Is a double stud wall with wood and steel studs a good idea?
lowelllodesign
| Posted in Green Building Techniques on
I am an architect from Toronto. I designed a one storey addition to an existing brick house. I am putting 3″ of rigid insulation on the outside of the new building envelope and a double stud wall using 2″x4″ wood stud for the outter stud and 2-1/2″metal studs on the inside as a chase space and using plywood between the 2 stud walls as vapour barrier thus eliminating 6 mil vapour barrier. I am insulating all stud cavities with roxul.
Do you see a problem in this assembly? Why is this assembly never done?
Thanks for your review and expert opinion in advance.
Lowell
BArch, OAA, LEED AP
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Replies
Lowell,
Whenever you install rigid foam on the exterior side of your wall sheathing, you need to make sure that the ratio of exterior rigid foam to fluffy between-the-studs insulation is correct. If the rigid foam is too thin, you can get moisture accumulation in your sheathing during the winter. This is all explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Toronto is in Climate Zone 6. (Here is a map showing Canadian climate zones.)
Your wall is unusual. It will hold 7 inches of insulation, whereas a normal 2x6 wall will only hold 5.5 inches of insulation.
In your climate zone, a 2x6 wall needs rigid foam with a minimum R-value of R-11.25. Since your framed walls are 27% thicker than a 2x6 wall, your rigid foam needs to have a minimum R-value of R-14.3.
If you use XPS or polyiso, your wall will be OK. (The polyiso is a little dicey, however, because it doesn't perform well at low temperatures.) If you use EPS, it would be better to increase the thickness of the rigid foam to 4 inches.
The metal studs act as thermal bridges, largely negating the value of the insulation installed between them, so the metal studs should be replaced with wood studs.
Lowell,
Oops. I just realized that your metal studs only measure 2.5 inches. That changes the math.
I still don't recommend the use of metal studs. Stick with wood studs for insulated walls.
Thanks Martin for the message. Can I also ask if it would be hard to rip 2"x4" wood studs into 2-1/2" deep members?
Thanks again.
Lowell
Thanks for the comment Malcolm. I think I tried calling lumber yards in the Toronto area looking for 2x3's but weren't readily available. I shall try calling again!
Lowell, Rather than rip down 2"x4"s, why not spec. 2"x3"s which are readily available? Or, for the space you save, just use 2"x4"s for the inner wall too?
Using 21/2' studs is problematic for a couple of reasons: You can't get the code mandated distance from the surface to wires run through the studs and have to provide mechanical protection at each hole, and 2"x3" wall feel really flimsy.
One potential difficulty I see with what you are proposing is that the sheathing and air barrier is on your inner wall. Usually when this is done the inner wall is also the load bearing one. In your case it isn't which means you will need to look carefully at the implications of having your sheathing, which provides the shear separated from the load bearing elements.
Lowell - You replied so quickly, you may bot have seen my edit.
Cheers, Malcolm
I will look into the wire clearance distance, Malcolm. The plywood sheathings are on both sides of the outer stud wall with the inner sheathing being the vapour retarder.
Lowell, I am curious, does the roxul comfort board with its vapor permeability, fire resistance, and no off gasing, not meet your performance needs?
I am not a builder nor pro. Just intrigued by the comfort board characteristics.
I need 3" of rigid foam outboard insulation, I think roxul comfort boards are thinner. It is at least twice as expensive and requires more hardware to install I believe.
"The plywood sheathings are on both sides of the outer stud wall with the inner sheathing being the vapour retarder."
Ah - I've got it now. That's a wall build-up I haven't seen. Usually if the sheathing is moved to the middle of the assembly it is on the exterior of the inner studs, and some other material is used as the outer sheathing - or no sheathing at all.
Having the outer wall sandwiched by plywood makes for an interesting sequence of trades with both the insulation and framing crews having to stop work and return after the other sub. If you aren't using the inner plywood for either backing or shear, why not substitute a smart membrane instead?
The clearance for wires is 1.25" to the hole (rule 12-516) leaving no room for the wire in a 2 1/2" stud, although perhaps the electrical inspector would waive this as there is no chance of nails penetrating from the outside.
Lowell, you didn't post the complete wall assembly including interior and exterior which I assume is brick with the proper detailing for the brick.
IMO you should get a detail from someone that works and is less complicated. I don't see any basis for what your have explained so far as to having any advantages worth doing. I agree with the others so far.
The Building science site has some great details and so does this site if you join as a member.