This is an interesting question and my initial gut reaction is to say 'go for it' regarding the SIPs walls of R15 and roof of R30.
But let's look at some facts.
I have been reading about a Passive House* that was built in Florida (Lakeland) and that house used R30 Batt insulation in the roof and only R10 for the walls.
- Those insulation measures in combination with light colored walls and roofing, and 36" overhangs provided 70% energy savings over an identical house that was built to meet minimum code standards next door.
- You can read the full study here on the Florida Solar Energy Center's website.
PV panels brought the overall energy usage of the more efficient home to 92% greater efficiency than the basic house.
Weigh the financials for SIP walls
Standard homes in Florida have walls that are insulated to about R5. The above study found that wrapping the standard R5 wall (usually 2x4's with fiberglass batt) with 1.25" of isocyanurate insulation on the outside brought the R value to R10 and as you can see did wonders for the efficiency of the house.
If you want to go to a 3.5" R14 SIPs for walls I wouldn't discourage you but I would advise you to weigh the financials.
If it does not cost much more to get the SIPS then I would go for it. Like you said there is an ease of construction and a savings on other things- like drywall installation costs. Also electricians find using SIPs much easier than normal construction after they get used to them. There are a lot of plusses.
SIPs for the roof?
Regarding the roof, I would also recommend SIPs. R30 in the roof will go a long way toward reducing your air conditioning costs. And they are the perfect use for a cathedral ceiling.
I would ask your SIPs panel supplier to make the overhangs longer than usual. Ideally up to 36" deep.
- The shading of your walls in combination with the R30 roof will truly make your home deeply energy efficient.
- The homes built in the study were only 1 story and they took full benefit of their over hangs.
- The more efficient home used 36" overhangs which left the sunny walls in 6' of shadow most of the year which were much more beneficial than the standard 18" overhangs on the basic home.
The only thing to watch with SIPs is the installation.
Water and air leakage at the seems are the biggest problems. Just make sure you have a very experienced installer who is not going to cut any corners regarding taping, water proofing and flashing.
Ground source vs Air-to-air systems
If you are getting this much efficiency in your walls and roof then I would recommend an air-to-air heat pump system instead of a ground source heat pump (Geothermal is a ground source system).
- It is becoming quite common in the green building industry to use something called minisplits (air-to-air systems).
- We use them on our houses up here in the north east. Our office never specifies geothermal systems any more.
Both systems work off of the electric grid. Both are very efficient. Some would argue that the minisplits are much more efficient. The minisplits are much less expensive to install as they do not require any drilling and associated piping, they simply have a small condenser on the outside of your house (like a traditional AC unit - only way more efficient.)
You can get the minisplits in both ducted and ductless. The ducted is more expensive but, when installed, it looks just like a traditionally ducted system inside of your home. The ductless systems hang on the walls inside of your space - it is sort of a long thick white or grey rectangle.
The great part about the minisplits is that they do heating and cooling but they are much much better at cooling than they are at heating. They are perfect for your neck of the woods.
- You can also work with humidity control and can be connected to air exchange equipment (like an ERV).
- They modulate constantly and keep a very even and comfortable airflow, temperature and humidity at all times.
- You hardly notice they are working at all.
For the money an air-to-air system is less expensive and equally if not more efficient than a geothermal system.
And last but not least don't forget to consider a Photo Voltaic system to help offset some of the (minimal) electric energy you will be using to cool your super-efficient house!
For more information:
Read "Are structural insulated panels (SIPs) the right choice for building a healthy home?" a Q&A answered by Dietmar Lorenz.