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Exterior rigid foam insulation

I am well into a 2 year build of a cottage near Ottawa Canada which I believe makes it equivalent to zone 6. I say cottage, but I am trying to build to the standard of a house.

The cottage is 2*6 framed 24 inch on centre with OSB sheathing. The exterior of the cottage is wood siding with a 3/4" of air flow behind it (installed on 1*3), then Typar house wrap and then 1.5" of R7.5 rigid XPS. I am still working on the inside of the cottage and thus the stud bays are still open. I planned to insulate them with Roxul batts as I can't find anybody who does dense pack cellulose in the area, and I don't think you can rent equipment needed to try a DIY install. After reading all your posts I am going to try to avoid the poly vapour barrier on the inside altogether and if forced will go Membrain which I think you are OK with if pressed.

I just read your article on calculating required XPS thickness and now I am worried mine isn't thick enough. I see all kinds of R7.5 installed around here but recognize that doesn't prove it is right. Is there anything I can do here (please don't say tear down my 20K siding) ideally from the inside to help with potential dew point issues? I read somewhere R7.5 was good enough for Dew Point in Canada (Jon Eakes I think) but your chart is pretty straightforward.

Asked by Mike 88
Posted Mon, 03/17/2014 - 13:03
Edited Mon, 03/17/2014 - 13:19


3 Answers

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Assuming that you are correct that your climate zone is similar to U.S. climate zone 6, then it would indeed have been better to include exterior rigid foam with a minimum R-value of R-11.25 rather than to have used R-7.5 foam. (To read why, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

At this point, there are two things you can do:

1. You can install MemBrain on the interior to limit the diffusion of moisture from the interior to the cold sheathing during winter; and

2. You can make sure that you keep your indoor relative humidity low during the winter, to lower the risk. (The easiest way to do this is with a mechanical ventilation system.)

A third approach, which I don't really recommend, would be to install one or two inches of closed-cell spray foam on the interior side of the wall sheathing. The reason I don't like this approach is because it creates the dreaded foam sandwich.

Finally, a fourth approach would be to reduce or eliminate the amount of insulation you install between your studs. This would have the effect of keeping your sheathing warmer during the winter, thereby lowering the risk of moisture accumulation. There are two downsides to this approach: (1) it incurs a significant energy penalty, and (2) it may be illegal, because it will reduce the R-value of your walls to below-code levels.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 03/17/2014 - 13:17

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Dew point control is mostly a function the winter season outdoor temperature averages- there is no one fixed exterior R that is "good enough" for all of Canada, given the range of climate. Ottawa's January mean temp is just slightly cooler than Minneapolis, but slightly warmer in December & February, so yes, your climate is very similar to US zone 6 locations, and the IRC prescriptive R11.25 min for dew point control using solely exterior R on 2x6 framing would be about right.!dashboard;a=Canada/ON/Ottawa!dashboard;a=USA/MN/Minneapolis

There are a couple of ways of dealing with it on the interior. An inch of closed cell spray foam on the sheathing would put a ~1 perm vapor retarder (the spray foam) between the sheathing and the interior air (it would air-seal the stud bay too), and would move the condensing surface (now the face of the spray foam, not the OSB) about R16 in from the siding (R0.5 for the OSB, R7.5 for the XPS, and R6 for the spray foam), which is HUGE margin for the remaining 4.5" of fiber depth.

Or you could just install MemBrain, which would run below 1 perm during the mid-winter weeks, then become more vapor open in the spring when the OSB releases it's moisture into the stud bay making the proximate air above 40% RH, and the drying rate would then be limited only by the permeance of your interior finish paint. (Standard latex would run 3-5 perms) The drying rate with MemBrain would be quicker than through 1" of closed cell foam, but the 1" foam would be more tolerant of 40% RH conditioned space air.

The third option would be to get super-obsessive about air-tightness on the gypsum and paint it with "vapor barrier latex primer" which runs about 0.5 perms. That would be the cheapest option and it would be enough, if somewhat less resilient, since both the exterior & interior would be under 1 perm year-round.

My personal preference would be the smart vapor retarder, with the higher resilience of variable permeance as the rationale for the higher cost. It's cheaper and greener than 1" of closed cell foam too due to the high global warming potential of the blowing agents for that foam.

(edited to add: Looks like Martin was posting while I was busy typing...again!? :-) )

Answered by Dana Dorsett
Posted Mon, 03/17/2014 - 13:38
Edited Mon, 03/17/2014 - 13:39.

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Thanks Martin/Dana,

Since you both seemed aligned on MemBrain, I'll definitely go that route and ensure that I monitor RH and use mechanical ventilation.

A couple of follow-ups:
*Would dense pack cellulose materially help here? Again I am not sure I can get it but would try harder for additional piece of mind.
*how much should I worry about this if I do the above (Menbrain, monitor RH, mechanically ventilate)? Should I rip open a wall sometime in the future to examine. This stuff keeps me up at night;-)

Answered by Mike 88
Posted Mon, 03/17/2014 - 14:46

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