Replies

1. Expert Member
   Dana Dorsett | Feb 18, 2015 11:28am | #1

   It's unlikely that you would have to go to a double-stud configuration to hit Net Zero in a Charleston SC climate, which is US climate zone 3A. In most of US climate zone 3 you can get there with 2x6 framing + a few inches of exterior side insulating sheathing (foam or rigid rock wool). Your heating degree days are barely more than half that of zone 6 climates where R40-R50-ish double studwalls are needed to get there, and cooling degree-days are more about roof-R and window gains, not wall-R.

   A 2x6 16" o.c wood sheathed wall with R23 rock wool batts, or cellulose or open cell foam, or blown fiberglass, with 3" of exterior EPS or rock wool with fiber-cement siding and half-inch gypsum comes in at a "whole wall" R of about R26-R28 after thermal bridging, which is already more than half the performance of a foot-thick cellulose insulated double-wall (~R40-R45, depending on details) and it's much easier to build. It may not even take R26 walls to get to Net Zero if you spend some time & money on optimizing your window size, U-factors, and SGHC for each side of the house, with appropriate overhangs on the south side to minimize unwanted summertime solar gains, but allow reasonable amounts of wintertime gain to offset heating energy use.

   Try teaching yourself how to use BeOpt, a freebie download energy use simulation tool courtesy of the US taxpayers. With that tool you can play "what if" games with the house design once you have most of the parameters entered, allowing you to fine-tune the window & insulation specifications, the house orientation, etc.

   In general, having little to no west facing glass will reduce both the peak cooling load and cooling energy use. South facing glass needs to be shaded by overhangs, but then you can play off U-factor against SGHC to find the right balance between heating energy use vs. cooling energy. East facing glass is still a net solar gain that may have to be reduced in size to limit cooling energy. North facing glass at U-factors lower than U0.35 will still have a net solar gain, but not a huge gain, and is optimal for daylighting, since the light is indirect and glare-free, casting no sharp shadows.

   What type of foundation are you contemplating?

2. GBA Editor
   Martin Holladay | Feb 18, 2015 11:31am | #2

   Rick,
   You may want to read this article: Hot-Climate Design.

3. 
   Rmfetter56 | Feb 19, 2015 07:08pm | #3

   Thanks for your input. I am going with a SOG raised to 18" above grade but I am trying to avoid using foam. I am doing the framing and air sealing myself so for double stud walls it's close to a wash. I will download BeOpt and go from there.