Good advice, thanks. Agree with excluding the summer sun &  we are designing to do so. Would you, budget allowing, insulate both the perimeter foundation wall and insulate under the slab?
What is PAHS design by the way?

Posted By Dana1 on 10 Mar 2011 04:47 PM
With slab on grade you 'd have to sink in much deeper footings and insulate the stem walls to get much in the way earth coupling benefit. It's usually less material and lower cost to simply insulate the slab.

The PassiveHouse recently built in Louisiana had ~ R16 under the slab, and ~R20 basement/crawlspace walls:

http://content.usatoday.com/communities/greenhouse/post/2010/08/souths-first-passive-house/1

http://www.greenbuildingadvisor.com/blogs/dept/green-building-news/following-passive-house-deep-south

Insulating the footings and the whole slab to at least R8-R10 (2" of EPS or XPS) and putting at least R6 of thermal break (1" iso) on an otherwise pretty standard stick-built house can go a long way toward boosting performance, but optimizing the glazing/shading for optimal seasonal solar gain/rejection and making the structure air-tight is what makes or breaks it in the end for homes in the southeastern US.

You didn't ask but...

A typical wood clad 2x4 framed house with batts or open cell foam or cellulose cavity fill has a whole-wall R-value of about R10 once the thermal bridging of the framing is factored in. It's pretty cheap to DOUBLE that by going 2x6 24" on-center framing w/spray cellulose cavity fill, 1" of exterior rigid iso. Once you have whole-wall R values of ~R20 and you've gone R40+ roofs/ceilings the windows and air leakage factors are going to dominate the heat gain/loss in most homes, and reducing that is also cheap:

Going with smaller and casement/awning windows on the E/W/N sides of the house rather than big double-hungs or sliders (and NO SLIDING DOORS PLEASE! They all leak like crazy in short years), and designing S side overhangs & window sizes optimal seasonal gains takes a big bite out of heating & cooling energy requirements. Think of every square foot of double-glass as an R2-3 hole in your R20 wall. The gain/loss from a single 24x60" ~U0.5 double-pane is the same as 50 square feet of unbroken wall. Going with better grade low-E U0.34 double-panes it's still worth 30 square feet of unbroken wall. Casement and awning windows seal better than double-hungs or sliders, and provide more egress area for the same sized window than a double-hung or slider, where that's an important code factor. If performance is a goal, where you CAN go smaller without impacting daylighting or function, DO. Think about it room by room. Use fixed windows where you can for daylighting or taking in the view- they inherently leak less. Shrinking the glazed area 25% or re-balancing it for winter solar gain is usually cost neutral or even cost-negative. Going with more glass you'd have to go to much higher performance windows to keep the cooling/heating loads well bounded, and it's easy to end up doubling the upfront cost for only 25% more glazed area to stay in the same heat gain/loss range.

[edited to add: I'm an idiot (or needed more coffee)-  the heat gain/loss for that 24"x  60" window is worth 70-100 square feet of unbroken R20 whole-wall, not the 30-50 square feet it would be for typical 2x4 construction, which is how window ratings can easily dominate the numbers.]

Designing the cooling/heating system so that all ducts stay inside the pressure & thermal boundary is also huge. If the ducts are in the attic, insulate the roof. Using 3-4" of sheet iso above the roof deck with a vented nailer deck above the iso puts a huge thermal break over the rafter or truss framing, and if it's a few simple pitches rather than a bunch of dormers & valleys it's about half the installed cost per unit R of closed cell spray foam. Finishing off the R with 4-5" open cell foam on the interior of the roof deck makes the roof deck the pressure & thermal boundary of the conditioned space- no venting necessary. But if the ducts can be routed elsewhere, 12-15" of low density cellulose on an air-sealed attic floor is a lot cheaper- just tryt to get at least 3" of blown over the tops of the joists or truss chords as a thermal break. (Low density cellulose is cheap, and usually outperforms low-density fiberglass under hot roof decks, and in cold-attic applications, ASTM C 518 tested R value at 75F notwithstanding. The performance differences in walls are smaller, but blown/sprayed fiber beats batts in real world installations since it's easier to fill in completely, with no gaps or compressions, and you can spray-fill micro-cavities as thin as 1/4" that you'd never stuff a batt-slice into.)

If you can make the slab/footing foam align on the exterior with the rigid iso on the walls and foam-seal/tape all seams, caulk the framing plates and studs to the sheathing as you go, and pay careful attention to sealing up all wall & ceilng penetrations it's possible to end up with very few thermal bridges and very low infiltration rates, and literally half or less the heating/cooling loads of a "typical" code-minimum house without expending a lot of money. You'll get modest amounts back on lower HVAC sizing, and your bills will be much lower. You'll need to have a dehumidify mode on the AC, since the sensible loads (temp only) will be a fraction of the latent loads ( humidity), but the tighter the house is, the lower the volume of sticky summer air you have to dehumidify.