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Double-Studs and Roxul

I am working with a builder to finalize design of a house on the great lakes in Canada (zone 6).
Our wall plan is 2 x 2x4 with 3 layers of roxul batts vertically in the stud bays and horizontally in between the stud bays and interior poly vapour barrier. The builder has a good reputation for energy efficient houses and reports no problems in walls he has investigated after many years in place.

From reading about monitoring moisture levels in double stud walls ( http://www.greenbuildingadvisor.com/blogs/dept/musings/monitoring-moistu...)
It seems as though this type has some risk associated with elevated moisture, particularly north walls.
My thought was to sheath the north wall with plywood, other walls with osb and use a ventilated rain screen behind unfinished white cedar siding all the way around. I am wondering if this is enough or whether we need to do plywood all the way around or put roxul boards on the outside to keep the sheathing warmer, or use a different wall system altogether with exterior foam or roxul board sheathing.
We will have Georgian bay to the south and east on our lot, so it is a little moist and moderated climate
Heating is in-floor with woodstove and an HRV will be installed.
We also have a mudroom with laundry located outside of this enclosure and a sunroom - thought being to keep the dryer vent separate and to have a sunspace that can be closed off from the house. Are there any durability issues with insulation choices in these locations - sunroom in particularly being warmer.

Asked by Jake VanDor
Posted Tue, 06/24/2014 - 07:29

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5 Answers

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1.
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Jake,
Everything I know about this issue is reported in two articles: the one you linked to, and this one:
How Risky Is Cold OSB Wall Sheathing?

So, the bottom line is: we don't know what the long-term ramifications are for annual moisture cycling in OSB.

How to respond to this uncertainty depends on your appetite for risk. If your appetite for risk is low, you should use plywood sheathing on all four orientations, as well as a ventilated rainscreen gap. That's what I would do.

If your appetite for risk is a little higher than mine, you can install OSB on three orientations as you propose.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 06/24/2014 - 07:44

2.
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Another possibility is what I'm planning: Move the OSB to the outer face of the inner wall, eliminate the outside sheathing replacing it with Greenguard DC14, an XPS drainage board placed over the house wrap. The DC14 also serves the rain screen function. BTW, since you are in Canada and likely will be forced to use interior poly you can gain about r1 by using staggered studs. An advantage of inline studs is reduced risk of condensation on the OSB where I've chosen to locate it.

Answered by Jerry Liebler
Posted Tue, 06/24/2014 - 10:13
Edited Tue, 06/24/2014 - 10:22.

3.
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Not that you asked, but I'm wondering why you're using all this Roxul. Do you have any insulation contractors who can blow in cellulose or fiberglass? Got to assume it would be cheaper, faster and easier than 3 layers of Roxul.

Answered by Dan Kolbert
Posted Tue, 06/24/2014 - 12:37

4.
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Hi Jerry: Good thought, That could work but it might make the insulating process a bigger headache!
We are going to do the staggered studs. I also just plain like having inedible sheathing on the outside.
Hi Dan: Good thought, We don't have good dense packers of either, and I like the fire, moisture, and vermin resistance of the roxul.

Answered by Jake VanDor
Posted Tue, 06/24/2014 - 18:07

5.
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Jake,
In your description of our wall you said the middle layer would be installed with the long dimension of the bats horizontal. This will make getting the middle layer in a bit of a struggle. I'm planning on using material that normally is used with steel studs for the middle layer, it is 16 " wide or 24 " wide so the long dimension can be vertical in all 3 layers. I think this will be much easier to install. I see the added "headache" is that the outer 2 layers need to be installed from the outside with my sheathing position while The "traditional" sheathing position requires that all layers be installed from the inside. Is that what you meant? Since I'm in the USA I don't need the interior poly so I'm choosing aligned studs and the OSB will be my primary air/vapor control layer. FWIW The mineral wool gives a higher r value for equivalent thickness by about 10% over dense packed cellulose or fiberglass.

Answered by Jerry Liebler
Posted Wed, 06/25/2014 - 00:23

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