I keep designing and re-designing the house we will be building in SC. In general, it is a cross shape, with a main gable, and two 'wings', one being the living room, and one being the master bedroom. I have three reasons for the shape- 1) visual interest in a small house. 2) The three major rooms have windows on three sides, which allows for excellent cross-ventilation (Our windows will be open between 55 and 85 degrees) and 3) The living room, kitchen, and master bedroom all face the mountain view.

Now the down side. This house has 168 linear feet of wall for a house of 1088 sq ft. The same area in a square shape would only have 132 linear feet, a 20% reduction. Aside from the obvious increase in building materials, and complexity (I'm building it myself, so that 'doesn't count'), I'm worried about the heating and A/C efficiency, for when those windows are closed. The walls will be about R-21, and I'm planning R-40 in the attic. The main gable will probably be 1-1/2 stories, with another bedroom upstairs, but there will be a door at the stairs, and the upstairs will be rarely used. I figure the upstairs will be an added buffer from temperature extremes. The house will probably have a full ICF basement (finances allowing). Am I over-analyzing this for the sake of efficiency? Obviously a styrofoam cube underground is going to be most efficient, but I wouldn't want to live there. How much difference would or could the layout make?

The cross shape is ideal for cross ventilation and taking maximum advantage of the outdoors. The climate is SC is, for the most part, very moderate. You should be able to open the windows, at least occasionally, 8-10 months per year. I would say, that if you like the aesthetics of the cross plan, then build it. While the plan will cost a little more to build, and to heat and/or cool, during the times when these are necessary, the cooling periods will actually be shorter, due to the greater ventilation. If possible, add porchs on the south and west sides to reduce heat gain, consider thermal shutters on northern and western windows to reduce heat loss, and plant fast growing trees and vegetation for shading. Then enjoy.

We are right at the edge of the mountains north of Greenville, almost at the NC border. We've been there many times, and found the outdoor weather pretty tolerable (coming from FL). They do get extremely hot at times, such as 105 last summer, and sometimes it can get cold, but those are the extremes, not the norm. Our land is heavily wooded with mature hardwoods, so shade is already abundant. I have a friend who lives near our property, and his uninsulated basement is mid 50's in the winter, mid 70's in the summer. I could live in that year round if push came to shove. If I could control the humidity, I would seriously consider living in the basement during the hot part of summer,and leave the A/C off. In FL, our A/c season is 6 months long, and we lived here for 6 years without it- talk about hot and humid!

Thank you.  I know the area.  One caution.  The humidity can be high enough in that area that mold can grow on the drywall ceilings and walls when the AC has been off for a few weeks.  You really should keep the humidity low enough in the home to prevent mold and dust mites.

I hear you on the humidity! Another thing we will have is an attic fan. We have one in our current house in FL, and it's great, as long as the night-time temperature gets low enough. Open the windows, turn on the fan, and woooshh! They are kind of noisy, though.

If you do install a whole house attic fan, you might want to consider installing a switch that can either vary the speed or select different speeds.  This can be very handy to have to meet different conditions.

Good point- the one we have now has two speeds- on and off!

Slower speeds are quieter for sleeping.

You might want to do a heat/cooling load analysis using one of the software programs available on-line such as HVAC-Calc. If you are concerned about maximizing energy efficiency within your design constraints, a heating/cooling load calculation can be very illuminating.

Coincidentally enough, my son is taking a course in A/C at college this summer as part of his Engineering degree. He has a great book (what little I could comprehend) and he should be able to do a pretty accurate calc for me. That should tell me how much real-life difference the various building shapes will make, and help me make decisions about insulation, and window area, as well as quality. From there, we can size the A/c system appropriately. I've heard that a lot of old-school A/C contractors put in way too big a system for some of the well sealed and insulated houses being built to day. Then humidity becomes a problem, because the system doesn't run enough.

You heard right. After doing a thorough load calculation to size my HVAC equipment, I was shocked at how little capacity I needed to heat and cool my well-insulated, tightly sealed ICF house. I bought the smallest mod-con boiler and A/C unit I could find. This winter, I confirmed that the equipment could heat the house with plenty of margin to spare. The boiler also heats my domestic hot water supply.

BTW, the load calc program convinced me to install an electric fireplace over a vented gas fireplace and to reduce the size of my windows to limit energy loss through the glass. Many don't realize how much energy is lost through even the most efficient triple pane windows. My Pella triple panes are U=0.25 which is equivalent to an R-value of 4. Typical thermopane glass is just R2! I installed rolling shutters on the SW facing windows to limit solar heating in summer and further reduce heat loss in winter. Skylights are also huge heat sinks in winter and solar collectors in summer. I installed one at DW's insistence. Any suggestions how to limit the energy inefficiency of a skylight?

HVAC installers that do not do the Manual D and Manual J calcs have a tendency to oversize the heat pump and undersize the ductwork.  This leads to a noisy inefficient system.