Geothermal vs spray foam insulation

VankmanSeptember 5, 2011

My wife and I are in the beginning stages of working with our home designer on our house plans. We have been discussing the use of some newer methods to insulate, heat and cool the home. I am not sure the up front cost of using both spray foam insulation and a geothermal heating/cooling system makes sense. I am not sure I could recoup both of the extra upfront costs and may choose one over the other.

Has anyone used these systems in a new home? If so which one do you think we would get the most return for our investment on?



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Location dictates everything.

Geothermal can cost $20k or $80k which is largely location dependent. This of course dictates payback.

HVAC costs can be $500 a year or $5000. This is dictated in large part by your location and dictates payback.

So you need to give a location, size of house, availability of NG (and cost), electricity cost, and then ball park estimates on Geo.

Generally because of the geo tax credit, that is your better deal with a million exceptions.

    Bookmark   September 5, 2011 at 12:54PM
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Reduced your heat and cooling loads. You only get one chance to do this to your home. Try flash and batt. It is an 1/2 inch of foam, and the remaining cavity with blown in spider fiberglass insulation. Atlanta has been doing this about 4 years now. You want to design your HVAC duct work inside your envelope, [inside the conditioned space], Mini Split systems are worth looking has 8 zones on One HeatPump. Many ways to look at your ROI. Reduce the size of your house..this will give you the most ROI! Good Luck

    Bookmark   September 5, 2011 at 3:34PM
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Thanks for the responses.

We are in North Carolina, the utility company for which we will be receiving our power from charges around 12 cents per kilowatt hour (based on information from people in the area, I am planning to call tomorrow for confirmation). Natural gas is not available from the utility company, we will have to place a tank to be filled by local companies.

Our house will be somewhere between 2500 - 3000 square feet. I have spoken to companies in the area about both spray foam and geothermal but they will not give me ballpark estimates until our house plans are complete and they can review them.

Thanks for the help.


    Bookmark   September 5, 2011 at 8:46PM
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Where in NC? The differences between the coast and mountains are pretty significant. It is a big state.

The tank of course gets filled with propane which is 3-4 times the cost of NG.

The inability to give ballpark estimates is stupid. Do you have a builder? Our builder gave us ballparks based on prior builds. They were always shocking because of the lack of competition in the area. The 2 I remember were $10k to foam the attic rafters and $60k for geothermal - both of which were non-starters.

Generally in NC (central and coastal), the HVAC costs are too low to justify either geothermal or foam. A well sealed house with above code insulation can cost $500 a year to HVAC without resorting to those more costly options. If you are in the mountains, then you change the picture a bit.

First the basics - ducts in conditioned space and good air sealing. Then bump the attic insulation. Then bump the walls. The ducts require appropriate house plans - and don't trust the designer alone. Plenty don't leave appropriate room for the units.

For a point of comparison, I have a ducts in the attic, 5000 sq ft. Conventional insulation but r-40 in the attic. Air sealed pretty well with ES standards. Seer 16 heat pumps and NG backup. I have $1000 in HVAC costs. Lose the NG backup and I'd probably go to $1200. That isn't a lot of money and 1/2 is a/c which is harder to cut.

The most important thing now is the ductwork. Then the solar orientation and/or shading. Then check for rate cuts for Energy Star certification. Progress gives us 5% for life. Given the merger, they should have 80% of NC but I know there are a lot of locals.

    Bookmark   September 6, 2011 at 4:54AM
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You will probably need a 3-ton geothermal heat pump, with a COP 4 the cost per kW of heating or cooling delivered into your home would cost about 3 cents/kW.

You should be able to at least get an estimate on a 3-ton system without the ductwork at this point. The important thing to know would be what any particular installer would put in the ground and how. That should give you a heads up of who to consider and who to stay away from.


    Bookmark   September 6, 2011 at 9:40AM
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I really think no matter if you live where it is colder or hotter you must pay utility bills . and when you are building a new house this is the one chance to make it energy eff-ent . I think its worth the cost of foam insulation it saves you so much in later energy bills because 1 reason they will always keep going up . and you can always upgrade with geo thermal and other things later . but it would not be cost effective to upgrade your insulation later . their is just no way that you can get any thing better than foam it is so air tight and seals all the small holes and openings . and I think you should go with the highest r value windows and doors as well . I think you should first make it the best at holding in or out the heat and cold . and 2nd make it cheaper to produce the hot or cold air in the house . I think the best would be to build with 2x6 walls with 5 in of foam and 5 in of foam in the attic to and use highest r value in the windows and doors . use geo thermal to heat and cool . and go with wind and solar power .and you can do the second part over time this may cost more but most likely with no electric or gas bills this has to be a big plus . and if you ever sale , telling the buyers they are no power bills , will sale the house quicker and at a higher price .

    Bookmark   May 25, 2013 at 4:32AM
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foam roofline.
walls 1" foam board on exterior
conventional insulation
air tight drywall to exteiror.
seal sole plates to slab & subfloors.

geo is outrageously expensive in my area.
my clients start out with geo in mind, then
switch to 15-17 SEER heat pumps.
in some areas in my state utilities are .13 per kwh.
others .08 per kwh.

if you build tight, and with unvented attic putting
ducts in semi conditioned space, then the above
works well.

but if in design stages you can locate both equipment
and ducts inside the living space..that is your big savings.
I save 25% of my utility costs by retrofitting ducts
& ahu inside my living space.
putting ducts in unconditioned vented attics is an
expensive design flaw.

hvac system should be sized with load calcs,
duct system sized & designed prior to install.
leaving details to be decided on the job results
in poorly performing duct systems.
oversized hvac systems & high cost problems
with little comfort for the homeowner to live with.
the trades people will be on the next job...and
you'll be stuck with the problems.

best of luck.

    Bookmark   May 25, 2013 at 11:26AM
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Good advice all the way around here. I have a comment on the mini split suggestion. It might be better not to have all your eggs in one basket for the outdoor unit(s). If you live in an area with frequent power outages, you can run parts of your system with a relatively modest genset if you have 2 or more outdoor units. You also have backup in the case of failure. You can cool parts of your house no matter what breaks.

Take this comment with a grain of salt, I am only an amateur and not an engineer. The other factor is that the compressor can only throttle to some minimum percentage. Assume that is 20%. For example, If you are at low demand and running one 6000 BTU indoor unit and it is driven by a 6,000 BTU outdoor unit, indoors you can run at 1,200 BTU. That would contribute a lot to comfort and the mini split producers use this to market their systems. The corresponding numbers for 18,000 and 30,000 BTU outdoor units are 3,600 and 6,000 BTU. You appear to toss away some comfort by consolidating.

In my opinion, a mini-split system should be designed in a way that makes takes low load operation into consideration because of this characteristic. That would mean, for example, putting bedrooms on smaller outdoor units if that is possible.

    Bookmark   May 26, 2013 at 3:16PM
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randall bumped a 20 month old thread, this ship sailed a long ago.

    Bookmark   May 27, 2013 at 1:48AM
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