Wall Assembly Design and Extra Moisture Protection
Martin,
As a home-owner about to build, we need a wall assembly design which will be 100% capable of managing moisture effectively year after year, without resulting in mold. We’re in Zone 5. Here’s what we’re thinking:
*Ox Engineered SIS (essentially the same as the Dow product GBA reviewed a few years ago). It’s a 4 in 1 solution for sheathing, exterior insulation (polyiso), WRB, and air barrier. It’s like Thermo-ply with an external layer of foam.
*Rock wool insulation for 2″ x 4″ wall cavity.
*Drywall with latex paint and no polyethylene barrier, so the wall assembly can dry to the interior.
Here are my questions:
1) What do you think of covering the polyiso with housewrap for extra bulk water “insurance?” Any compatibility issues? Since the SIS product is relatively new, I have some discomfort in relying on the chemical adhesion of taped joints alone over time to maintain the WRB above grade. Self-healing nail holes also seem to be a potential point of long-term moisture entry. Its perm rating is 0.3.
2) We’ll have 20% brick for the front (under porch) and the rest of the house is vinyl siding and vinyl shake. Is it advisable to build a rainscreening gap between the vinyl siding and the exterior foam, again for additional bulk water insurance?
3) If yes to the previous question, how do you recommend installing the siding in such a way as to not create waviness over the vertical furring strips? Is 1/4″ the ideal thickness for this application?
4) If we did either house wrap or rainscreening as a second line of bulk water protection but not both, which of the two would you recommend as most cost effective for a new, 2 story?
5) There is a substantial price difference between 1/2″ polyiso (SIS) which is R-3, and the 1″, which is R-5.5. We’re thinking of a 2″ x 4″ cavity. Is it too risky with the 1/2″ given our climate and our critical need to avoid condensation (sorption) on the inside surface of the SIS?
Thanks in advance for your help in thinking this through. I’ve learned a lot from your blog and from community discussion. Keep up the great work!
-David James
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Replies
The rock wool in the cavity will be r15. Indoor wintertime dew point will be near 40 f . With r6.5 on the outside you'll be seeing moisture issues whenever outside temperatures drop below 27 degrees f. With r3 outside it starts at 34 f. I think neither meets your needs! With r 15 in the cavities, to avoid moisture issues with 10 degree outdoor conditions you need at least r 15 of foam!
David,
In your climate zone (climate zone 5), the minimum R-value for exterior rigid foam over a 2x4 wall is R-5, so your foam will be thick enough, as long as you choose the thicker version of the Ox SIS product. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Yes, it's a good idea to install housewrap on the exterior side of the Ox SIS sheathing.
Vinyl siding is inherently well drained, because there is always air behind vinyl siding. When vinyl siding is installed over rigid foam, the usual reason for using furring strips is to have something firm to nail the siding to -- not to provide drainage. For more information, see Can Vinyl Siding be Applied Over Furring Strips?
Will all house wrap materials be equally compatible with the surface of the SIS polyiso? Any different installation technique from wrapping OSB?
The SIS is rated to bear 1,180 lbs per stud (16" centers) for a 9' wall, which is what we're building. Would it be advisable to use plywood in the corner cavities or periodically to beef up sheer and compression strength?
Thanks much!
David,
Q. "Will all housewrap materials be equally compatible with the surface of the SIS polyiso?"
A. Yes, as far as I know.
Q. "Any different installation technique from wrapping OSB?"
A. No.
Q. "Would it be advisable to use plywood in the corner cavities or periodically to beef up sheer and compression strength?"
A. You'll need to consult an engineer to answer that question. The answer depends on the height of your building and the wind loads that it needs to be designed to resist.
As Martin notes, in US zone 5 the R3 version wouldn't meet code for 2x4 construction without interior vapor retarders if you were using wooden sheathing. But SIS isn't wood, it's a fiberboard and but would be fine from a dew-point aspect since it's integrity & performance isn't degraded by moisture during condensation events, and would meet code as long as all siding is back-ventilated.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
Mind you, it has fairly low permeance at any thickness, (under 0.3 perms), reducing but not eliminating drying toward the exterior. (It's about the same as the dry conditions permeance of asphalted kraft facers on batts) and the assembly will be more resilient if you can keep the winter season condensing hours lower, and the interior side more vapor-open.
However, the R3 version and R15 rock wool in the cavities would have slightly lower thermal performance than the IRC prescriptive R13 + 5 c.i. for zone 5, due to the higher thermal bridging (R7 rather than R10.5 over the framing ) too. Even though R15 + R3 is the same center cavity value as R13 + R5, the thermal bridging difference undercuts performance significantly. With the R6.5 version it's thermal performance would be about ~R1.5 better whole-wall performance than a code-min OSB/ply sheathing with the prescriptive R5 on the exterior and R13 in the cavities.
A ~25% framing fraction is typical for 16" o.c. standard framing construction, and at that you'd be looking at:
R12.5 whole-wall with the R3 SIS + R15 cavity fill
R16 w-w with the R6.5 SIS + R15 cavity fill
R14.5 w-w for a wood sheathed R13+5.
Bottom line, spring for the R6.5 version for both the dew point margin & moisture tolerance as well as significantly better thermal performance.
Dana, thanks for the additional insights. I just realized I originally stated the 1" SIS was R6.5 and it is actually R5.5. The original entry above has been edited to show the correct value.
R5.5 would still be fine with R15s in the cavities, and you'd be at about the same whole-wall R of a code-min standard-sheathing R13+5, with wood ship-lap siding, not more.
Adding an inch or two of polyiso to the exterior of the SIS would buy you a lot of dew point margin, and put you in the ~R20-R25 whole-wall range. It's a far easier to do now than after the brick goes up. Not that a tight IRC 2012 code-min wall is an energy-pig, (it's clearly not), but in zone 5 there's usually a longer-term economic rationale for going as high as R30 whole-wall. See Table 2, p10 of this document (and read the whole first chapter):
http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones
Thanks for the link, Dana. Very helpful information.