Question about flash and “batt” in conjunction with Zip R6 in Zone 6.
Question about flash and “batt” in conjunction with Zip R6 in Zone 6. I am referring back to a previous post. (https://www.greenbuildingadvisor.com/article/insulated-sheathing-cold-climate#0).
I am building in zone 6 (eastern Idaho). 6300 elevation, cold, but fairly dry.
Continuous insulation is not yet the norm here for new builds; it is undoubtedly coming, but not here yet. Moreover, finding a builder is hard (rural area) and I am finding I have to meet my builder in the middle with some of my ‘wants’, such as continuous insulation for a thermal break. (You might say that I am in this situation: https://www.greenbuildingadvisor.com/article/cant-find-contractor).
My middle-ground so far is getting him to use Zip R6 for a CI. This is in contrast to an open layer like Roxul which would require strapping/furring with screws and additional blocking or bucking on the window openings. I would prefer the open exterior insulation in this cold climate, but I cannot get my builder to go with the additional strapping and window work.
We have 2×6 framed walls, so to achieve the 35% mark on the exterior foam, we will apply 2 inches of closed-cell foam (and the remainder will be blown-in cellulose); this follows the blog from North Dakota. I plan on a class-3 vapor barrier (undecided on whether we will go solely with an appropriate primer+paint, or whether the building dept will require some sort of membrane, in which case we will use smart membrane). Combined with some humidity monitoring, I’m hopeful this is a reasonable course of action I(as opposed to just filling the wall cavity with blown-in).
I’ll take comments on this general plan. However, my main question relates to the areas of the exterior walls where the majority (or entirely) of the 5.5” depth of the framed wall is e.g. headers (r multi-ganged studs). I have a lot of windows on some walls, and hence the amount of area where there is very little to no cavity for spray foam is not trivial. In these areas of the wall, there may only be that R6 Zip (no other insulation). Therefore, in principle, the % of exterior insulation is 100% (above that 35% threshold).
Can I assume that, while undesirable from an R-value, these areas are not an issue from a moisture control perspective (i.e. condensation on the interior face of the Zip foam).
Thanks
m
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
If I was stuck on Zip R-6 as being the most complicated exterior layer my builder could wrangle, and I needed more overall wall performance, I would abandon the Zip R and look to move to a 2x8 wall with R-30 conventional insulation, combined with an interior vapor retarder such as MemBrain.
I say this as someone who has Zip R-6 on his entire house. But, if I had needed closed cell spray foam inboard of it to make it work, I would have done something different.
The 35% ratio in zone 6 is with a class III vapor retarder (ie painted drywall). If you go with a class II (faced batts or smart VB), the R6 is fine as is and you can skip the spray foam. I'm in the north end of zone 5 and this is a pretty common assembly here built with interior poly. It has been built for a long time and holds up just fine.
If your builders are not familiar with exterior insulation, the 2x8 with R30 batts suggested above is definitely the simpler option.
Adding rain screen under the siding is a good idea with either assembly. Besides providing extra drying capacity for the sheathing, a rain screen also increases the life of any painted siding.
Thanks Akos (and PO'S above).
My thinking (only 'thinking' since I have zero personal experience with these techniques) was that I was trying to address *to some degree possible* (not in this specific order, but perhaps prioritization is a good idea):
> R22 (or 13+5) code - easy enough (note: this county still has older code in place; which seemingly leads to the builders being less familiar with many topics found on this site)
> pay attention to vapor with whatever assembly is chosen
> thermal bridging - hence the CI
> air-sealing - since this is not a high-priority for builder in my local, I am not sure I can depend on super detailed (e.g. taping) attention by my builder, but I can hope
Since the builder has no experience with exterior insulation, the insulated Zip panel was as close as I figured I could get to traditional sheathing, and R6 (as opposed to R12) would still enable nail guns for the Zip and the cladding (horizontal cedar and vertical metal).
> This checks the thermal bridging box (I hope to a good degree)
While I can 'hope' for attention to air sealing on the exterior (e.g. Zip tape), the use of closed-cell foam provides me with additional sealing, and gets me the 35% (and foam is readily available in my locale)
> checks the air sealing box (to some degree)
> based on that 2018 post inre North Dakota, it would seemingly manage the vapor.
-----------------------
I am willing to pay the extra$ for the Zip and the foam. Materials-wise, it is not too much of a bump compared the base of >$400/sq-ft for traditional framing here.
But if your suggestion is that the inclusion of foam is not much better at achieving those goals compared to R6Zip+facedbatts, or 2x8 R30 and no CI, (and I can avoid the ancillary negatives of spray foam), then that is food for thought.
Thanks
> thermal bridging - hence the CI
Overall R value is the better metric to consider since its assembly independent, and a 2x6 wall + 1" foam is very similar to a 2x8 wall with no foam as far as overall R value.
> air-sealing - since this is not a high-priority for builder in my local, I am not sure I can depend on super detailed (e.g. taping) attention by my builder, but I can hope
If you have concerns about your builder not being diligent about taping Zip, you should be concerned about more than just air sealing. The bigger risk to your building will be water that could enter improperly taped seams and cause far greater damage than the energy penalty from air leakage through the same seams.