Code official wants polyethylene for interior of EPS insulated wall
Hello,
I’m planning a new house construction in New Brunswick, Canada and am looking for some input from the knowledgeable folks here. The house is located in the southern part of New Brunswick. (Climate zone 6A I think)
My planned wall assembly is:
Siding
Strapping
WRB
3″ of EPS foam on the exterior (My builder is recommending 3″ Silverboard XS, which claims to be R-5/inch)
OSB Sheathing
2×6 framing with Cavity Insulation – cellulose or blown fiberglass
Interior Drywall w. Paint (Air barrier)
Based on the information I’ve read on this site, I had planned to omit any interior vapor barrier due to the exterior foam thickness. The silverboard XS claims that it has a permeability of 3.48 perm (https://www.amvicsystem.com/silverboard-rigid-foam-insulation) at 1 inch, so I imagine 3″ of the product would be below 1 perm.
The local code officials want a vapor barrier on the inside against the stud wall, but my concern is that this would create a risky wall assembly that won’t be able to dry.
Do I have any options? Would a “smart” vapor barrier like certainteed’s membrain work if I can’t convince them to omit the interior poly?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Chris,
Canadian building inspectors' understanding of vapor barriers is about 30 years behind the times, and many building inspectors in Canada aren't aware of building science principles. I feel your pain.
Usually, it's easier to throw in the towel than argue. In most cases, Canadian building inspectors will accept the installation of an interior "smart" vapor retarder like MemBrain instead of polyethylene. With your wall assembly, the MemBrain is unnecessary (and a waste of money) but relatively harmless from a moisture-management perspective.
Thanks Martin. If I need to, I'll go with the MemBrain if that's what it takes.
Did I miss anything in my planned wall assembly?
Chris,
You wall plan sounds fine to me.
Southern NB is definitely zone 6. Even Fredricton has averaged fewer than 5000 HDD-C/base 18C (the zone 6/7 boundary) over the past 20 years:
https://fredericton.weatherstats.ca/metrics/hdd.html
https://www.greenbuildingadvisor.com/blogs/dept/building-science/all-about-climate-zones
Even if it were zone 7 the R15 of exterior EPS is sufficient for dew point control at the sheathing with R20 or R22-ish cavity insulation without an interior vapor retarder.
https://up.codes/viewer/utah/irc-2015/chapter/7/wall-covering#R702.7.1
If you can sell them on MemBrain that would take less argument, but an engineering analysis would still be able to prove the (nearly) obvious, if it came to that.
The perforated facer version of SilveRboard® Graphite XS would offer at least some drying capacity toward the interior, but the unperforated version even at 1" thickness has a permeance of 44 ng/Pa.s.m² (0.77 US perms) would qualify as a vapour barrier under Canadian code definition (= anything under 57 ng/Pa·s·m2 is a vapour barrier, per the NBC.)
At 1" the perforated version tests at 157 ng/Pa.s.m², but they don't have separate data for 3". Most Type-II EPS (with or without graphite) would come in slightly less than 57 ng/Pa·s·m2 even without a perforated facer, so you can't count on MUCH drying toward the exterior, but some.
https://www.amvicsystem.com/wp-content/uploads/2017/02/Copy-of-Copy-of-SBGXS-Physical-Properties-metric-imperial.pdf
Amvic's EPS Silverboard is really R-4.9/in, ±2%, according to their tech department. Not that it matters in this case, but their rounding up to 5.0 is a stretch in my opinion. Their graphite version is probably a true R-5.0/in.
At the outdoor temps that matter, with the average temp through the foam less than 40F, and the SB-35XS (2lb foam, no graphite) performance is probably going to exceed around R5/inch in this stackup. They are claiming R5.0 at 75F mean temp through the foam in an ASTM C518 test plate:
https://www.amvicsystem.com/wp-content/uploads/2015/07/Physical-Properties-of-SilveRboard.pdf
Even generic non-graphite loaded 1.5lb and 2.0lb EPS foam runs R4.5/inch or more at a mean temp through the foam of +40F (R0.3 /inch higher than it's +75F performance) and close to R4.7/inch when the mean temp through the foam is 45F.
The mean wintertime outdoor temp in Fredriction is about 25F, so with a warm side of 70F and R20 cavity insulation the warm side of the foam is going to average in the low to mid 40sF, and the mean temp through the foam will average in the low mid-30s
https://weatherspark.com/m/27856/1/Average-Weather-in-January-in-Fredericton-Canada#Sections-Temperature
So, it should beat R15 average by quite a bit, probably even better R16 ( unless the published 75F ASTM C518 number is pure BS.)
Dana, that's the chart that led me to contact them, as I had never seen EPS advertised at R-5.0/in. Their VP of Tech got back to me with the info above, tested by a third party using C518 protocols (including mean 75°F). But I agree, it should perform better as the temperature drops, and with a gap, the silver facer should add a bit more R-value as well.
Curiously, they only claim R4.5/inch for the graphite loaded stuff at +75F mean temp (unlike the R5.0 for SB-35XS) , R5.4/inch at a 0F mean temp:
https://www.amvicsystem.com/wp-content/uploads/2017/02/Copy-of-Copy-of-SBGXS-Physical-Properties-metric-imperial.pdf