Thermal bridging with steel studs
dagould2000
| Posted in GBA Pro Help on
I’m thinking about installing steel studs to the exterior of a concrete block single story home. Is thermal bridging still going to be a major issue if the stud bays are filled with spray foam. I’m considering proseal closed cell foam. Also the bays would be covered in Doug fir plywood siding for a board and batten look. What weatherproofing methods would you recommend above and below grade for this application. Thanks
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Replies
Yes, so much so that you might as well not fill them. Use continuous, external rigid foam.
Seriously- what Jon R said!
At 16" on center an R13 open cell foam wall layer comes in at about R7 after thermal bridging, which is about what you'd get with a continous layer 1" of foil faced polyiso strapped to the wall with steel furring.
With credit for the air films, block and siding the open cell foamed wall still comes in at no more than R8- R9 "whole assembly", R10-ish if 24" o.c.
Filling it with R25-28 of closed cell foam at some ridiculously high expense still wouldn't bring it up to much over R11 for the layer, due to the severe thermal bridging, maybe R12-R13 ish for "whole wall R" after thermal bridging. If you're going to insulate between steel framing keep to the cheap stuff, save the high R/inch goods for continuous insulation layers.
Continuous layer of rigid foam and steel furring through-screwed to the block wall works well.
For more on the thermal performance of insulation in steel framing see:
https://cfsei.memberclicks.net/assets/researchreport/RP18-1.pdf
Besides the high thermal conductivity issue, steel studs usually are formed from relatively thin steel sheet that is flash galvanized. That translates to “not very durable in the elements”. I’ve seen many steel studs installed on block/concrete walls that have rusted badly. I think it’s due to moisture in the concrete since this has always been indoors, but on exterior walls. If you do use steel studs, at least put some kind of moisture barrier between the studs and the concrete.
Rigid foam on the outside and some kind of furring over that is probably a much better option for you all around.
Bill
The "exterior grade" steel studs suitable for hanging exterior grade gypsum sheathing & siding onto seem to go the distance in commercial construction. The flash-galvanized thin stuff used in commercial interior partition walls aren't normally used in commercial curtain-wall construction. (And should not be used here.)
If for some reason you need to support your cladding on something like a steel stud without the thermal bridging, there are products that exist to solve that problem.
Here's one example: http://smartcisystems.com/
And another: http://www.cascadiawindows.com/products/cascadia-clip
However, if continuous rigid insulation with furring strips is viable for your project, that's probably a more economical choice than the fiberglass z-girts.
David,
Here's one more vote for a continuous layer of exterior rigid foam, followed by furring strips or synthetic stucco (EIFS).
EIFS would be a VERY different look from the described "...board and batten look..." goal, but a lot cheaper & easier.
Exterior grade steel furring/hat-channel girts over the foam could be used to mount board & batten, but 1x wood girts would also be fine, and easier to deal with for most.
Thank you very much for the quick responses. An insulation company quoted this portion of the project as their "best" option. I was curious about this and It seems that it was for a good reason.