I am building a brand new 2400 Sqr Ft single level home in South Carolina. I will have spray foam open cell insulation encapsulating the house  as well as Energy Efficient  doors and windows. The home will have a fireplace in the family room (not efficient,I know), and an on demand tankless water heater.  I have to determine how I heat and cool this home.

I will have Propane for the cooktop and tankless water heater

The open cell insulation is costing 2.5 times more than roll insulation but I have been told it is very efficient and the company installing it is very reputable

Does it make sense to invest the money in a  Geothermal system or  should I just purchase an highly efficient heat pump and air exchanger that would cost less?
Does it make sense to use propane for the Tankless water heater or is there something mor effcient? 

The plan is to live in this home for at least 30 years.

I really need some expert advice.

Thanks everyone for your words of wisdom and comments.

We have found, in our moderate climate, that a high efficiency heat pump (16+SEER) is pretty much equal to a geothermal system, when combined with a tight, well insulated structure, and energy efficient windows and doors.

A condensing on demand water heater is more efficient than a standard efficiency, and depending on the venting configuration may be cost-neutral if you have many feet of exhaust vent to run. (Condensing units use plastic vent, standard efficiency require $tainle$$ $teel.) but depending on your electricity prices a tank-top heat pump may cost less to operate, and will cut down on the cooling load (while adding a bit to the heating load.) The first-hour gallons ratings are still pretty low though- you won't be filling a spa or soaking tub with any of the current offerings.

If you have all Energy Star water-sipping dish/clothes washers a tankless will short-cycle, and may not be able to deliver hot enough water in those very short draws due to the ignition delay. If you go that route, get a version with a mini-buffer tank inside.

If you're a showering rather than a tub-bathing family, a drainwater heat recovery heat exchanger on the drain down-stream of the main shower will be cost-effective, more than doubling the output of any tank system, and lowering the fuel/electricity costs for heating the water. You need at least a 4 foot section of vertical drain to get 50% energy return on them though (taller and fatter are both better- a 4x48" heat exchanger is roughly as effective as a 3" x 60" version.) This often can't work with slab-on-grade homes, unless the shower is on the second floor, but if you have even a deep crawlspace to work with it even a stubby ~ 30% return version is worthwhile. If you're going with a slow-recovery beast like one of those heat-pump tanks this can be much cheaper & more effective than marriage counseling after the first round of cold showers. :-) The difference in propane use is as significant with drainwater heat exchange as going from a standard efficiency 40 gallon tank to a condensing on-demand if 2 or more people are showering every day. (If you're going full basement or crawl, and you think this is a possibility, search this forum for "drainwater heat recovery", or ask.)

Also, even 1" of insulating XPS sheathing over the structural sheathing is worthwhile to keep limit the performance-robbing theramal short-circuiting of the insulation by the framing. A 2x4 framed building with 1" of XPS or iso on the exterior and open cell foam cavity fill will usually equal or slightly outperform a 2x6" framed building with open cell foam and no exterior insulation.

Using insulated concrete forms for foundation walls/stem-walls is also worth it, from a thermal performance point of view. If termites are a concern, use copper flashing as a sill gasket extending beyond the foam as a termite-block. At the very least insulate the slab edge to R8-R12 to at least 18" below grade on the entire perimeter, if it's slab-on-grade. (Use borate-loaded rigid EPS to keep the ants & termites from burrowing through it in termite country.)

Think about solar gain & shading issues where it's sited, and size the windows & overhangs/awnings accordingly. It takes a lot less window area to get daylighting than you might think, and placing them higher under over hangs reduces summer solar gains but preserves some wintertime heating factor. A solar survey of the site can be useful, even if you never intend to use active solar equipment/panels. A good design can optimize winter & summer passive solar heat gains by quite a bit.

I almost forgot- in SC you'll definitely want to AVOID composite shingle roofing, and go with a ceramic, cement, or metal "cool roof" material to limit summer solar gains through the roof. To find products that meet the specs see: http://www.coolroofs.org/index.html (high aged reflectivity and emissivity numbers are both important.)

So would it make more sense to upgrade to a metal roof, go with a high efficient 16 SEER heat pump, and skip the geothermal costs?

How much more should a metal roof cost if the 30 Year Architectual Shingled roof installed is $8200? Double, Triple the cost???

Is a metal roof and a 16 SEER heat pump part of the 30% credit from the government like the Geothermal system is on a new construction home?

Thanks again

In the northeast a standing seam metal roof is somewhat over double the cost of 30 yr achitecturals. There are no tax credits for the metal roof. Ditto the heat pump unless it is part of a geothermal or some other system based on renewable resources which will supply 65% of your needs. (I think the 65% is right, but...)

In SC the difference in upfront costs for geothermal vs. air-source heat pump can be hard to rationalize on a lifecycle cost basis. For the money it takes to do geothermal, in a mixed climate like SC you may be able to go all PassiveHouse on the envelope design, and get by with just a dehumidifier and a small amount of resistance element heater on the ventilation air. (See: http://www.passivehouse.us/passiveHouse/PHIUSHome.html ) Lowering the heating/cooling loads to near-zero is sometimes similar in cost than the systems required to support those loads. To find the cost effectiveness break points can take a very sharp pencil and a good thermal/solar simulation of the structure, but if the extra cost of the more-expensive thermal envelope in the mortgage payment is less than what you would have otherwise been paying in utility costs, you''re money (and usually comfort) ahead.

Outside design temperatures for heating SC locations are in the mid-20s F, and a PassiveHouse level of energy use for space heating can probably be designed with clear-wall R values in the ~R35-R40 range, which is easily doable with simple 2x6" studwall + exterior foam layup. A 2x6 studwall with spray cellulose or o.c. foam + 3" of exterior iso board would yield ~R40. Last time I priced it out 3" iso was going for about $1.75/square foot- not cheap, but if it means swapping geo or air-source heat pumps for a dehumidification & ERV ventilation package at less than 1/4 the cost, how expensive is it really? Assuming a simple footprint and 100% markup for additional installation costs that 3" iso overcoat is still well under $10K for a 2400square foot ranch. (Figure 100' of wall, 10' tall, x $3.50 per square foot comes in at $7K). Using energy-heel trusses for the attic to allow for R60-R75 of blown cellulose or similar would add a bit too, but if the end result means you don't need a heating/cooling system, and you never get significant winter/summer peaks in energy bills, it can be worth it. Optimizing the cost/benefits in any design is an iterative process to be sure, but that first-ton of geo is a doozy, and buys quite a bit of insulation and higher-efficiency windows & doors, etc. It takes a lot less insulation to go this route in SC than it does in MN, and I can't imagine doing a full custom design in your climate without at least considering it.

WAG type numbers: If a minimal geo system starts at $20K, spending half on insulation, half that on ventilation & dehumidification systems could be enough to take care of the cooling load, then add another $5-10K for more insulation, air sealing, better windows & doors, and your heating "system" could be as small & dumb as putting a thermostat on your toaster. ;-) And your power bill may never go over $100...

Your neighbors might think it odd, but in 30 years I'd anticipate at least some significant energy-cost inflation, and doing a high-performance envelope as new construction is a lot cheaper to do a high-performance building envelope as new construction than as a retrofit. Whatever code is on insulation, double it, as a starting point. Then pay close attention to the glazing heat gain/loss issues in a site-specific way. Heating a place 90% or more by passive gains & a low, controlled heat loss isn't out of the question. You might have to skip the fireplace though , or open all the windos & doors when you light a fire. (If you really are going that route, use a fireplace insert for 70% efficiency vs. 15% on the wood burning, and far less parasitic infiltration losses from the flue stack effect.)

Mop-on cool roof coatings & membrane roofs can be cheaper than metal roofs. You really have to factor in a lot of things, including architectural appropriateness, etc. Ceramic and cement roofs also tend to have lower solar-gain specs than composite shingles, but also at higher upfront cost. In CA there are tax & rebate incentives for using cool roof materials (as well as minimum-standard mandates under CA Title 24 rules). I'm not sure what, if anything is available in SC, but it's usually cheaper than going to an R100 roof to achieve similar reductions in solar gain on a low-slope roof. Whether or not you can get a kick-back on it in SC may depend on which electric utility you're working with.

There are some real snake-oil sales droids out there in the coatings world. Refer to the CRCC to find out the real performance specs (and if it isn't listed there, don't buy it.). http://www.coolroofs.org/products/search.php

I am not a big advocate of geothermal, especially on an energy efficient structure. But I am doing it because of the 30% subsidy. I was going to do all in-floor hydronic radiant throughout, even in the upper level. I will get the tax credit on the entire system, so when factored in, the geothermal will only add a couple thousand dollars. Otherwise I would probably have used a wood boiler.