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Wall Assembly-Very Cold Climate Zone 7

I live in northern Maine and will be building a passive solar home. I want to achieve r-50+. Here's a rundown on my proposed wall assembly with three thoughts in mind; 1. high thermal resistance, air tightness, economical as possible. Any ideas/suggestions would be appreciated.

Pine board cladding
Drainage plane
WRB housewrap (ie. Tyvek)
3/4" fiberboard
2x4 outer stud wall (3.5" Roxul insulation) 24" OC
5.5" (or 7.25"?) cavity (Roxul insulation)-chase for electrical
Primary air barrier attached to inner wall (taped OSB treated with liquid vapor barrier)
2x4 inner stud wall (3.5" Roxul insulation)-load bearing wall 24" OC
5/8" sheetrock

Thanks.

Asked by Matthew Michaud
Posted Dec 29, 2012 9:54 AM ET
Edited Dec 29, 2012 11:29 AM ET

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6 Answers

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1.

Matthew,

This in my opinion is the perfect wall for a cold climate, is a warm side poly air/vapor barrier used in that part of the country? If so I would scratch the OSB and consider Teno or MemBrain right under the drywall to complete the wall assembly.

Answered by Doug McEvers
Posted Dec 29, 2012 10:48 AM ET
Edited Dec 29, 2012 11:03 AM ET.

2.

I was considering the OSB to be an important structural piece of the load bearing inner wall. Additionally, I didn't want to place the vapor barrier in an area where it could get pierced by many screws (drywall, hanging pictures, shelves, etc.). I figure the vapor barrier is on the inner 1/3 of the wall, so I'm safe. Thoughts?

FYI-the studded walls are 24" OC

Answered by Matthew Michaud
Posted Dec 29, 2012 11:23 AM ET

3.

Doug,
In my opinion you are asking Matthew to consider an inferior option! First off he needs the OSB, though I'd prefer plywood, to provide racking or shear strength. Second He has a very good vapor barrier positioned in the ideal location for cold climates and away from likely abuse by future homeowners, on the outside of the inner wall. With the excellent drying, in both directions of the wall Matt described the choice of liquid applied vapor barrier to seal the air barrier will give the best results.

Mathew,
It sure looks good to me! I think I've seen this wall before (chuckle)! With such thick walls, have you planed your window installation?

Answered by Jerry Liebler
Posted Dec 29, 2012 11:38 AM ET

4.

Matthew,
I'm presently building something similar (in climate zone 7) - you can find some details here, if you're interested.

Have you considered making the service cavity the inner wall (ie on the interior side of the sheathing)?
You may avoid a lot of penetrations that way.

Are you committed to using OSB?
In my opinion, for only a marginal increase in cost, plywood is a superior product for an application like this.

Given the wall assembly you've described, have you determined yet how the air barrier system will transition at the ceiling?
Will you build a ceiling deck (and service cavity) like in Sunrise Home?

Personally, I think you're on a good track.
I agree with Doug that assemblies like this are just what you want for a cold climate.

Answered by Lucas Durand - 7A
Posted Dec 29, 2012 11:42 AM ET

5.

if shooting for an R-50+ wall in a passivhaus-ish house in northern maine, why not do what chris corson is already doing? if it wasn't nearly double what we need, insulation-wise, we'd probably go w/ similar system...

http://www.greenbuildingadvisor.com/blogs/dept/green-building-news/striv...

for the knox house, assembly was (in to out)

5/8" GWB
2x4 24oc OVE w/ roxul batts
1/2" OSB (air barrier)
12" TJI's insulated with 3.5lb cellulose
1/2" high perm sheathing
3/4" strapping
siding - ship lapped pine/cedar shakes

i think the last time i talked to chris he was planning on removing the outermost sheathing altogether and utilizing a sarking membrane instead.

Answered by mike eliason
Posted Dec 29, 2012 8:17 PM ET

6.

Thanks for the info. Should I tape my Tyvek WRB and fiberboard layers even though I have a primary air barrier on the inner plywood/OSB wall?

Answered by Matthew Michaud
Posted Jan 1, 2013 12:37 AM ET

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