Exterior walls in Dallas Texas
Hi all,
Working on designs for my own home, and I’m at the point where I have to decide on the exterior wall construction to get an idea of wall thicknesses.
My plan is 5/8 Drywall –> 2×6 –> Zip-R9 –> 3/4″ furring strip rain screen –> siding.
My alternative is to use regular Zip with 1.5″ foam boards on the outside. I understand that this provides benefits when dealing with shearing and bracing, but would be an additional step over using the Zip-R.
So my questions boil down to:
1. Am I way off on this assembly in the first place?
2. Am I going to overcomplicate/overprice the design due to the shear/brace limitations of Zip-R? It’s a fairly large two story house (3300 sqft first floor).
3. Is the price premium of Zip-R offset by the simplification of having the outer layer of the wall assembly be OSB instead of foam (from the perspective of window/door details, etc).
4. Am I missing something else entirely?
Of course I will be using an engineer for putting all the plans together, but I’d like to get as many of these decisions dialed in before hand, especially in an area that does not seem to do much in the way of “building science.”
Thanks
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Replies
1. You are overthinking your wall assembly for Dallas. Dallas Co. is in the 2015 Code now, and we’re on CZ3, but on the 2021 Code, we’re on CZ2; having said that, I still recommend my clients to apply CZ3. Neither code requires minimum outside rigid foam for condensation control, but that also depends on the ΔT and humidity readings. Not knowing where clients set their homes thermostat, humidity and ventilation, to be “safe”, I always design all my homes in NTX with 1” rigid foam, but ½” is also acceptable.
2. Huber has engineered reports for Zip-R9 to be accepted as structural sheathing, as long as you follow all installation instructions.
3. Not to me. A) I prefer the rigid foam on the outside, and B) The rigid foam with the approved tape can be a second moisture barrier.
My assembly would be 2x6 24" o.c. with dense-packed cellulose, TAPED OSB/CDX, WRB (Spray-on or membrane), TAPED 1” polyiso R6.7, 1x4 rainscreens with bug shields/screens on top and bottom, siding.
Use 1x4 window bucks on top of the rigid foam to install flanged windows and doors. There are plenty of details on-line.
4. Structurally and building science speaking, there is no "wrong" with your assembly, I just look at it differently.
Achieving success with the installation of any system in your house depends 99% on the quality of the work, regardless of the products you choose.
I know BGA is about building green, but isn't that area something like 2000 heating degree days? Even a simple 2x6 wall with high density batts is good enough.
Assuming 3300sqft/floor, so 6600 total, 9' height, say about 5500sqft of outside walls.
2x6 wall with HD batts, so R18 assembly your heat loss is about:
-2000 HDD *24 h * 5500 sqft / R18=146 Therms
2x6 with HD batts + R9 Zip R, so R27 assembly:
-2000 HDD *24 h * 5500 sqft / R27=97 Therms
The energy savings is essentially squat. Add in there that making foam has a carbon footprint, even on environmental front it is probably not worth it. Chances of find trades that can deal with exterior rigid in an area where it is not code are also pretty slim.
The difference in cooling energy use is even smaller as the temperature delta is much smaller even when on the hottest of days.
If you want better than 2x6 wall, I would look at 2x8 with R30 batts, but even that is hard to justify in warm climate.
I think in your case, your energy savings would come more from good design details, things like keeping HVAC out of unconditioned attic, getting a well sealed envelope and integrating proper exterior shading for all your windows.
in addition to what Armando Cobo and Akos said,
if you are not putting in windows of at least r5, then the additional insulation is essentially doing squat, as the windows will still make up the majority of the wall heat loss and additional insulation of the wall will add very little overall r-value