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Double stud wall

Getting ready to build my own home. Looking at a wall assembly of 2" exterior EPS, plywood, 2x6 exterior wall with r22 fibreglass batts, 6 mil poly vapour barrier, 2x4 interior with r14 fiber glass batts and 1/2" drywall. I maintain r30 to the exterior side of the vapour barrier and r14 on the interior side so the 2/3 - 1/3 rule is maintained. I will not have to be concerned with electrical box and plumbing penetrations through the vapour except for items venting or take air from exterior.

Would do it the regular double stud wall but I don't like penetrating VB and blown in cellulose is expensive here and I can,t do it my self. Northern canada. Where should I install the water barrier on the exterior over the plywood or over the foam before strapping for siding. Will be using 2 stage variable fan speed furnance as well as erv and ground source heat pump to ensure air quality and energy efficiency. R60 in roof. Anyone see an issue with this assembly?

Asked by Sheldon Lee
Posted Sun, 08/24/2014 - 18:52

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4 Answers

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It appears you have chosen an assembly that has the cost of a double stud wall without the usual benefits ( straight forward, conventional, window & door flashing and installation) and still has the hassles (furring and framing windows and doors) of exterior foam.

If you can move the outer (2x6 stud) wall out 2" you can put the plywood on the inside of the outer wall where it will actually e a bit warmer then the EPS on the INSIDE of the plywood, then poly over the EPS then build the inner wall. This would be a bit unconventional but would eliminate furing (AKA strapping) over the EPS. and have the WRB attached to the outer face of the 2x6 wall. I'd recommend adding an exterior XPS drainage plane (Greenguard DC 14) as it'll add r1 to the assembly for 1/4" thickness and assure drying of the wood parts without a conventional "rain screen". This still puts the polly outside the inner 2x4 wall with all the benefits.

I hope you mean mineral wool bats, they are much easier to fit properly than flimsy fiberglass bats.

Are you sure that a ground source heat pump is an economically astute choice? Geo systems too often under perform air source "minisplit" systems (the ones that keep heating below -25C) at WAY greater cost.

Answered by Jerry Liebler
Posted Sun, 08/24/2014 - 21:24

2.
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Sheldon,
There are several problems with your proposed wall assembly.

1. Your exterior rigid foam is not thick enough to keep your plywood wall sheathing above the dew point in your climate. Your design is likely to result in damp wall sheathing, and the wall sheathing will be unable to dry to the exterior.

2. Furthermore, your plan to install a polyethylene vapor barrier will guarantee that your damp wall sheathing will be unable to dry to the interior.

If you want to install exterior rigid foam, it's important to (a) limit your wall framing thickness to 5.5 inches, and (b) omit the polyethylene vapor barrier, and (c) ensure that your rigid foam is thick enough to keep your wall sheathing above the dew point in January.

For more information on these issues, see :

Calculating the Minimum Thickness of Rigid Foam Sheathing

Are Dew-Point Calculations Really Necessary?

If you want to build a double-stud wall, you really can't install any exterior rigid foam -- because it's essential that the plywood sheathing be able to dry rapidly to the exterior in April and May. This is best done by installing a ventilated rainscreen gap between the plywood sheathing and the siding.

For more information on designing double-stud walls, see How Risky Is Cold OSB Wall Sheathing?

Answered by Martin Holladay, GBA Advisor
Posted Mon, 08/25/2014 - 09:17
Edited Mon, 08/25/2014 - 09:20.

3.
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Martin,

I an designing a residential addition in Toronto.

I am proposing a 3" expanded rigid with ventilated rainscreen gap for siding, 2x4" stud exterior stud walls fill with Roxul no sheathing on the exterior side and OSB sheathing on the interior side, 2x2" chase stud wall for plumbing and electricals and drywall finish with no vapour barrier.

Can I ask if you foresee any problems with the above construction of an exterior wall in Zone 6?

Thank you in advance.

Lowell

Answered by Lowell Lo
Posted Thu, 09/04/2014 - 13:40

4.
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Lowell,
Q. "Do you foresee any problems with the above construction of an exterior wall in Zone 6?"

A. No.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 09/04/2014 - 14:00

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