What wall setup?
K_M
| Posted in General Questions on
So I’m currently designing my new home build. I live in the Blue Ridge Mountains of VA so we see a wide variety of weather. I’m torn on what setup to run for my exterior walls. I’ve looked at doing a 2×4 with 1″closed cell spray foam and R11 bat and then 2″ exterior rigid foam or possibly jumping up to 2×6 with rockwool R23 and then just 1″ of exterior rigid foam…. I really can’t nail down what is going to be the best bang for your buck situation here. I am sitting basically right on the line from CZ4 and CZ5. Maybe both setups are overkill for my climate?
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Replies
I personally would go with the 2x4 wall with R15 mineral wool batts and 2” of exterior rigid foam (polyiso). This gives whole-wall performance of about R25. Your 2x6 wall configuration gives whole-wall performance of around R24.5. You can do a bit better with the 2x4 wall assembly and get a slightly thinner wall too. On my own house, my retrofit is to replace the existing R11 fiberglass batts and R3 exterior foam with R15 mineral wool batts and 3” of exterior polyiso for about R31 whole-wall. I’m in CZ5.
I’d skip the spray foam step. You can’t really fit 1” closed cell spray foam and an R11 batt in a 2x4 wall anyway. If you want higher wall performance I’d just add thicker exterior foam until you get to where you want to be. That goes for either the 2x4 or 2x6 wall assembly.
Bill
+ 1 on skipping the 1" spray foam. Compressing the R11 to 2.5" delivers about R9, so the cavity fill is still only going to be R15-ish in a 2x4 wall, about what you'd get with a rock wool batt. There are cheaper and greener ways to air seal a wall than an inch of closed cell foam.
The lower cost ease of construction of 2x6/R23 rock wool and 1" exterior polyiso trumps the marginal performance improvement of 2x4/R15 + 2" exterior polyiso, and neither are overkill for a zone 4/5 boundary location. The R25-ish whole-wall figures are right in there for the recommendations for zone 4 found in Table 2, p10 of this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
At those levels it's usually possible to hit Net Zero Energy with a PV array that fits on the house using cold climate heat pumps for space heating.
Dana, I’m curious why you think the 2x6 wall would be cheaper and easier to construct. I’d guess you’re thinking the thinner exterior foam would make details like window mounting/flashing easier, but I’d think the overall cost would be pretty similar between the two wall configurations (2x4 / 2x6). The foam is slightly more expensive on the 2x4 assembly, but the batts and lumber will be slightly more on the 2x6 assembly. I’d think labor would be pretty close either way.
I think you have more residential expierience than I do so I’m interested in your opinion here. I live mostly in the commercial world and don’t get to play with wood as much.
Bill
Rigid foam is fairly expensive stuff per board foot, and you'd be using twice as much with 2" than with 1".
Most siding can be mounted on 1" foam simply using longer fasteners, whereas with 2" foam furring through-screwed to the framing with pancake head timber screws is mandatory.
At 16" o.c. there is more board feet of lumber in 2x6 framing than 2x4, but at 24" o.c. it's comparable in board feet with fewer boards to cut, and still has structural capacity comparable to 2x4 16" o.c. framing
Thanks Dana. Sounds like you’re thinking is that the additional material cost in lumber is more than offset by the higher cost for extra foam when using 2x4 construction. I hadn’t thought of using wider spaces studs with 2x6 construction either. Always new stuff to learn :-)
Bill
The other advantage to 24" o.c. framing is a lower framing fraction, less thermal bridging overall. Taken to the advanced framing optimum-value extreme with single top plates and advanced framing corners etc the framing fraction drops to about 15% compared to the ~25% typ. for 16" o.c. framing. That adds ~R2 to the "whole wall R" in most designs.
Thank you guys so much for the input!!! So seems recommendation is 2x4 framing, R15 rockwool insulation, 2" exterior rigid foam board R10. So would you guys recommend utilizing the Zip sheathing? I appears to me that it makes flashing much easier for the novice as well as creating a good air/vapor seal if taped correctly. Thoughts?
I assume also doing a 2x4 wall gives me the ability to run a standard interior window casings and just make custom size exterior to account for the 2" of foam.
Or better yet.... Could you not just do a normal jamb for a 2x4 wall and then add a 2" brick mold to get the added width? Maybe that's a dumb idea??
I suppose you could do that, but I’m bit sure why?
Bill
Kevin,
This may be a regional thing, but every exterior door I order I have to specify the jamb depth to the supplier, taking into account framing, drywall, and sheathing thickness. We don't have standard jambs.
Most places here offer standard jamb widths which coincide with normal 2x4 or 2x6 construction. They do all offer custom jamb widths however prices jump up at that point fairly significantly. I suppose another conversation is in order with my door and window supplier to see what can be done to simplify installation but not increase costs to much.
HI Kevin -
great exchange here but just putting in my $.02 to spend as much effort, time, and money on air sealing as you are on R-value...
Peter