Stick-framed Roxul SIP
From my previous posts many know that I’ll soon build a “pretty good” house using double stud walls insulated with mineral wool. I think I have finally “invented” a construction method that is “reasonable”. It starts with ripped plywood top and bottom plates that are the width of the total wall, inner layer (with sheathing on it’s exterior face), mid-wall insulation and outer wall. The plywood ( I don’t trust OSB for this) is the bottom and top plate, second top plates of 2x lumber will be added as sections are erected. The wall(s) will be built in sections, 8′ the plywood plate length is the obvious first choice but not a constraint. The process of building a section begins with framing the inner wall then it is sheathed openings are cut and rimmed with plywood “picture frames then the sheathed layer is detailed with tape and caulk etc. as the primary air barrier. That done, the inter-wall insulation is “positioned” then the outer wall framing, insulation and, if used exterior sheathing. At each section boundary doubled studs (one in each section) are used on at least the inner wall to make a fast easily sealed joint. The sections go up and are caulked and sealed as they are added just like it was using foam SIPs, the inspection questions are also no different. The walls I’m planning use the process described with 2x4s 24″ OC stud spacing, staggered studs, is that really 12″ OC? I’m treating both as ” load bearing” in terms of headers etc. With the staggered studs section boundaries will not be insulated, in the outer wall before erection and exterior sheathing must be delayed or better, not used (my choice).For headers I’m planning on a simplified and minimized version of insulated box header framed with 2×3 and 1/2″ plywood insulated with 2 1/2″ Roxul my longest header is 70 1/2″. In my case once erected the house-wrap goes on then An XPS drainage plane (Kingspan Greenguard DC14) layer then steel panels (Amrico EZwall) that support “thin brick’. BTW I have a very detailed 3 D model in “Sketchp” a lmost done but I keep making changes. What do you think?
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Thanks for the detailed description. A drawing would be very helpful as some parts are hard to visualize.
Jerry,
The biggest issue with double-stud walls is assuring that the exterior sheathing can dry quickly in the spring. I don't think your decision to use exterior XPS with a vapor permeance of only 1 perm is a good idea. You want fast drying to the exterior in the spring, and this XPS layer will slow the drying rate.
For more information on this issue, see Exterior Rigid Foam on Double-Stud Walls Is a No-No.
Jerry,
Thanks for explaining that there is no exterior sheathing.
If you are treating both walls as load-bearing, and there is no exterior sheathing, you might want to have an engineer verify whether the plywood in the center of your wall assembly provides adequate wall bracing.
I understand that the XPS product has ventilation channels -- like Delta-Dry, it appears to use ventilation drying rather than diffusion drying to get rid of extra moisture -- but that fact alone wouldn't give me enough confidence to specify it. I think that a more permeable membrane -- one that uses diffusion drying -- might make more sense.
Martin, there is NO exterior sheathing other than the DC14 XPS! Also DC14 is no ordinary XPS, it ventilates and allows drainage on BOTH its inner and outer surface. The moisture perm of 1 only affects moisture that is attempting to pass through it, like solar driven moisture being released from exterior masonry not up or down it's inside surface which is the path for moisture from drying what,s inside it. The only wood sheathing in my double stud walls is OSB with a 3 1/2" layer of mineral wool and a 2x4 wall filled with another 3 1/2" of mineral wool on it's outside. This wall has 7 inches of mineral wool outside the OSB!
Regarding shear or racking bracing: The outer wall's top and bottom are coupled to the inner wall by the plywood top and bottom plates. It is a single wall assembly and the OSB is the bracing. The DC 14 is installed over a vapor permeable "house wrap".