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Seeking feedback on double stud wall with hybrid insulation strategy

We preparing to build a house in Ottawa, Ontario (climate zone 6), and we’re looking for feedback on our insulation strategy. We are going with a double stud wall design, (2x6 load bearing to the exterior, 7” insulation space and 2x4 non-load bearing on the interior), for an insulation space of 16”. We are considering a hybrid of Open Cell Spray Foam and dense pack cellulose. A roughly 4” layer of Open Cell Spray Foam will be sprayed into the 2x6 stud cavities, and the remaining 12” space will be dense packed with cellulose. Insulweb netting will be used to hold the cellulose in the wall. Additional open cell spray foam will be applied to envelope the bottom plate, and fill the headers. Our city building inspectors require a vapor barrier on the interior, so we will go with polyethylene vapor barrier installed over the netting.
All other aspects of the the wall system will be typical - siding, house wrap, plywood on the exterior, drywall on interior.
We have opted for Open Cell Spray Foam because it is a good air barrier, is semi-vapor-permeable and it’s blowing agent does not contribute to global warming. Due to spray foams higher cost, we’re using only what we need to create an air barrier, and will use dense pack cellulose to insulate the majority of the cavity. This strategy requires that the spray foam not be a vapor barrier, as are required by code to have the interior polyethylene vapor barrier, and we do not want two vapor barriers.
We have tried to balance cost, air sealing, and build ability, keeping in mind the products and trade skill sets available in our area.

Any feedback, suggestions or comments would be appreciated!

Asked by Jeremy V
Posted Wed, 01/23/2013 - 12:23

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2 Answers

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Jeremy,
A couple of issues:

1. In your climate zone, polyethylene isn't a great idea. See if you can get your building inspector to accept a "smart" vapor retarder instead -- for example, a product like MemBrain. More information here: Do I Need a Vapor Retarder?

2. With such a thick wall, you end up with cold wall sheathing. I think that any thick high-R wall like this (especially one without a layer of exterior rigid foam) needs a rainscreen gap between the siding and the sheathing. More information here: How Risky Is Cold OSB Wall Sheathing?

Answered by Martin Holladay, GBA Advisor
Posted Wed, 01/23/2013 - 12:37

2.
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Thanks for your feedback Martin.
I will inquire as to weather my inspector will accept a "smart" vapor retarder.
And I forgot to mention in my original post: we are planning to include furring strips under the siding to create a rainscreen gap.

Answered by Jeremy V
Posted Wed, 01/23/2013 - 12:51

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