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Exterior insulation in zone 7

Bonai | Posted in General Questions on

Hi there,

Here is a good one.

My issue is that I discovered this website when the house was halfway framed. It is now framed, roof and windows are in.

We live in Winnipeg, Manitoba Canada in what would be a good zone 7 in the US, winters are dry and cold. Code for above ground walls is….R16. The plan was to have R22 batt in the interior walls (2″x6″). Now I realized how inefficient the walls will be. 

Because the windows are already in, my options for exterior insulation are limited. I wanted to use Comfortboard 80 in 1 1/2″ thickness, which gives R6 value and finish with 3/4″ strapping as rain screen and Fiber cement sidings. Going that route makes us already needing custom casings for all windows to receive the siding. I can not really go any thicker than that as we already have wood corbel installed.

I read that too little insulation is not good in cold climates and will lead to walls moisture accumulation. According to the article I should have at least R15 based on my location.

Am I better skipping on exterior insulation or not?

Edit: Code says we need interior vapor barrier polyethene.

Cheers

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    With the high vapor permeance of rigid rock wool it's fine to go thinner, as long as you have the code-mandated vapor barrier on the interior side. From where you currently are, if 1" rock wool isn't a construction problem it would bring the whole-wall R up to R20-R21 (up from R16-R17 "whole wall") while lowering the overall moisture risk.

    Were it not already sheathed with windows installed it would have been good to consider using let-in or strap steel bracing on the walls and do the exterior sheathing with a vapor permeable fiberboard, which adds to the total R. I'm not sure how available it is in your area but MSL's 1.5" thick R4 SONOclimat ECO4 is pretty good stuff, vapor permeable (26 perms- more permeable than some housewraps!), moisture tolerant, and not outrageously expensive compared to similar European imported products.

    https://www.mslfibre.com/Upload/Documentation/T12670-106_SONOclimat_ECO4_En_08-14637348086622519116.pdf

    With 26 perm sheathing and rainscreened siding you could even skip the interior side vapor barrier, since standard interior latex paint on gyprock would be the most vapor-tight layer in the stackup. If it could then tolerate another inch of thickness 1" rigid rock wool would add another meaningful performance boost, a total R-adder of R7.5 (now in the R24 range) over OSB sheathed 2x6/R22 type construction , and only 2.25-2.5" thicker.

  2. Expert Member
    BILL WICHERS | | #2

    I'm really surprised your code for walls is only R16! I would have thought at least maybe R21-23 up there. Wow.

    If your windows are just framed in but not flashed and detailed, it might be worth seeing if you could move them around a bit to be able to accomodate more exterior insulation. Another option (depending on the window) is to try making them into "innies". I would absolutely try to get as much exterior insulation up as you can.

    Bill

  3. Jon_R | | #3

    For more R value, adding foam (continuous or strips) on the interior side is an option.

    Exterior insulation is usually beneficial for moisture, but good walls can be built without it.

  4. AlexPoi | | #4

    What you are doing (1.5 inch exterior continuous insulation + 2x6 walls) is code in Ontario and Quebec so you'll be fine. Make sure to detail a rain screen behind your siding though. And if you plan on running AC in the summer, I would switch to a smart vapor membrane.

  5. SCChris | | #5

    Just want to echo what Jon R said about interior insulation. Google Swedish Platform Framing fo more details.

    From outside in: siding/rain screen/wrb or equivalent/sheathing/studs and batts/(maybe optional if you choose foam or adequately detailed drywall with correct paint) vapor barrier tacked to studs, lapping ceiling plane/run girts horizontally (fastened to studs) to run your smaller services in there, and fill those bays with foam and tape (and that can be your vapor barrier if you need that for code) or you can use comfortboard as you wish/drywall.

    Make sure you get credit on the electricians bid because they won’t need to spend time drilling holes to run their cables.

    Image from:
    http://blog.lamidesign.com/2018/05/usa-new-wall-never-ending-question-of.html?m=1

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