Wall assembly for Climate Zone 6
I’m located in NY State, Climate zone 6A. I’m going to be building our home this upcoming spring. I would really like to use Huber’s zip r-6 sheathing for my 2×6 wall assembly but understand that I will not have the correct ratio of outsilation to insulation. I read another question posted on this site, and there were recommendations to use a flash and batt to remedy this. I understand the zip r-12 could solve my issues here, however, it is not readily available in the area. If I were to get it here, I would probably be paying a hefty price to get it here.
Can anyone critique or give me any other ideas or advice.
Here is what I am thinking for a wall assembly, outside to in. LP smarsiding, hubers zip r6, 2” of closed cell spray foam, unfaced r-15 fiberglass insulation.
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Replies
Dennis,
Your proposed flash and batt with Zip R6 wall would work great. You're going to want a rainscreen gap for the LP smartside and paint to hold up though. It makes a big difference in durability by drying things out faster. Specify water-blown or HFO closed cell foam for lower global warming potential.
Alternatively, you could use Zip R6 with full cavity fiberglass or whatever you like, paired with a class 2 vapor retarder on the interior of the framing. This could be Membrain, Intello, or even asphalt-saturated kraft paper/facing. If you go with the latter, I'd recommend airsealing the drywall ("airtight drywall approach"). The other membranes are less messy to seal but more costly.
Best,
j
Thanks for the suggestion of the rain screen Jason. As I replied to Akos. I didn’t realize that I could use other cavity insulations in conjunction with a class 2 vapor retarder.
Hi Dennis.
I like Jason's idea to use the ZIP R-6, I'd suggest cellulose in the cavities, and an appropriate vapor control layer on the inside of the wall. Spray foam is not a climate-friendly material. Cellulose is. If you do use spray foam, Huber recommends a rainscreen siding detail. I'd recommend that no mater what is in your walls.
If you haven't seen these articles, check out Walls that Work and Working with ZIP R-sheathing.
Thanks Brian, I actually haven’t seen either article. I just read both. I’ll have to see the availability of using cellulose with the insulation contractors in our area. For some reason I haven’t heard of it being used much here. I’m going to be framing the house, and ultimately the budget isn’t there for a high performance house. But I’d like to do the best I can. I really do not want to fiddle with outside insulation, especially at windows and doors. Since zip r6 is the readily available I am trying to build a wall that will not rot and have good performance. I will be looking at class II vapor reatrders more to see if that makes more sense financially, which I think it will.
With the spray foam and a Class II (preferably smart), you meet the guidelines here*. Without the spray foam, you (in Z6) can't - because Zip's R6 isn't "exterior" and isn't > the 1 perms (according to some tests) or 5 perms needed for Table 2A. When it comes to wall moisture, be conservative - don't stretch the wording of experts or code.
Also consider a smart Class II, cellulose, plywood, 1.5" of exterior unfaced EPS. Exceeds the guidelines nicely, dries in both directions, environmentally friendly and should be less expensive.
+1 on a rain-screen gap.
* - IMO, they are very well thought out, accounting for the trade-offs between interior and exterior perms and the warmth of sheathing. On the other hand, there could be some bias.
Thanks for the reply and the link Jon.
This is a very standard assembly in my area (zone 5) without any spray foam but with an interior vapor retarder.
There absolutely nothing wrong with going with the VB+2x6+R6 ZipR. The spray foam in the cavity would barely budge the R value of the assembly but would add a lot of cost. The ROI on it is probably never.
Thanks for the reply Akos. I was under the impression that I will have condensation problems if I don’t utilize closed cell spray foam in conjunction with the zip r6. If I understand correctly, I could fill the cavity with fiberglass/mineral wool/cellulose and would not have any issues as long as I have a vapor barrier between the drywall and studs?
Yup, that is all you need. A reasonable air tight drywall or vapor barrier install is also needed.
You can read more about it bellow:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-092-doubling-down%E2%80%94how-come-double-vapor-barriers
Note that BSI-092 includes "If the outside is “leakier” than the inside we reduce the risk". I suggest that taped Zip and any kind of interior side sealing is likely to result in the opposite.
Elsewhere Joe L talks about interior Class I vapor barriers being a bad idea if AC is used.
Jon,
We have to be careful with recommendations like that. The brief refers to keeping the outside foam layer "leaky" by not taping the seams of it. The sheathing still needs to be air tight underneath.
The biggest risk for condensation on the cold side of walls comes from air leaks not from foam thickness. Dealing with that is the most import part.
Thanks for your input. So does the kraft face on the fiberglass insulation work in this wall assembly. Outside to in. LP smarsiding, rain screen, Huber zip r6, 2x6 stuff with fiberglass batt faced, drywall.
I’m shooting to build an airtight home utilizing Huber’s products. Since I will he framing I will have a lot of control of this. I am also thinking to use Aerobarrier if my blower door test results aren’t great. In that case I would probably have them come out after framing is complete.
You can certainly get airtight without Zip, but OK, there is regular Zip with exterior unfaced EPS or mineral wool to warm it. Can exceed all the guidelines/recommendations without artful interpretations.
Intello has a good risk reduction effect on just about any design.
That should work great.
If you are doing the framing, getting near passive house levels of air tightness is pretty easy with taped sheathing, I doubt you'll need any something like Aerobarrier.
Just make sure you clearly define your air barrier and it is continuous along your whole assembly. That is, you should be able to draw it on any cross section of your house without lifting a pencil from the sheet.
The transitions from foundation to walls and from walls to ceiling always take a bit of work, so it is important to figure these details out ahead of time.
Question for all parties - is Zip R even considered “outsulation” since it’s still inside the sheathing? I know it qualifies for continuous insulation as it breaks thermal bridging but from my understanding it does nothing to lower the dew point of the inside of the sheathing other than moving the location of the dew point to the backside of the polyiso against the studs? I wonder if the ratio of continuous to between the stud bays ratio even applies here other than Huber trying to jump on it from a marketing perspective? It certainly doesn’t keep sheathing warm which is the chief concern of the CI ratios in the code, no?
The inside face of the sheathing is not exposed to interior moisture sources, so I wouldn't consider it a 'condensing surface of interest'. It's only meaningful exposure is to the exterior hence the seam tape and rainscreen. The inside face of the foam still gets a bit cold at only R6. That and the code is what needs satisfying with the class 2 vapor retarder on the interior.
So yes, I'd consider it exterior continuous insulation, and a good wall strategy. "Outsulation" is a trademarked EIFS product.
You're exactly right. "outsulation" means outside the sheathing.
Lots of opinions on that, but what is needed is data.
Some favorable Zip-R data here, but I wouldn't consider it definitive.
If you can find a cellulose installer, look at the numbers for just going double stud with regular zip. May be a wash, and you'll have an assembly that's easier to build, will perform as well or better, and will be able to dry to the exterior.
This is all really helpful for me as well living in Upstate Vermont. It is quite difficult to find out the "right" answer for the North East.
We insulate somewhat uniquely, from outside to In: Vinly siding-->Homewrap-->OSB sheathing-->2x6 walls with Kraft Faced Fiberglass-->1" Greenguard XPS (taped)-->sheetrock. I have found this to be a much tighter setup then putting the foamboardon the outside and it does break the thermal bridges without losing the ability to "breathe" on the outside. I guess my biggest question is can anyone see a reason why going my method is bad? Thanks in Advance.