Single-story 30x40 hip roofed house with 9 foot ceilings. Double 2x4 (24 inch OC, double top plate) walls 5 inches apart with both walls fully sitting on the sub-floor. Outer wall sheathed with 7/16 Zip Panels. Cellulose insulation, not dense-packed.
1. Is it advisable, and permissible, to run the house wiring on the outside of the inner-wall studs, or, should the inner-studs be drilled for the wiring?
2. When building headers for window & door openings, do you use double 2x header material on both the inner and outer walls, or, do you use single 2x header material on each wall?
3. Should the outer side of the inner wall be sheathed as well?
Thanks for any input.
Rob.

Rob,
Why the double top plates? What type of foundation are you planning? What is your exterior finish system? You can define either the inner or outer wall as the "load bearing" wall. Only load bearing walls need headers. With both walls sitting on sub floor you have a lot of thermal bridging in the floor joist and sub floor, there are better ways. Sheathing the outer surface of the inner wall can become the primary air barrier and give you the a " service cavity" but complicates the insulation system. If it's cellulose insulation and not dense packed, how are you dealing with the settling that will occur?

Hi Liebar, thank you for your input.

1. I believe that double-top plates are required by the building codes, for 2x4 24 inch OC walls, here in Canada.
2. The crawel-space foundation will ARXX ICF's with a 6” cement core built directly on bedrock.
3. The exterior will be, from the ZIP Panels out, Tyvec, 1x3 strapping and then cement board siding.
4. The rim joist will be insulated.
5. The space between the double walls will be open to the attic insulation, the idea being that I can 'top up' the insulation in the attic as settling occurs.
6. The spaces above and under the windows will be spray foamed to R-40.

drawing.

Please note that this drawing is based on Richard Lachance's drawings from: NB Super Insulated House.

Rob,
If double top plates are required, I'd sure fight such an absolutely unnecessary waste of wood! With properly aligned framing and metal corner ties there is simply no justification for a second top plate. With the ICF foundation you should be able to eliminate the thermal bridging of the floor joist by making the inner wall the load bearing one and building the outer wall taller.
Have you considered supporting your floor framing by a 2x4 that has say 1" of it's width over foam, placing the rim joist flush with the outer edge of this 2x4. This would stop the floor framing and sub floor 6" inside the outer edge of the ICFs. Once the floor is decked with sub floor the outer stud wall would be built and erected sitting on 1 1/2" of concrete and foam on the outside. The outer wall would not be the load bearing wall and would be built taller ( by the depth of floor joist + sub floor). Then the inner, structural walls would be built on the sub floor, sheathed (use the ZIP system here) and erected. If the outer face of the inner wall is sheathed, it should be detailed as the primary air barrier and it is simpler to use Roxul bats in the inner stud cavity. Spray foam under and above windows is expensive and may well cause moisture issues. I'd look at filling these areas with Roxul instead. Also if the inner wall is sheathed you could use fiberboard for the exterior sheathing, it would require adding a building wrap WRB but would give your wall outstanding drying ability in both directions and save a little money.
The last question is how are you going to fill the walls, if it's from the attic I'd be real generous with the "energy heal" on the roof truss system to give the insulator some head room.   I'd also advocate a ceiling service cavity made by screwing and gluing OSB to the bottom chords of the roof truses then ,using long screws, attaching 2x4s to support the drywall. This would mean building the exterior walls 4" taller yet but would give a safe walking surface in the attic and allow an excellent air sealing job.

Rob,
Sorry I shoulda looked at your wall cross section before I wrote the above. But the Pressure treated 2x12 is a nasty thermal bridge as your crawl space is "conditioned". That's a lot of spray foam around the rim joist. To get the insulation around the rim joist it looks like you need to delay installing the exterior sheathing until after the insulation is placed. Again cutting and fitting Roxul bats gives good insulation without trapped moisture issues. You might consider insulating the crawl space 'floor' with perlite left in the bags.

Hi Liebler:
I took your advice and reduced the amount of spray foam around the rim joist and adjusted my drawing.

I thought I had the problems of cellulose licked, ie settling and windows, only to find out from you that my walls will rot and that I need to find a 'height-challenged' cellulose installer!

Is there a better solution than to use spray foam under and above the window openings that will ensure proper R-values?

Thanks,

Rob.

Rob,
Another issue with deep spray foam is it can become a fire hazard if applied in too thick layers, heed the instructions!  To avoid the moisture entrapment issues you could use open cell foam, it has at least some drying potential, or (my preference) cut and fit Roxul bats in the inaccessible areas.  Why not make the walls different heights? That would eliminate the spray foam outside the rim joist entirely and eliminate the thermal bridge of the sub floor/bottom tie plate . BTW the sketch shows a top tie plate which would restrict the ability to have the attic insulation "refill" the walls. I'd try to convince the local officials that the interior poly wasn't needed and put the zip sheathing on the outer face of the inner wall creating a service cavity and becoming the primary air boundary. Perhaps you could fill the walls through the overhangs/soffits from the outside,over the top and complete the soffit after installing the cellulose.

Well, I submitted my Sketchup drawings to the technical drawer to create AutoCAD drawings and his response was:
1) The quoted price of $1.50 per sf would not stand, it would be considerably more as he would have to adjust all his AutoCAD settings to accommodate double-wall construction.
2) Here in Quebec, there must be 1 inch of foam insulation on the exterior under the siding.

Trying to build a house, yourself, is not easy...

1. Sounds reasonable - more time, higher cost. Will be well worth it for a good house.
2. Also a good idea for a thermal break - just make sure that the amount and type of foam does not create a moisture barrier - so perhaps EPS?

Rob,
If you are building it yourself, do you really need Autocad drawings? I'd contact the building official directly especially about the ALEDGED exterior foam requirement, which is simply ABSURD with your thermal bridge free wall. The foam definitely will REDUCE the drying ability to the exterior and create a worse wall! If you are building yourself you may want to consider using all Roxul bats in your exterior walls. The material cost is on a par with dense pack cellulose so it'll be a bit higher than the loose cellulose but the installation is much more DIY friendly than dense pack cellulose.

Thanks Bob.
Hi Liebler:
The municipality requires 'stamped' drawings prepared by a professional, my Sketchup drawings won't do.
I am embarrassed to say this but, I am an English-speaking person in a prominently French-speaking Province - this makes it very difficult for me to navigate the whole 'house building world' here, much less present arguments.
I just ordered the National Building Code of Canada and I hope that it will make many of my novice questions on building this structure go away - I'll try not to ask anymore 'basic' questions on this forum - I have so many.
I am a little afraid of 'fiberglass' because I have read so many posts on the web about how cellulose is superior, especially when it comes to air leaks and proper fitting.
I have modified my drawing to show the Top Plates, an 8" core ICF and the removal of the pressure treated sill plate.
Rob.

Having many "English" relatives & ancestors in Quebec, I understand entirely. Many of the towns where they live or once lived (Maple Grove, Sawyerville) have disappeared entirely or have had the names changed. Would help if you can find a bilingual friend; I can't imagine what it must be like.

Basic questions are easy here, but as you've found, you'll get 10 different answers & opinions which you have to distill.

Rob,
There is a HUGE difference between fiberglass and mineral wool like Roxul. Roxul bats hold their shape but are compressible enough to assure a snug friction fit. Roxul bats are cut with a hand saw or a serated knife, not at all like fiberglass bats that are so compressible they are routinely cut with a utility knife. Roxul is r4 per inch, better than dense packed cellulose.

Hi Liebler:
You've got me interested and I have some questions:
1) Have you built a more or less super insulated house with Roxul batts? If so, what where the wall details?
2) Assuming that I use Zip Panels for the walls, sealed with Zip Tape, I shouldn't have much air leakage through them, right?
3) Is there a specific product name for the Roxul batts that you are referring to?
4) Is there anything special that I must watch out for when constructing a wall with Roxul batts?
Thanks,
Rob.

Rob,
A1: No I haven't, yet, built a "super insulated" house using Roxul bats. I'm in the planning phase for my super insulated "dream house". However I have used Roxul bats in a more or less ordinary house I built some years ago.
My dream house walls will be double 2x4 24" OC advanced framing with 5 1/2" space between stud walls, the inner stud wall is structural and it's exterior face is sheathed with 1/2" plywood, the plywood is attached to the wall with construction glue and the bottom plate is glued to the sub floor. The outer wall is erected without sheathing. After window and door openings are completed between the walls the insulation will be installed. The exterior sheathing will be 3/4" fiberboard covered with house wrap then Greenguard's DC14, then Ambrico's EZbrick substrate for thin brick then the thin brick (1/2" nominal thickness) My windows will be set in 4" from the outer surface of the finished wall.
A2: The devil is in the details! Corner caulking top and bottom sealing with caulk or gaskets, sealing around penetrations all leakage paths need to be closed off. With proper attention to detail you can achieve a VERY airtight house.
A3: In the US they go by Roxul "Comfort Bats". They generally are sold at the 'big box' home improvement stores. Lowes is probably the biggest US retailer of Roxul. They come pre-cut to fit either 24" OC or 16" OC framing with available thickness to match 2x4,2x6 or 2x8 standard lumber
A4: Nothing special, take the time to fit them well & do a good job air sealing

Hi Liebler:
Nice wall!
I have some questions, again:
1) Is it required, in your wall or mine, to place sheathing on the exterior of the interior wall. In my case, both the inner & outer walls are fully supported by the foundation.
2) Roxul, between the walls, is 5.5 inches thick - can it be crammed into the 5" space in my case, or, the 4.5" space in your case?
Thanks,
Rob.

Rob,
A1: One layer of "shear bracing", usually sheathing, is required somewhere in the wall. In my wall I placed it on the outside of the inner stud wall to accomplish the bracing and serve as the primary pressure boundary and form a service cavity & eliminate the need for air sealed drywall which I can't see being very durable.
A2: I wouldn't try to cram it as it makes for much more effort, but it would, most likely fit. My walls do have 5 1/2" between the sheathing and the inside of the outer studs. I've considered reducing the space between the walls to 3 1/2" but.  The major reason I've still got the thicker wall is it fits better over my foundation which is also double wall. The outer basement wall is 8" poured concrete. Three inch foam separates the inner & outer basement wall. The inner basement walls are 2x6 24"OC and sit on an insulated slab floor. The inner basement walls carry the entire load of the house. My outer first floor walls sit on the concrete outer basement walls.

Robert, most building supply sell Roxul.It is a trade name for a product known all over the world as "mineral wool". It is made at a plant just west of Toronto and you can get it in the same sizes as fibreglas (2x4, 2x6, 2x8 etc) but you get less in a bundle because it doesn't compress like fibreglas.

2) Roxul, between the walls, is 5.5 inches thick - can it be crammed into the 5" space in my case, or, the 4.5" space in your case?

It doesn't compress much, certainly not like fiberglass batts. You might be able to compress it a half inch, but more than that and you run the chances of having the sheetrock bow out between studs. I just used it in some cavities with 1" of cc spray foam applied to the back and had to shave the Roxul batts to get a fit.