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Geothermal vs. Spray Foam


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By Eric in Starkville, MS on 1/29/2009


A question for you readers out there. My wife and I are in the planning phase. Through our research, it appears that the "tighter" a house is built, the longer the ROI for a geothermal system would be. For example, using spray foam for a complete building envelope would, by its nature, virtually eliminate air infiltration. Since this is the most costly loss of heat (or cooling in our case) in a typical house, if a geothermal unit were selected, it would take much longer for it to pay for itself vs. a home built more "loosely." Is this correct?

Thanks guys. Love the forum.

Eric


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By Grant in Blacksburg, VA on 1/29/2009


A geothermal systems saves energy while heating and cooling.  The tighter the house, the less heating and cooling you will need.  As a result, the system will take longer to pay for itself.  That's good news, as far as energy efficiency goes, but bad news for justifying the payback on the energy-efficient geothermal system...

Note, however, that if you build a tight house you MUST have air exchange to maintain interior air quality.  If you don't use an ERV/HRV system for air exchange, a tight house with sufficient air exchange may not be that much more energy efficient than a normally built house.  The REAL advantage to the "tight house" with air exchange, however, is that you control the source of air entry and keep moisture out of the insulation and the walls, and get to filter the air rather than the air coming through the nooks and crannies, where the humidity can breed mold, and where the dust and mold can infiltrate into the house with the air.  The controlled ventilation keeps the air "clean" even if it still must be comparably conditioned as compared to a non-tight house.

In a hot and humid environment, dehumidification can account for 50% of the cooling loads.  Unless you prevent the humidity from entering your house by dehumidifying the ventilation air with an auxiliary system, then the tight envelope will still suck almost comparable air conditioning energy just to dehumidify the air.  In fact, if the "tight" house is properly designed to stay passively cool and ventilation air is constantly bringing in humidity, then you can actually end up with significant moisture issues, if the air conditioner can't run long enough to keep the house dry and yet not over-cool the house. 

It is critical to undersize whatever conditioning unit you select to run constantly enough to dehumidify the air in the house, or else have a supplemental dehumidification system. I'm struggling with these issues myself.  With a tight ICF wall system and an ERV for air exchange, the payback on the higher-cost geothermal systems seems too long to justify... But then again, "something" has to run continuously to dehumidify the air around my parts, whether or not the active systems need to be cooling/heating the air... 

So, I'm looking at a tight envelope, designed for passive cooling and heating, with an HRV to help maintain the "equilibrium."  The HRV will likely have supplemental electrical cooling/heating (cheap to install but highly energy consumptive if run constantly) As I've preliminarily crunched the numbers, such a system running only infrequently for extreme conditions seems to win the cost/benefit analysis, versus seldom-used, expensive to install low-energy consumptive systems like ground-source heat pumps...  But for this to work in hot and humid Alabama, I will likely also need something like a dessicant wheel to control the humidity when active cooling systems aren't running. With the addition of the cost of dehumidification, I'm not quite sure if I'll still come out ahead...

I'm eventually going to cost out this alternative approach against the cost of a constantly running geothermal heat pump without the HRV and dehumidifier...  I don't know yet which one will actually use less cumulative energy and/or be the cheapest to operate...

If I had all of the money in the world, I would put in the expensive geothermal heat pump, with the HRV, AND the dehumidification system and consume the lowest possible energy cost (just to be nice to the environment), but maybe never get a "payback" for my initial investment.  While I was feeling so generous, I'd probably also decide to produce the required electricity with a large PV solar array <grin>.  While I'd love to live such an environmental ideal, it's just not financially feasible at this point...

If you ever want to get to net zero energy in the future, pick the lowest energy load system now.  The supplemental electrical heating/cooling could always be removed from the HRV later and replaced with a more efficient and more expensive geothermal system.  Or perhaps the HRV could be added to the geothermal system in the future...  Which is the best route up front???  I sure wish I had the answer...

The German PassivHaus design experience would suggest that on a smaller scale house, the ERV system wins on up-front cost and life-cycle energy savings.  The benefits of the geothermal heat pump, however, likely scale well to larger houses requiring larger supplemental heating and cooling loads.  The bigger the loads, the more expensive the electrical supplemental heating and cooling systems attached to the ERV will become.  At some point, logic tells me there will be a crossover where the "economies of scale" will make the geothermal system win out...

When I get around to it, the number crunching to make the decision should be fun.  I'm willing to go either direction. Eventually, I'll probably get the various vendors to help argue their own cases, and make my decision from numbers they help to provide. I'll have the geothermal system vendors and the HRV vendors help design the optimal system for my house (including the all-important control of humidity!) and let them crunch the numbers accordingly. I'll make the best up-front cost decision, and then hopefully add additional energy conservation to the system in the future.

Does this make my answer "clear as mud???"


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By Grant in Blacksburg, VA on 1/29/2009


Just an additional thought...  Regardless of the rest of my arguments above, the tighter house allows (actually REQUIRES) you to size for a much smaller geothermal heat pump.  This lowers the up-front equipment costs and even further lowers the energy consumption over time.  Any installed conditioning system in Mississippi MUST be designed to run nearly constantly to control the humidity level in the house.  When you have a tight house that doesn't need as much cooling loads, that means sizing the geothermal heat pump significantly smaller in order to keep it running consistently.

So regardless of the rest of my "life-cycle cost" arguments (regarding which systems are best WITH the tight house), paying for the tight envelope probably pays much of its own cost by justifying a smaller and less expensive geothermal heat pump (or alternative).

With all of the additional benefits of a tight envelope (air quality: avoiding dust and mold, etc.), I think the tight envelope, itself, should never be in question.  The only questions relate to which systems will be the most cost-effective "in conjunction" with the tight envelope.


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By Kenneth in Lees Summit, MO on 1/30/2009


I built ICF, and I would agree with your assessment that as you get more and more efficient, the incremental cost for that next bit of efficiency increases. Eventually the incremental costs exceed a reasonable return on investment.

Given the option (as you laid out) of all things equal would I choose spray foam or geothermal, I take spray foam every day. The reason, HVAC units are always being developed for more and more efficiency. When I was building, the "base" AC had SEER of 10, but if you wished to invest more money you could get SEER approaching 20. Not too many years back, these AC units were operating at 6 SEER. At some point in the future, you will replace the HVAC and have an opportunity to go with something much more efficient than you can purchase today, and this is common. Yet at no point in the future will it be easy to retrofit spray foam.

With one decision, the compromise is reasonable term and easily fixed in the future with a retrofit. With the other, the compromise is pretty much permanent.

Always, ALWAYS, build the box right first. Install energy efficient appliances later.


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By Jon in Ellicott City, MD on 1/31/2009


I'll add another vote for placing the highest priority on the envelope. A tight, high-R envelope is a choice you'll never regret. And, as Ken points out, it's easy to do now and hard to add later

In terms of GSHP payback, I've finally gotten some firm numbers on my own house. For heating costs alone, I'm estimating $1,350/yr for the geo system, compared to $3,367 for oil @ $2.80/gal and $4,810 for oil @ $4/gal. 

I haven't calculated AC numbers. Any savings there, and on the DHW side, I count as a bonus.

There are also some nice tax breaks right now. If you get an Energy Star rated unit, you'll get $2,000 back from the feds. My local gov. also gives a $5,000 credit towards property tax.

For my house in Baltimore, I expect to pay back the initial cost within five years or so. Beyond that, I should be saving $2,000 or so per year. Of course, these numbers are entirely dependent on the prices of electricity and oil.

If I had natural gas available at my site, it would be a tougher choice. But geo vs. propane/elec./oil, at least for my house, is easier to justify.

BTW, Grant, Ken makes a strong case for ventilating dehumidifiers (or is it dehumidifying ventilators?) instead of HRV/ERV. I think it depends a bit on your climate, but it's definitely worth looking into. Do a search and I'm sure you can find the discussion. 

Jon

P.S. Manual J for my house estimates 68,000 btu/hr heat loss and 42,000 btu/hr heat gain. HDD in my area is approx 4,450.


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By Holly in Massillon, OH on 1/31/2009


We are in the planning phase for an owner-built ICF geothermal home. We too were wondering if it was unwise to invest so much extra money into a geothermal system when it will take so long to pay for itself. A green builder we met with sends all of his house plans to energywisestructures.com for a complete design of the HVAC system. He said that the HVAC guys around here don't understand a tight ICF house and recommend a larger system than is necessary. The site has some good info and we are planning to send our plans for HVAC design.

In the following document energywisestructures.com/downloads, they say they are not in favor of air-to-air heat exchangers, which we were planning to install. I'm not sure what we will decide to do there, since humidity is not a significant issue in our climate.
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By Brian in Dome-ville, FL on 2/13/2010


I am building a very tight, efficient, dome house in hot, humid central FL. It's got R-36 all the way around, and should be almost perfectly sealed. It's small; 1,200 s.f. Should I consider an ERV? Since the dome collects heat at the top and is much cooler at slab level, I was planning to use a small 4" duct exhaust fan drawing from the apex where it's hottest and blowing that air outside, and drawing air in from the side of the dome away from the sun. I also considered a dehumidifier, would that save me any energy? I have not looked into dehumidifiers. 

The entire dome will be cooled by a single 12K Btu (11-SEER) window unit; we've tested this and it's plenty of cooling power. I just wondered if I could run the A/C less if I had a dehumidifier, and if it would save any energy.


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By Kenneth in Lees Summit, MO on 2/14/2010


Yeah, I considered an ERV, considered the cost and payback and decided not to. That said, I have the ductwork already installed, I considered that cheap insurance in case I changed my mind later. You need ventilation, an ERV is but one way to accomplish your need (ventilation), but by no means the only way. In FL, your need for ventilation and how to best accomplish it are different than in other parts of the country.

I do have a dehumidifier though (I really liked the Ultra-Aire, but they came with ultra-price, so I chose the Aprilaire). With tight construction, I use dehumidification in seasons others do not. If you can provide complete cooling with a window unit, I doubt this supplement will be much value to you. OTOH it uses very similar ductwork as the ERV, so if you need it later it is easy to retrofit.

As to dehumidifier vs. air conditioner, they really aren't much different. One exhausts heat outside, one exhausts heat inside. Unless you use a dessicant dehumidifier, and when I built these weren't common.


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By Pablo in Mullica Hill, NJ on 5/23/2010


My two cents: I believe that for the pure purpose of dollars and cents the geothermal system is a better investment. Unless you are in a real cold area, in my opinion you could use other insulation systems such as blown-in insulation at a fraction of the cost of spray foam and for the difference you can upgrade to the geothermal system and get the 30 percent tax credit. In the end, you should have a very energy-efficient home!!!
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By Kenneth in Lees Summit, MO on 5/24/2010


Pablo brings up an interesting approach - you can get a very tight house without the expense of spray-foam insulation. However, this takes attention to detail (spray foam takes attention to detail as well) that few builders understand or appreciate.
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By Arne in Houston, TX on 8/10/2010


Regardless of the materials used to seal a home, people often forget to test the effectiveness of the sealing once the building envelope is complete. A "blower door test" will pressurize the home, allowing you to identify and seal leaks that still remain in the structure instead of assuming that the different contractors did their work as designed or you intended it to be done. It's a good idea to do this before you have installed drywall, but after all penetrations from HVAC, plumbing and electrical have been done. 

Achieving a tight home that is well insulated will reduce the size and cost of any HVAC system and allow a better cost/benefit analysis of HVAC options.

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By Brian in Asheville, NC on 9/27/2011


Some topnotch advice here. The only thing I can add is to follow up on what Arne has said and question the effectiveness of spray foam in wall cavities.


Spray foam is awesome. However, if you are only using it to insulate your wall cavities, then you could be missing out big time. Spray foam is not the answer to air sealing. Air infiltration through the walls is actually small compared to the amount that goes through everywhere else; between plates, transitions, ceiling plane, etc...

The blower door test is crucial for finding and addressing these air leaks. Airtightness is THE most cost-effective approach to energy efficiency. See blower door test if you aren't familiar with this test or what constitutes good results.

Also, in terms of a well-insulated building envelope, spray foam in wall cavities does nothing for thermal bridging, which is more important than R-value. It's possible that a 1/2" layer of insulative sheathing will have a bigger impact than upgrading to spray foam in the wall cavities.

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