@Stephen, since you plan to
@Stephen, since you plan to install R3 continuous, the main benefit for 2×6 24oc would be the energy savings from the additional cavity R-value. Whether or not it’s worth the additional cost depends on the particulars for your project (local lumber, insulation, energy costs). Keep in mind that in hot climates like yours, walls are relatively less important than roof, windows, orientation, overhangs and ducts in conditioned space. I think it’s safe to say that if you have to pay a premium for a framer willing to do 24oc then you’d be better off going with competitive 2×4 framing. But don’t forego the exterior (continuous) insulation.

BTW, if you happen to be going with stucco cladding, that system typically already includes 1 inch EPS, and at a lower cost than the R3 ZIP panels (compared to non-insulated ZIP).

In my experience, bowing isn’t an issue for walls. We previously had to use 5/8 drywall for 24oc ceiling joists to avoid bowing, but most of today’s drywall, which is a lot stiffer, can span 24oc joists @ 1/2 thickness without any issues. So even if there ever was any issues with wall bowing @ 24oc, that’s certainly not the case today.

@ Arlene wrote: “you will have shifted the sensible to latent ratio so equal that there is no unit on the market that exists that can handle it.”

I’ve designed a LOT of mechanical systems for beyond-code homes in the Southeast and I don’t have that problem. We can achieve close to 50% SHR if required (through airflow control and/or reheat techniques) but operating at a such a low SHR imposes a severe penalty on sensible efficiency. It’s always better to reduce the moisture load to begin with rather than wasting energy to remove it!

Stephen Definitely 2X6 on 24
Stephen Definitely 2X6 on 24″ centers. Thousands of houses have been built this way with no drywall problems. With respect to the plenum truss, is it not possible to use two or three ducted multi-splits in closet or hallway drop ceilings? Also pay attention to the comment about equipment above.

So, in the couple of years
So, in the couple of years since this blog was first put up, I’ve been doing a LOT around the impacts of building the ‘igloo’ in the south. What I would say is which every you do, you want to look for an HVAC company that understand how to run a Manual J and knows what a SHR is and is willing to pull OEM data on the unit. None of this AHRI crap. In the south, it won’t get you what you need. And just gird your proverbial loins, you’re going to be in a multi-stage unit and/or a dehumidifier.

Because if you are going to build that good of a building envelope in the south, and you are harboring any kind of delusions of grandeur of getting a SEER above 16, you can kiss the idea of a humidity removal in HVAC equipment goodbye. You will have shifted the sensible to latent ratio so equal that there is no unit on the market that exists that can handle it. Best you can get is a 3 to 1 sensible to latent ratio, at least last I looked. And I look a lot. And last I looked, DOE High Risk R&D for these products still didn’t have them close to hitting the streets.

@Richard: I concur with the
@Richard: I concur with the builder. The Zip-R minimizes thermal bridging, which is a big deal. But the main benefit of cavity foam is air sealing (plus a small bump in R-value in the case of ccSPF). You can seal the cavities MUCH cheaper (and equally effective) by ‘picture framing’ all the sheathing-stud intersections with caulk or closed cell foam (via Hilti type applicator). You’d get a lot more benefit by upgrading your sheathing to 1.5″ Zip (R-6.6).

As for cavity insulation… unless you have another reason to use Rockwool, I think you’d get more bang for the buck with blown-in insulation (Spider, BIBS or cellulose).

BTW, if attic is vented, be sure to seal all the top plates, especially along the seam between top plates and ceiling drywall, as well as seal the ceiling fixture back-boxes. Ceiling light cans (even those rated AT) need to be sealed. This is typically with site-constructed boxes made of EPS or with specialty covers (https://tinyurl.com/yxcqogvs).

Many of my peers will disagree, but in my opinion, full sub-slab insulation is a waste of money in your climate (exception: radiant floors), since most of the action is at the perimeter. In all but the coldest climates, for at-grade floating slabs I typically specify 2 to 4 feet horizontal along the perimeter edge and 2 inches vertical on the inside edge (between notched stem wall and slab). Exposed insulation tops may or may not be an issue depending on your floor coverings. If so, the top of the edge insulation can be beveled to make this work. If the slab is monolithic (turn-down), only the outside face needs to be insulated. If part or all of the slab is below grade (i.e., basement), then omit the horizontal insulation.

In your climate, slabs with good ground contact provide a passive cooling benefit. Any heat lost to the soil during heating seasons creates a thermal zone that, over time, gradually reduces the already small floor loads. With an effective perimeter insulation wrap, the only loss is the residual heat that remains at the end of the heating season.

I am building in Blairsville
I am building in Blairsville GA at 2300 ft on mountain ridge facing ENE over Lake Notterly. Would love your advice. Using 2X6 framing with 2″ closed cell, 1″ exterior Zip-R and Rockwoll. Builder says way overbuilt. Also, I insulated slab which he never does!

Slab insulation now has boric
Slab insulation now has boric acid in it and proper stainless steel mesh at edges and penatrations physically blocks termites.