External Insulation and Condensation Concerns
Last Post 22 Nov 2014 03:18 PM by kenora. 33 Replies.
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edithUser is Offline
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09 Oct 2014 09:40 PM
Dana,
I am getting from your advice that in a slightly warmer climate, such as Ottawa, there is no need for a vapor barrier if you have sufficient EPS on the exterior to keep the sheathing warm. We are planning on 4" and already have 8" of dense pack cellulose in our stud bays. Simple tyvek though. Sound safe? And painted gyprock should do the trick as a vapor retarder?
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10 Oct 2014 02:38 PM
Posted By edith on 09 Oct 2014 09:40 PM
Dana,
I am getting from your advice that in a slightly warmer climate, such as Ottawa, there is no need for a vapor barrier if you have sufficient EPS on the exterior to keep the sheathing warm. We are planning on 4" and already have 8" of dense pack cellulose in our stud bays. Simple tyvek though. Sound safe? And painted gyprock should do the trick as a vapor retarder?

It's possible to work from the 25 year weather history to come up with an exterior-R that would be protective in all but the high-arctic/antarctic locations.

With 4" of EPS (~R16.8) and 8" of dense packed  cellulose (~R29.5) you're looking at about R46.3 total, with about 36% of the R exterior to the sheathing. The January mean temperature in Ottawa is about -10C, with total heating degree-days and mid-winter temps comparable to the warm edge of US climate zone 7 or the cool edge of climate zone 6.  In a colder than average year you might have about 4900C HDD (8820F HDD), which would make it more comparable the cold edge of US zone 6 (still less than 9000F HDD). The average heating season is more like 4600C HDD/ 8200F HDD, which is still would be the colder half of a zone 6 climate.

If you refer to the IRC prescriptives for going with exterior foam and no interior side vapor barrier those climates you'll see that in US climate zone 6 you need about 33-35% of the R to be on the exterior of the sheathing to work, and in zone 7 it's about 40-42%. For Ottawa you probably don't need 40% of the total on the exterior, but you surely need more than 30%. At 36% you're in the right range, and adding another half-inch to inch of EPS would give you margin.

In Canada the national building code doesn't absolutely require interior vapor retarders (news to some people). This document seems to imply that 20% would be OK for Ottawa, (per Table 9.25.1.2 in the NBC 1995), though I'm skeptical that would be sufficient during a cooler than average season. Some inspectors may not sign off on it unless you put the printed page in front of them. The National Building Code website has been hacked, and the code isn't currently searchable, but Table 9.25.1.2 may have been updated since 1995 with more conservative ratios.
edithUser is Offline
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11 Oct 2014 07:57 PM
Thanks Dana.
I was just told by someone working as an architectural technician that my wall is "risky" because I am not putting in a vapor barrier. In his opinion moisture will condense in the EPS. My understanding is that EPS is not particularly sponge-like...Also, we have detailed our sheathing so that it is an effective air-barrier, and my thinking is that there should not be a significant amount of moisture moving through our walls given that there shouldn't be mass air moving through the wall. An effective "pressure boundary" is our sheathing. We will also have a high efficiency HRV running. All good things in my opinion. I am pretty committed to leaving out the plastic sheet in our wall.
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23 Oct 2014 07:23 AM
you can always use a smart vapor barrier or "retarder" like intelloplus or membrain by certainteed.
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23 Oct 2014 10:58 AM
our walls given that there shouldn't be mass air moving through the wall.


People not using significant external foam (unlike edith) should note that air doesn't need to move though the wall - moving from the interior, to a cold surface and then back into the interior would be enough. So in such cases, the interior side should also be air sealed.

Also note that using significant exterior foam along with a more permeable inside layer is, like most designs, a "works pretty well" vs a "failure proof" design. Ie, condensation is likely to occasionally occur and moisture levels that could support mold growth will be even more common. So other things that lead to a resilient wall (cellulose, low interior humidity, plywood or no sheathing vs OSB, overhangs, stucco wrap, etc) should still be considered.
HP HomeUser is Offline
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23 Oct 2014 07:06 PM
If you are concerned about vapor why not use mineral wool exterior insulation?
SurfsupUser is Offline
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24 Oct 2014 07:56 AM
If you furr out the outside of rigid foam, consider using 1x6 furring around windows/sliders if you will be putting finish trim (3.5" wide) to box out the windows. You'll want the furring strip wider so the siding has something to nail into.
edithUser is Offline
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09 Nov 2014 10:38 PM
We are considering a smart vapour barrier. Our plan was to use Roxul board but it is twice the price of foam unfortunately.
kenoraUser is Offline
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15 Nov 2014 08:34 PM
Same question please. for clarification ..Kenora has about 5800-6000 HDD.. I plan 4" EPS on the exterior with an interior vapour barrier (which can't be removed in the older part of the house (800 sq/ft over conditioned crawlspace) but can be omitted in the new side (800 sq/ft over walkout basement) if appropriate).

2x6 construction with batt FG on existing side, I plan 2x6 construction with either roxul or dense pack cellulose in the addition.

Is 4" EPS -enough-... I know more would be better

(Kenora Canada P9N0E7)
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15 Nov 2014 09:12 PM
Kenora has about 5800-6000 HDD
You might want to check that again. It would be surprising if it was that low.

ETA:  Oh, that's probably Celsius.
Dana1User is Offline
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17 Nov 2014 12:01 PM
Posted By kenora on 15 Nov 2014 08:34 PM
Same question please. for clarification ..Kenora has about 5800-6000 HDD.. I plan 4" EPS on the exterior with an interior vapour barrier (which can't be removed in the older part of the house (800 sq/ft over conditioned crawlspace) but can be omitted in the new side (800 sq/ft over walkout basement) if appropriate).

2x6 construction with batt FG on existing side, I plan 2x6 construction with either roxul or dense pack cellulose in the addition.

Is 4" EPS -enough-... I know more would be better

(Kenora Canada P9N0E7)

With 4" (~ R16) of unfaced Type-II on the exterior of a 2x6 framed studwall w/R20-R23 that has a poly vapor barrier on the interior you will still be OK in Kenora, as long as it is truly air-tight to the interior, and you  have a rainscreen vent space between the foam & siding.  With 4" of foam would leave you with about 0.6-0.75 perms of drying rate into the rainscreen, so bulk-water management is critical- any moisture that makes it into the wall cavity could take months to dry, but that's far better than it taking years.  Check the window & door flashing carefully and correct as-needed, ensuring that is continuous with the housewrap or felt, and lapped correctly at every detail.

For the new section you would be able to skip the interior vapor retarder according to IRC Chapter 7, (R15 would be the minimum) but a smart vapor retarder would only help, not hurt.  By retaining 3-5 perms of drying capacity toward the interior should the stud cavities take on water means what would take a month to dry through 4" of foam only takes a matter of days.
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18 Nov 2014 11:12 PM
Dana - ABC 2006 still uses / up to 4999 - .20 / 5000-5999 - .30 / 6000-6999 - .35
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19 Nov 2014 05:03 PM
Posted By FBBP on 18 Nov 2014 11:12 PM
Dana - ABC 2006 still uses / up to 4999 - .20 / 5000-5999 - .30 / 6000-6999 - .35

Thanks for that reference!

So, in 5800-6000 HDD Kenora, with ~R16 foam on the outside of an R23 filled stud wall you have R16/(R16+R23)= 0.41.

If he only needs 0.30 to meet code (0.35 on the coldest winter of the century)  it's huge margin from a compliance point of view.
kenoraUser is Offline
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22 Nov 2014 03:18 PM
Thanks all; that is EXACTLY the information I needed.

You folks don't know how much I/we appreciate the time and effort to answer these technical questions.

Cheers
Kenora
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