Wall Insulation for a Mixed-Humid Climate
Mauro_Zammarano
| Posted in General Questions on
Which wall/insulation makes most sense in a Mixed–Humid Climate (Zone 4A)? Building a pretty good house and aiming to net-zero with solar panels (to be installed later).
I am sure that there is an optimal compromise for durability/cost/performance in this climate zone. Any suggestion?
Also what would be a good target for enclosure tightness? 1 ACH50 sounds reasonable to me
Thanks!
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From a carbon and dollar cost standpoint, double stud walls filled with cellulose seem to be a good combination. Or, for more traditional construction, 2x6 walls with cellulose and 2" or so of exterior insulation, recycled if possible. Anything that gets you into the R30-R40 range is about right. And yes, aiming for 1.0 ACH does also seem about right, and it's a target that a competent builder should be able to hit without too much effort. Since you are aiming for zero energy, you should take a look at the DOE's ZERH guidelines. There's a lot of good information there for you and your designer/builders.
Since Peter mentioned the DOE guidelines, I thought I’d share this article because it paints a good picture of what that route entails (it also includes a link to those guidelines): Why Zero Energy “Ready” Makes Sense.
I think the PGH R40 walls are a bit overkill in zone 4.
Say you have 4500 heating degree days. A two story 3000sqft house with 9' ceilinghas about ~2500 sqft of walls (if you take out glazing). So over a heating season, the heat loss through an R20 assembly is roughly:
4500 HDD * 24h * 2500 sqft / R20= 135 Therms.
If you are heating with a heat pump with a COP of 3 off $0.1 electricity it costs you:
135 Therms * 29.3kWh/Therm* $0.1 / COP3= $131/year
An R40 wall would half the loses and the heating costs, so you save $65/year.
I can't see a double stud wall having any reasonable ROI.
I think a 2x6 wall with high density batts or dense packed is good enough giving you an R18 assembly. If you want a bit better you can look at 2x8 24" OC framing. With R30 batts that brings the assembly up to R24 for minimal extra lumber cost over 2x6 16" OC.
Your dollars are better spent on higher efficiency equipment and getting your air sealing right.
Good point. The whole idea of the "pretty good" house is balancing energy efficiency and its cost against the cost of additional onsite generation. In zone 4a, the small amount of additional generation could easily offset the cost of going to R40.
Thanks all for you reply! I like the simplicity of a 2x8 24" OC framing.
I was also looking into phenolic foam from Kingspan
https://insulation4us.com/products/kingspan-kooltherm-k12-internal-insulation-board-all-thicknesses?variant=32604708536369
It sounds like a great solution and I believe it is not expensive when cost is normalized by R-value.
I expect aa 2x6 plus a 1.18 inch phenolic foam (R9.5) to outperform a 2x8 but no idea about cost and drawbacks.
Also, EIFS looks like a prepackaged solution that my builder should not have problems installing and it seems to be popular in this area especially for contemporary houses. I honestly prefer the look of a synthetic stucco like EIFS to siding.
You can also stagger 2x4 studs with a common 2x8 plate to get the benefit of a double wall's reduced thermal bridge. When I attended the 2nd Annual Passive House Conference in 2007, a discussion was had about what constitutes superinsulation in a heating dominated climate. The consensus was heating degree days divided by 180 for walls and 120 for ceilings. With this in mind, R-25 for the walls would be a good target (180 into 4500 hdd).
Something that doesn't force your builder to have to reinvent the wheel. IMO that would be ZIP-R or equivalent then whatever interior insulation you want just make sure it's installed properly.
Better yet look for a builder in your area who might already have the required expertise. (https://www.energy.gov/eere/buildings/maps/doe-tour-zero)
I advise being extremely cautious with stucco or EIFS in this climate. It is popular mostly due to reasonable cost, but I’ve seen many fail due to poor moisture control within the envelope. EIFS has a good reputation for its continuous insulation, mostly made of EPS, but in humid environments, particularly with wood frame construction, air permeable barriers are advised to keep the envelope dry. Not considering this with an EIFS system can do more harm than good. I recently observed significant EIFS degradation on a 20 year old building in Georgia. It eventually caused indoor air quality issues and now failing mechanical equipment.