're worried about drying potential -- rain screens really help with drying.
Bill
I wouldn't worry about the foil faced drywall. House have been built around me (zone 5/6) with interior poly and exterior rigid foam insulation for a long time. I've opened up 25 year old walls built like this and the inside was pristine, as good as the day it was built, better than any wall without exterior rigid.
I've only ever done strapping over steel studs and hanging siding off the strapping. This is pretty quick with the right screws.
Siding can also be nailed onto sheathing but this does mean adding a layer of OSB/CDX as it won't work with fiberboard. This way you don't need to attach to the studs directly. Most siding can be nailed up like this directly through 1.5" foam.
Rain screen strapping is always the better idea with any painted siding as it will make it last much longer plus avoids having to re-sheath the house as well.
I would try to get as much insulation onto the wall as possible. Because of the huge thermal bridging of the steel studs, you only have about an R6-R7 assembly currently, aiming for around an R20 assembly is reasonable. That means around 2.5" of GPS or 2" of polyiso. Around me roofing polyiso available from most commercial roofing supply places tends to be the lowest cost in terms of $/R value and since it is usually fiber faced it is still somewhat permeable to allow for a small amount of drying to the exterior.
Thank you both for the advice.
The existing aluminum siding is attached with ring shank nails to the fiberboard, and it hasn’t fallen off. As a quick test, I hammered in a couple smooth shank nails and easily pulled them out by hand.
Wood strapping attached over foam to the metal studs sounds like it will make for an easier install.
I personally feel people worry too much about thermal bridging with wood studs, but with metal studs it's a real problem. Steel is so conductive that it has an r-value of essentially zero. Cavity insulation with steel studs is going to give very little insulation. If you have exterior insulation it has to completely cover the steel, it's so conductive that if part of it is exposed the rest acts like a giant radiator.