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Roxul / foam EIFS wall system in zone 6A (Canada) - vapour barrier placement

I am working on a house plan for approx. 1800 s.f. The views appear well suited to mostly passive solar with some windows (dining and 3 bedrooms) facing east.

For the wall assembly I am proposing (outside in)

Siding
1x3 furring strips
4" of foil-faced polyiso foam board (est value R6 per inch or R24, taped and sealed.
OSB sheathing
2x6 wall on 24" OC with Roxul R22 cavity insulation
sheetrock

Total installed R value: R46.

based on average winter temperature of -10C or 14F I calculate and inside temp/humidity of 70F / 35% i calculate:

delta T = 56
insulation distribution inside/outside = R22 / R46 = 0.478
Temp @ sheathing = 70 - deltaT x 0.478 = 43.22
based on psychrometric reading dewpoint is at 40F.

Is this a good enough cushion to warrant vapour barrier @ the polyiso sheeting level, or should I be using:
-Less interior insulation (ie R14)
-more exterior insulation (ie 5-6" foam)
-a different (vapour permeable) foam sheathing and a poly vapour barrier behind the sheetrock?

Any help appreciated !

Asked by Barney Northrup
Posted Tue, 02/04/2014 - 01:49
Edited Tue, 02/04/2014 - 10:05

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2 Answers

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1.
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Barney,
Your proposed wall assembly will work well. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/04/2014 - 08:49

2.
Helpful? 0

FWIW: The 2x6 /R22 Roxul 24" o.c. wall plus sheathing plus sheetrock comes in at about R15 as a whole-assembly average after factoring the thermal bridging of the framing.

The R24 polyiso will under-perform it's rated R by quite a bit in a zone 6A climate due to the severe derating that occurs at low temp. At center-foam-depth temp of +8C/46F or colder polyiso will begin to underperform EPS. Any polyiso in the stackup that's averaging below 0C will be something like R3/inch, or even less.

If you design your stackup with the outer layers of foam begin EPS, such that the mean mid-winter temp of the outermost polyiso is at least above 0C (+5C would be even better) you can count on the EPS being at least R4.5/inch, and the polyiso averaging at least R5.5/inch. Since the EPS will continue to gain performance at lower temperature, it offsets the falling performance of the polyiso, but it's not an entirely linear model. Assuming an outdoor mean temp of +14F, and an interior mean temp of 70F, with R22 cavity fill and R20ish foam that's R42/(70F-14F)= R0.75 per degree. To keep mean temp of the exterior skin of the iso at 0C/32F would take exterior EPS performance of R0.75 x (32F-14F)= R13.5, which is almost 3". If you did a stackup of 1" polyiso and 3" of EPS you'd have a labeled-R rating of R12.6 EPS + R6 iso for R18.6, and your average mid-winter performance would be about R13.8 EPS plus R5.5 iso, which adds up to R19.3., but at the mid winter extreme temps you're still performing in that range, whereas with an all polyiso solution it's slipping a bit.

That said, even an all-polyiso stackup at 4" will provide sufficient performance to meet your design goals- figure the outer 2" to average about R7-R8, and the inner 2" at about R9-R10 in mid-winter call it R17-ish average performance, maybe even R19 for the 3-month average. (Adding that to the R15 framed wall portion you're looking at R32-ish whole-wall performance, which is pretty good.) Since the performance of the polyiso rebounds with higher outdoor temps (= warmer polyiso temps) the drying season begins sooner than you'd get modeling it at constant R17. From an IRC prescriptive value point of view you only need R15 on 2x6 framing even in zones 7 & 8, so you have some margin here.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_p...

Answered by Dana Dorsett
Posted Tue, 02/04/2014 - 17:48

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