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Advanced wall construction idea. Will it work.

I live in a zone 6a and plan to build this summer. I have been researching and drawing different wall construction details, and keep coming back to this design. However I can't find much about it online or in discussion with other professionals.

My wall section outside in would consist of:
siding
house wrap (breathable drainage plane)
3/8 plywood
2x4 stud wall 16 oc with roxul batt. Framed to underside of floor decking.
1"-1-1/2" extruded styro with ship lap joints taped. Again to underside of floor decking. This will be my vapour barrier and thermal break.
2x4 stud wall 24" oc less joist height to support floor and run electrical. Will also insulate with fibreglass batt.
drywall and paint.
Note that I realize that the rule of thumb is 2/3 of the insulation should be to the cold side of the vapour barrier, and this design is close. I could easily achieve it by opting for 2x6 exterior, increased styro, or decreased batt to the inside. however I'm not to concerned as the design allows for drying to the warm side if or when condensation occurs.
My reasons for considering this design are:
-I will have quite a bit of exterior detail and I don't want the hassle of working over top and around insulation.
-I don't like the idea of a vapour barrier on the cold side of my wall for obvious moisture issues.
-I know that a continuous vapour barrier to the inside of a wall is virtually impossible after all the mechanical and fastening penetrations
-I like the fact that this design can dry to both the inside and outside.
With all the benefits that I see with this design I wonder why I can't find more about it. Is there something that I'm missing??? I realize that framing a second wall is more work but after factoring in the fact that you would have to strap over the entire outside of the house for something to fasten siding to (unless you plan to brick or stuco) I don't see that a second wall to the interior as such a big deal. In my opinion as a framer building walls is the fun part! If you have ever strapped and sided over styro you know that it's a pain in the a...
Maybe it's the r value that people are hung up on? But I would suggest that r means nothing if your walls leak... and something that I have encountered with these super tight wall systems is that the less leaks you have the greater concern they become. You can end up with a really high concentration of condensation where you have a small leak and most of these designs don't allow for the wall to dry in or out.
Does anyone have first hand knowledge of this design? or see an obvious flaw in my reasoning that I could be missing?

Asked by Kevin vesey
Posted Mon, 02/17/2014 - 14:17

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Helpful? 0

Kevin,
Triple posting is confusing. It's best to have one thread, so all participants in the forum can read the same back-and-forth.

So, as I noted when you double-posted: let's stick to one thread. For interested GBA readers, here is a link to the page where Kevin first posted his question: Wall design questions.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/17/2014 - 14:29

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