So, if this looks anything like what it looks like on my computer when I post it, it will be a miracle...

Anyhow, average temperature in Calgary is 51 degrees fahrenheit, but we are subject to radical weather fluctuations... (wikipedia has a great writeup on Calgary actually, if I didn't live here, I'd want to after reading it... my reference for the data below...) I agree, I'm sure the inside is hotter than the outside probably 75 percent of the time...
I agree on the rainscreen idea just as good practice. Typically we build with ICF and then acrylic stucco right onto the foam after the fibreglass mesh and cementatious scratch coat, but this funny little flat roof addition will be stick built with 2x6 exterior walls over a heated ICF crawlspace. I don't want to follow the city blindly, and end up with more problems down the road!!

Climate data for Calgary International Airport [hide]Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

Average high °C (°F) −2.8 (27) −0.1(32) 4(39) 11.3(52) 16.4(62) 20.2(68) 22.9(73) 22.5(73) 17.6(64) 12.1(54) 2.8(37) −1.3(30) 10.5(51)

Rainfall mm (inches) 0.2(0.01) 0.1(0) 1.7(0.07) 11.5(0.45) 51.4(2.02) 79.8(3.14) 67.9(2.67) 58.7(2.31) 41.7(1.64) 6.2(0.24) 1.2(0.05) 0.3(0.01) 320.6(12.62)

Snowfall cm (inches) 17.7(7.0) 13.4(5.3) 21.9(8.6) 15.4(6.1) 9.7(3.8) 0(0) 0(0) 0(0) 4.8(1.9) 9.9(3.9) 16.4(6.5) 17.6(6.9) 126.7(49.9)

Sunshine hours 117.4 141.4 177.6 218.8 253.7 280.3 314.9 281.9 207.7 180.5 123.9 107.4 2,405.3

WUFI (Fraunhofer Institute for Building Physics) consistently shows water content and humidity issues without a vapor barrier behind the wall board in cold climates. The software is quite complete in that it allows one to build a wall and then shows RH, water content and temperature at every point in the wall for every day of a year of loaded weather data.

http://www.ornl.gov/sci/btc/apps/moisture/

As it says, the dew point method of analysis is outdated and one needs to account for hygrothermics.

DOE  (Office of Building Technology) agrees - only a few places in the SE US should omit an inside vapor barrier.  Building Sciences Corp - a couple of guys selling consulting services from what I can tell - probably in their best interests to say that the experts have it wrong.

Man this whole thing is making my head hurt... I'm afraid, that as a Mac user, I don't seem to be able to use the WUFI Simulation software, so I am downloading it to my old PC to give it a spin.

Read the DOE brochure and looked at Building Sciences Corp. I am not sure what to believe at this point. The DOE Brochure seems to essentially treat the vapour barrier question the same for all insulation options. Not sure I can buy that. The permeability of the different materials is so radically different. I am having a really hard time because even the envelope engineers I have spoken to here, refuse to sign off on a wall detail that includes 2lb foam and poly vapour barrier, but candidly, I have learned more from you guys than I have from them!!! (Frustrating since the engineers here, think the city is crazy...)

With the WUFI software, are you able to use data for Calgary specifically?

Posted By jonr on 27 Apr 2010 09:49 PM
WUFI (Fraunhofer Institute for Building Physics) consistently shows water content and humidity issues without a vapor barrier behind the wall board in cold climates. The software is quite complete in that it allows one to build a wall and then shows RH, water content and temperature at every point in the wall for every day of a year of loaded weather data.

http://www.ornl.gov/sci/btc/apps/moisture/

As it says, the dew point method of analysis is outdated and one needs to account for hygrothermics.

DOE  (Office of Building Technology) agrees - only a few places in the US should omit an inside vapor barrier.  Building Sciences Corp - a couple of guys selling consulting services from what I can tell - probably in their best interests to say that the experts have it wrong.

But jon, the definition of "vapor barrier" in that DOE lOffice of Building Technology link is "less than 1.0 perms" (see the bottom of page 3, and elsewhere in the document).  Closed cell foam at 2lb density meets this definition at a thickness of 2" or greater.

The vapor retardency of kraft-facers used on fiber insulation batts is ~0.4-0.5 perms but so is 4-4.5" of 2lb foam.  But the AIR SEALING of the foam is far more assured than kraft facers, which leak at the edges, as a rule, and the amount of moisture moved by air leaks is orders of magnitude higher than via vapor diffusion through 1.0 perm material.

The WUFI hygrothermic enhancement of going beyond dew-point-only models is primarily accounting for differences in capillary draw between materials.  High capillary draw insulation protects the wood materials during wetting or condensing conditions since it redistributes the liquid moisture away from the wood.  While there are huge differences in capillary draw between different fiber insulation type (cellulose being the clear winner of those most commonly used), foam has very low capillary draw- it's not at all hydrophillic.  Wood studs, sheathing, and gypsum board will draw moisture from foam insulation, and not the other way around, so closed cell foam won't be your friend if used improperly.  But this property of foam also makes it an excellent capillary break- it won't wick liquid moisture into the structure either.  But the same is true in SPADES with poly vapor retarders.

Capillary breaks are very important between masonry or concrete foundations (with moderate to high capillary draw) and wood foundation sills (with high capillary draw.)   A foam-filled stud cavity on a concrete foundation without a capillary break between foundation & sill is at higher risk of sill rot than a fiberglass-filled cavity that can vapor-release the moisture from the sill wood, or a cellulose fillled cavity that can both wick and vapor-release moisture from the sill, etc.  But an interior side vapor retarder would only make that worse, denying any possibility of vapor or capillary drying toward the interior, since poly sheeting has ZERO capillary draw, and extremely high vapor retardency (permeance on the order of ~0.05 perms.)  Proper attention to capillary breaks on foundations is important, independent of insulation type and climate zone.

But I'm dubious that a proper model of 2lb foam cavity fills shows saturating levels of humidity in the studs with or without an interior vapor retarder in Calgary's climate. With a rainscreen the outward drying at the exterior stud edge & sheathing will be significant, even in winter, and in summer & shoulder seasons both interior & exterior stud edges have far more drying time than absorption time.  It may make sense to paint the interior stud edges with vapor retardent latex paint (~0.5-1.0 perms) on cc foam-filled cavities in very cold climates, to allow at least some inward drying, but poly over the whole thing would be overkill at best, or even a step backward, lowering the overall drying capacity of the structure by a large measure.

But jon, the definition of "vapor barrier" in that DOE lOffice of Building Technology link is "less than 1.0 perms" (see the bottom of page 3, and elsewhere in the document). Closed cell foam at 2lb density meets this definition at a thickness of 2" or greater.

Note that I wrote "inside vapor barrier".
Yes, WUFI does allow local climate data to be loaded.

Posted By jonr on 28 Apr 2010 03:35 PM

But jon, the definition of "vapor barrier" in that DOE lOffice of Building Technology link is "less than 1.0 perms" (see the bottom of page 3, and elsewhere in the document). Closed cell foam at 2lb density meets this definition at a thickness of 2" or greater.

Note that I wrote "inside vapor barrier".
Yes, WUFI does allow local climate data to be loaded.

Duly noted!

When closed cell foam is the only insulation, the insulation IS the inside vapor barrier (and the outside & middle...), the question in this thread being:

"Does 2lb foam need a 6mil poly vapor retarder on the interior."

Everything further than 2" into the foam could be defined as outside the interior vapor retarder, if 1.0 perms is being treated as some sort of magic number. In a 4" or greater 2lb fill,  half or more of the R is outside the "interior vapor barrier", and the average winter temp at that 1 perm transition point is above the dew point of the interior air- it isn't loading up with humidity all winter unless the exterior side has a strong vapor retarder on it to prevent all outward drying.  A strong interior side vapor retarder will surely prevent summer & shoulder season drying toward the interior, and the overall drying capacity of the assembly.  All drying & moisture accumulation in cc-foam insulated assemblies are months-long (years long?) processes, and interior poly will slow it down both directions.  Since the annual hours in Calgary for inward drying conditions are  more than the hours of winter vapor uptake from interior air, and outward drying will be happening much of the winter as well,  methinks the foam & wood will stay drier without the poly than with it.

I am having ~1 inch of closed cell foam followed by R-19 batts placed within the 2 x 6 walls.

There are several proposals in this thread, but I don't think only foam + PE (no other insulation) is the concern.

The whole "moisture will get in but not continually enough to cause problems and will then be trapped by multiple retarders" is flawed, but WUFI will simulate a wet wall drying out too.

Dana, I recommend that you just run WUFI - or find some references that indicate it isn't accurate.

Posted By jonr on 28 Apr 2010 08:51 PM

I am having ~1 inch of closed cell foam followed by R-19 batts placed within the 2 x 6 walls.

There are several proposals in this thread, but I don't think only foam + PE (no other insulation) is the concern.

The whole "moisture will get in but not continually enough to cause problems and will then be trapped by multiple retarders" is flawed, but WUFI will simulate a wet wall drying out too.

Dana, I recommend that you just run WUFI - or find some references that indicate it isn't accurate.

I apologize- my bad! I was referring only to the question as to whether a class-I interior side vapor retarder was necessary in Calgary Alberta,  not flash & batt in Bettendorf IA.  (Flash & batt in cold climates clearly needs some level of interior vapor retarder, using anybody's model.  But 2" of foam + 3.5" of batt might work in Bettendorf, if not Whitehorse Yukon.) 

The WUFI freebie doesn't run on any operating system that I'm currently using, but I'd be curious to how it models 4" of 2lb foam under the roof deck of the membrane roof with & without poly on the interior, as well as 4-5" of 2lb foam in a 2x6 stud cavity with/without poly on the interior in Calgary's climate, which is what I THOUGHT you were referring to.  Under the membrane roof I'd expect it could be problematic either way, but worse with the interior poly.  In a stud cavity I'd expect it to be fine without the poly, but possibly problematic with.  2lb foam is a powerful impediment to drying at 2"+, but not nearly as powerful as 6 mil poly.  The rate at which 2lb foam lets moisture is very low indeed. 

Calgary's climate is fairly dry, and the winters not super-long or deep, and since the convection & capillary capacity of 2lb foam is near-zero, a WUFI model isn't likely to be very different from an ASHRAE model.  The vapor drives of 30-50% RH 70F air just isn't very high, the summertime drying toward the exterior is quite high in Calgary.  If built to code with 10mm rainscreen, vapor drives from the exterior are never very high.

The folks at BSC make extensive use of WUFI and other models in their design process, and have trended away from use of class-I vapor retarders like poly in their design recommendations. Any recent stackup of theirs for (wetter colder) Minneapolis should be adequate for Calgary.  I found their recent research writeup simulating various un-vented roof stackups a decent read. The roof deck rot risk was non-zero with deep closed-cell foam under a less-permeable metal roof,  but was zero under a shingle roof, which may be relevant to to the 4" of closed cell under membrane roof situation.

Jon and Dana -

That was amazing.  Sorry for not checking in again sooner, I am muddling my way through the whole thing with the city.  Dana, your analysis is spot on with what my impression of the whole situation was.  It is great to have some of the science behind it to digest and further explore.  Jon, I truly appreciate you raising the WUFI example, because it is at least something tangible that I can push back to the city and ask them whose model they are using and why... At this point, my impression is "Bruce, the building inspector, says so..." is the answer I will get, but given Calgary has crested well over a million people, and we are building 20k homes a year, we need to do better than that!  If you guys have any questions that relate to sourcing green materials, or that relate to ICF construction, I'd be happy to return the favour!

Sincerely,
David.