Replies

1. 
   Riversong | Jun 09, 2010 04:20pm | #1

   Mark,

   Sounds minimally OK. It's definitely NOT a good idea to build a slab in zone 6 without frost protection, and the 2" of XPS is the minimum required for that climate (it also must extend a total of 4' vertically downward and/or outward).

   The cross-hatched cellulose walls should offer the code minimum R-20 but I would advise leaving out the poly vapor barrier since most cellulose manufacturers discourage its use (cellulose works best when it can breathe, use a vapor retarder primer instead).

   The IECC requires a minimum of R-49 for the ceiling, which would require at least 13" of dense-pack cellulose. Do you have that much depth in the truss-joists? If not, I would recommend building your vent cavity above the framing with a double layer of sheathing separated by strapping.

   Putting a capillary break between concrete and wood framing is always a good idea. Be sure to air seal this joint as well.

   You won't get much acoustic isolation from horizontal strapping unless you use resilient channels.

2. 
   Lyndon | Jun 09, 2010 04:34pm | #2

   Dear Mark,
   I would consider placing the poly VB between the 2x6 studs and interior strapping. To co-ordinate with this, I would do everything reasonably possible to keep the wires, electrical boxes and plumbing (if any) within the furring cavities and outside of the 2x6 framed and poly-wrapped wall cavities. This will help to keep the VB continous. Use standard (non vapour-retarder) paints on the drywall so the furring cavity can dry toward the inside.
   The 2" insulation under the slab is definitely needed, and I would consider thickening it if finances allow. In addition, provide lots of insulation on the slab edges and wings of insulation under the finished grade adjacent to the slab, sloping away from the slab. The detail between the slab edge insulation and wall framing must be carefully considered and executed to stop water, insects and flow of heat.
   Truss joist on the roof is good - The deeper the better. In Ontario, the code requires a 2.5" minimum space between the underside of the roof deck and insulation. I would consider adding 2x3 or 2x4 purlins on top of the joists (across the joists) or sleepers (along the joists). Place them on the flat for easier nailing and an excellent surface to attach roof decking. Then, working on the inside, use the underside of the top chord of the truss-joist as the nailing surface to attach boards to make the vent space, which would then be about 3". This board should be fairly vapour-permeable, I would think. Both plywood and OSB are pretty vapour impermeable. I'm actually not sure what would work best here - I would think wood-fiber board - but I've never used that stuff before.
   Lyndon

3. 
   Lyndon | Jun 09, 2010 04:39pm | #3

   Oops, my answer was posted at the same time as Robert's and it seems to be in contrast to some of his points - I hadn't read it yet.
   LT

4. 
   Lyndon | Jun 09, 2010 04:48pm | #4

   I agree with Robert about the insulation values. I'm working on a Passive House in the Toronto area and the design calls for roughly about R60 walls and foundation, R100 roof, R 20 under the basement slab. Of course, there is the high-performance south facing windows, with proper overhangs to go with those r values.
   Sorry, I didn't know much about cellulose insulation.
   LT

5. 
   Doug McEvers | Jun 09, 2010 05:06pm | #5

   I agree with Robert, you should go with ADA in a marine climate. These folks have really mastered ADA and the details with ACH50 numbers under 1.

   http://www.energetechs.com

6. 
   Mark Pennell | Jun 10, 2010 08:48am | #6

   Thanks for the advice everyone. I have not ordered the roof joists yet, so I can specify the depth; I think 14" would do the job.

   As for the slab, I will do a thickened edge slab and skirt out or down for frost protection. The local building inspectors claim that frost protection is not needed on a slab less than 600SqFt, only supporting one level; any insight?

   I will ditch the poly, and instead of strapping the wall I will staple the cellulose net back slightly to prevent the bulging out to far, and that way air tight drywall approach is much more simple. Is vapour retarding primer readily available at most building supply stores?

7. 
   Riversong | Jun 10, 2010 11:50am | #7

   Mark,

   Moorcraft Latex Vapor Barrier Primer Sealer (or SuperSpec) 260 creates a less than 1 perm film with a single coat. I'm sure other major paint brands have also come out with similar products now that codes require only 1 perm retarders.

   While there's little reason to build a frost-protected foundation for an outbuilding. Any building that must remain plumb and level and has utility connections that can be disrupted should be on a foundation or slab that isn't going to heave.

   However, soil must contain at least 5% silt and be saturated with water in order for frost heave to occur. If you're building on well-drained gravel, then there's little worry about frost heaving.

8. 
   Mark Pennell | Jun 10, 2010 11:55am | #8

   Thanks Robert, your advice has been great and well appreciated.