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1. buildABILITY Corporation

1.
Aug 11, 2015 7:26 AM ET

Wall R value
by Reid Baldwin

"Exterior walls with R-values between 38 and 44. Wood-framed walls have cavity insulation of fiberglass batts and an exterior wrap of extruded polystyrene rigid insulation between 2 and 3 inches thick."

How does a fiberglass batt plus 2-3 inches of XPS yield between 38 and 44?

2.
Aug 11, 2015 7:40 AM ET

Edited Aug 11, 2015 7:44 AM ET.

Response to Reid Baldwin

Reid,

Let's do the math: An R-38 wall with R-15 of exterior foam has R-23 of insulation that is not foam. If the wall is framed with 2x6s, then you can put R-20 of fluffy insulation between the studs. The rest of the insulation (R-3) would need to come from insulated siding (insulated vinyl, for example) or some combination of interior finish materials and air films. Otherwise, the walls would need to be framed with 2x8s.

An R-44 wall with R-15 exterior foam has R-29 of insulation that is not foam. If the wall is framed with 2x8s, there is enough room for R-26 or R-27 of fluffy insulation between the studs. The rest of the insulation (R-2 or R-3) would need to come from insulated siding or some combination of interior finish materials and air films.

In Climate Zone 6, where most of the projects are located, at least 36% of a wall's R-value needs to come from exterior rigid foam (if exterior rigid foam is being used). So R-15 exterior foam will work for a wall with a total R-value of R-41.6. If you are aiming for R-44, the R-15 foam becomes marginal.

I can't think of any easy way to make these walls work with R-10 exterior rigid foam unless spray foam insulation is installed between the studs.

There is one other possibility: these Canadian builders are using polyiso, and are assuming that 2 inches of polyiso has an R-value of R-12, and that 3 inches of polyiso has an R-value of R-18. That's what the labels say, so I could understand the assumption -- but polyiso doesn't perform well at cold temperatures, so it's not a good choice for exterior rigid foam in Canada.

3.
Aug 11, 2015 8:17 AM ET

Are the costs referenced in \$US or \$CAN?
by John Semmelhack

Are the costs referenced in \$US or \$CAN?

4.
Aug 11, 2015 8:44 AM ET

Wall R value
by Reid Baldwin

Thanks for the response Martin.

I was thinking in terms of whole wall R values, but they are probably reporting nominal R values. Your analysis shows that the nominal R values could at least approach what is stated.

Since they are using interior polyethylene vapor barriers in combination with exterior XPS foam, I suspect they are not reading GBA.

5.
Aug 11, 2015 9:06 AM ET

Edited Aug 11, 2015 12:35 PM ET.

Response to Reid Baldwin

Reid,
I agree with you completely on the issue of exterior rigid foam plus interior polyethylene. Even if those Canadian designers don't want to subscribe to GBA because of the high cost (in US dollars) of subscribing, they could learn a lot from our free articles.

Et si quelques-un de nos amis canadiens veulent poser des questions en français -- c'est possible aussi. Nous répondrons.

6.
Aug 11, 2015 9:44 AM ET

Costs
by Scott Gibson

John,

7.
Aug 11, 2015 11:23 AM ET

Did someone say "Free Cotton Candy"?
by Greg Labbe

Years of consistent funding for building energy conservation since the 1970's by both GOP and Dems has fostered an incredibly rich discussion south of the 49th. I'm jealous and the Yanks should be proud of this achievement; pragmatism over partisan dogma - something the Cons here can't get past.

The GBA is the result of vision (FHB) and meticulously curated content that people might love to hate, but in the end usually presents both sides of technical divides with articles on building science and pragmatic construction techniques.

Many Canadian contractors still don't know how to dense pack cellulose properly and you can see from the article above that we eat free cotton candy without question like a polite Canadian! It's quite possible that some of the builders and some of the designers involved in the NetZero projects above have never heard of the Pretty Good House or GBA for that matter. In the last decade, Canadians have slipped back to the 1990s and we're a fragmented group spread over a vast area of land.

It is for this reason, I'd like to call upon GBA to help Canadians see the light and let's start by designating Scott Gibson as an Honourary Canadian.

8.
Aug 11, 2015 1:06 PM ET

poly and exterior insulating sheathing
by Michael Lio

The question of whether low perm insulating sheathings cause moisture problems has been recently addressed in a comprehensive study by the National Research Council of Canada's Code Centre. Dr. Hamid Saber has shown that the sheathing's R-value is the key factor that influences interstitial moisture and not its permeability. (Look for a possible code change that relaxes the current rules). The wall assemblies for these net zero homes meet the outboard/inboard ratio requirements of the Code's in each of their four provinces. Tens of thousands of houses have been built across Canada with exterior insulating sheathings (XPS) and poly without consequence.

9.
Aug 11, 2015 2:08 PM ET

Response to Michael Lio

Michael,
Thanks for the information; I appreciate it. Searching for more info, I figured out that Dr. Saber's first name is Hamed, not Hamid. I found one relevant web page:
http://www.nrc-cnrc.gc.ca/ci-ic/article/v18n2-10

If anyone has a link to a report on Dr. Hamed Saber's study, please share it.

10.
Aug 11, 2015 2:32 PM ET

Found a paper by Hamed Saber

Here is a link to one of Hamed Saber's papers -- but it's one that makes no mention of polyethylene:
"Risk of Condensation and Mould Growth in Wood-Frame Wall Systems with Different Exterior Insulations."

Dr. Saber wrote, "The results showed that decreasing the air leakage rate resulted in lower risk of condensation and mould growth. The values for the overall average mould index of walls configured with structural sheathing are lower than that of walls configured without structural sheathing. St John’s appears to have the most severe climate in comparison to the other three locations investigated (Ottawa, Edmonton, and Vancouver); the greatest values of the overall average mould index of the wall configurations amongst the four locations occurred in this location."

11.
Aug 11, 2015 4:23 PM ET

It wasn't about polyethylene (response to Martin, #10)
by Dana Dorsett

The point of that paper was to study what happens if they adjust the required maximum vapor permeance was relax from about 1 US perm to about 5 US perms. Ergo, no mention of polyethylene, which is two orders of magnitude more vapor tight than 5 perms, which is basically the difference between a minimalist Class-II vapor retarder and standard latex paint. It's spelled out in the first paragraph of the Abstract:

"The work originated from a Code Change Request “CCR-802” in which it was suggested that the Water Vapour Permeance (WVP) limit be raised from 60 to 300 ng/(Pa•s•m²) while leaving the limit for air leakage of building envelope materials unchanged at 0.1 L/(s•m²) at a pressure difference of 75 Pa. "

One US perm= 57.2 ng/(Pa•s•m²), so 300 ng/(Pa•s•m²) would be 5.24 US perms- roughly the vapor permeance of a single layer of interior latex primer.

What the paper reports is that with only 5 perms on the interior, a 2x6 R19 wall with no exterior foam has significantly higher mold risk than the same wall with R4 foam on the exterior in a number of climate zones. (We kinda knew this, but they quantified it.)

Even though Canadian code only requires a minimal class-II vapor retarder as the interior side vapor barrier, 4 or 6 mil polyethylene is cheap & ubiquitous, and has become the industry standard despite being an order of magnitude tighter (for both good and ill.) 2-mil nylon meets code as long as the air next to the sheet is reasonably dry (and becomes more vapor open at high humidity, a desirable feature) but it costs twice as much as 6 mil poly.

12.
Aug 12, 2015 8:57 AM ET

Response to Dana Dorsett

Dana,
My reference to polyethylene was in response to Michael Lio's comment that "Tens of thousands of houses have been built across Canada with exterior insulating sheathings (XPS) and poly without consequence."

I agree with Lio's statement, but I wonder whether Dr. Hamed Saber has looked into the question of the riskiness of walls with exterior rigid foam and interior polyethylene. Michael Lio implied that Saber had conducted research on that issue, but I haven't yet found a link to a paper that directly addresses the issue.

13.
Aug 12, 2015 7:12 PM ET

Edited Aug 12, 2015 7:13 PM ET.

Interior polyethylene
by Kye Ford

I have yet to ever see a problem here in Climate zone 6 with interior poly and exterior rigid insulation. Never not once! I have taken apart lots of these wall assemblies years later and have never seen any issue of mold or significant condensation.

In theory the double vapor barrier sounds scary enough but in real life in cold climates I am not so convinced!

14.
Aug 12, 2015 9:22 PM ET

flashing
by Charlie Sullivan

Kye,

The usual argument about double vapor barriers is that if water leaks in somehow (e.g. a bad flashing job), it will accumulate without any way to dry. Perhaps Canadians do a better job of flashing things well than Americans do.