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Community and Q&A

Geothermal+DHW+Radiant Heat

marcussax | Posted in Green Products and Materials on

As a newcomer to GBA I ask for pardon if this question is worn out. I’m in a very preliminary design phase for an 18K square foot residential home in central North Carolina. The design is to incorporate precast insulated concrete panels, and SIPs with radiant floor heat on 2 levels. Does adding radiant heat along with air conditioning and hot water make a better case for geothermal? I’ve gotten conflicting information and recommendations from several HVAC folks. All experienced and educated feedback is welcome.

Kind Regards,

Mark Payne

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  1. charlie_sullivan | | #1

    Here are three points that are pretty standard responses to what you are asking:

    1) Geothermal is very expensive, and isn't much more efficient than the best performance alternatives. You can get to the same bottom line as far as energy consumption or carbon footprint cheaper by improving your envelope, or after you've done all that makes sense there, buying PV panels. You can often get all the wet to net zero electric consumption for what it would have cost to install a geothermal system that only got you part way there.

    2) Radiant floor heat is a great way to make a poorly insulated house feel comfortable anyway. But if you are building a well insulated house, it isn't going to be as luxurious as it sounds. If the floor temperature is warm enough to feel luxurious, a well insulated room will overheat. It can still work--you can run low temperature water in the tubing and avoid overheating the room, but the floor will be cooler than body temperature and won't feel noticeably warmer to walk on than a simple well insulated floor. In summary, there's nothing wrong with radiant floor heat, but it's not particularly beneficial. I would say the biggest benefit is that you can do heating more quietly than with forced air and with less intrusion on the space than with panel radiators. Radiant ceilings are another option to consider that provide about the same comfort level probably at lower cost.

    3) SIPs are something that works, but not necessarily the most cost effective way to get excellent insulation nor the most durable. There's a new blog post with an excellent overview of the most cost effective ways to make a wall that has truly excellent performance, both in terms of energy performance and long-term durability.

    If you like the modular aspect, assembling on site instead of building on site, makes wall panels in a factory to assemble on site, but uses construction much more along the lines of what is recommended in that blog post.

    You'll probably also get some flak for the size of the house, if that's for a single family, not a dozen families. But my take on that is that using the best available techniques for a large building is actually even more important than using them for a small house that would have much lower energy consumption to start with.

    That's just my summary of the conventional wisdom you'll find here, no really an answer to your question. I'm more of a fan of geothermal than most people here--I think it's fundamentally a good idea, but it takes a lot of expertise to design a good system, and the result is high costs and systems that underperform, often both at the same time. Installing loops is fundamentally expensive, and once you add the fact that small sales volumes make the equipment expensive, and the fact that you need lots of specialized design work, the cost can be unreasonably high. But I think that there is a role for those systems in a future energy scenario based on a lot more renewable electricity, in part because they have lower peak demand on very hot or very cold days, compared to air-source heat pumps. And they probably make more sense in a large building than a small one.

    They are not, however, the only reasonable option if you are set on doing radiant floors or ceilings for heat. You can do a hydronic system with air source heat pumps, as reviewed here:

    If you do hydronic, you'll need fan coils for cooling. I'd recommend the ones made by chiltrix for their low fan energy consumption and much lower cost than the also nice ones made by Jaga.

  2. marcussax | | #2

    Hello Charlie,

    Thank you for your input. I looked at the EcoCor site and particularly liked their Super Slab. My home design is motivated by many factors:
    1. Frustration with tract homes I've lived in.
    2. Sloppy trades work noticed after completion and on the jobsite.
    3. Not being able to find quiet, private space in my home when family is present.
    4. Hot and cold air from a minimum of ceiling registers.
    5. Inconsistency of air temperature and quality.

    The radiant floor idea came from the thought that heat should come from the floor and cold should come from the ceiling. I appreciate your point of overheating in a well sealed enclosure. I am rethinking hydronic based on what you've said. Perhaps mini splits will make sense yet they are not very pretty.

    I am a recovering perfectionist with an engineering mind, a head for spacial relations, above average mechanical reasoning, and a lot of experience with tools, materials and have done much of my own trades work. I would like to start a trend for net zero here in North Carolina and have a challenge with "entrenchment" from the mainstream building community. Then there's always the client's budget and short term thinking. Currently I am not a licensed contractor.

    You mentioned durability around SIPs and would like it if you would expand on that. One of my goals is less labor on site, less margin for error (sloppiness) and speed of completion. Looking at the EcoCor project photos, I still see a lot of sticks and what looks like more on site labor than with a ready to assemble SIPs package.

    I would very much appreciate any more perspective you may have.

    Kind Regards,


  3. SwitchgrassFarmer | | #3

    Mark, based on your interests, experience, etc, I recommend you go to one of these seminars before you make any decisions. It would be money well spent for the scope of what you are planning.

    At a minimum, read Dr. Lstiburek's books and/or white papers on every building technique that you are considering.

    PS I live in a SIP home with geo and radiant heat. None of those topologies are as easy as the marketing folks would have you believe.

  4. marcussax | | #4

    Thank you Andrew.

    Would you do another SIP home with geo and radiant heat? I will study the materials you suggest.

    Kind Regards,


  5. user-2890856 | | #5

    Mark ,

    What type fuel is available there ? Prices per unit of fuels available ? Where in NC is this home to be built ? Net Zero is a concern or criteria for this home ? Are you panning an integrated project design and delivery strategy , , team approach as opposed to a bunch of buffoons who claim they are experts all calling the other an idiot ?
    You don't particularly need radiant for the reasons mentioned , then again the floors will feel the same regardless until you get into the lower temp ranges . The house is large , I work on many houses in this size range . Multi Aqua from S C has some very nice alternatives for heating and cooling that require water as the medium as opposed to refrigerant , that can be kept outside for the most part with only minimal invasion into the envelope . Piping material is less costly , water is better than environmentally unfriendly refrigerants . 1 to 2 diffusers can probably handle the largest rooms , quiet and less UGLY alternatives .

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Does adding radiant heat along with air conditioning and hot water make a better case for geothermal?"

    A. A "geothermal" system -- more accurately called a "ground-source heat pump" -- almost always provides air conditioning as well as space heating. Since these systems are already quite expensive, an increasing number of these systems include a desuperheater to provide some of the home's domestic hot water -- because what's a few extra hundred or thousand dollars if you are paying $15,000 to $40,000 for your heating system?

    A ground-source heat pump system has to be engineered and site-built. A fairly high number of these systems don't perform as well as advertised, and the problems are usually due to design errors or installation errors. Common problems include ground loops that aren't sized correctly; unexpected issues related to soil characteristics (issues that effect thermal transfer); and oversized pumps that extract an energy penalty. These problems can often be solved, but they usually involve head-scratching and lots of consultants -- as well as buckets of money -- to fix.

    You're designing an enormous house (18,000 square feet), so it's a fair guess that your budget is large. If you go ahead with your plan, make sure that you budget enough money for an experienced mechanical engineer with wide experience designing ground-source heat pumps.

    Here at GBA, we recommend that designers spend most of their time perfecting a building's thermal envelope. That means focusing on airtightness, high R-values, and high-performance windows. If you invest a little more money than average in these features, you end up with a building that is easy (and relatively inexpensive) to heat and cool. If your mechanical engineer understands how to accurately perform a Manual J calculation, you'll end up with low loads, and you'll find that your building is easy to heat and cool with conventional equipment.

  7. SwitchgrassFarmer | | #7

    I laugh at the warm radiant floors thing. Yep, the glossy magazine advertisements show happy barefoot people frolicking about their home. You know what, that was great for the first year, then the podiatrist told me that walking around with no arch support was ruining my feet, and so now I wear ugly orthotic sandals inside anyhow. (The cats do like it though.)

    All of these construction methods (SIPs, radiant, geo) are wonderful, but only within the context of your project, and, more importantly, within the abilities of your project team. Even seemingly unrelated things like what kind of siding you plan to use will have a direct impact on your choice of systems.

    Mark, your personality traits sound like my own, so I think you could end up being your own worst enemy. Perfection and residential home building are an uncommon duo. Everything is a compromise. Even if you do build a perfect home it will be based on your interpretation of same. Inevitably future owners won't value your work similarly, thus making the "ideal" home an unwise financial investment.

    I think you have two possible avenues:
    A. Learn everything you can while working with reputable experts to design your dream home. Then find that rare contractor(s) that is(are) willing to work with someone like you (and me). I can tell you first hand, there aren't many such folks. I was extremely lucky to forge a relationship with such a builder. And yes there was "fire" in that forge.
    B. Go on a year long world expedition, and have a well regarded architect design and manage the construction of the home while you are away. You will be ahead financially, and emotionally.

    Oh, and yes, if you do decide to go with "A", buy an actual hard copy of Hank Rutkowski's Manual J. "Learn it. Know it. Live it."

  8. charlie_sullivan | | #8

    Mark, you asked about SIPs. I don't have direct experience with them--I only know what I have read and heard, so I was hoping Martin would answer your questions, but a quick search for SIP on GBA yield plenty of reading, with the top 3 results discussing rot. This recent thread gives some perspective:

    and it includes a link to a more detailed article Martin wrote.

    As I understand it, one issue is the fact that sealing seams well is critical, and so even though the quantity of site work is reduced, the expertise needed for the site work is still high. Another issue is that OSB in general is less moisture tolerant than other wood products. That's a concern for many construction types, but it is a bigger concern in SIPs because the OSB is a more critical structural component.

    If your local expertise is low, it might be better to use a robust wall system that is tolerant of mistakes rather than aiming for a low labor content design. I think robustness is a key attribute of both of the favorites to come out of Martin's recent wall blog--the Lstiburek incarnation of the double-stud wall ( and exterior foam on a 2x6 wall. And although the EcoCor approach requires some of the same on-site expertise that SIPs requires, I think their system is more robust, and you also have the advantage that the expertise at EcoCor is, I believe, substantially higher than a typical SIP company.

    The idea that heat should come from below and cold from above makes sense, of course. But you'll find that the better the insulation, the less important that is.

    Andrew wrote:
    "Your personality traits sound like my own, so I think you could end up being your own worst enemy."

    LOL. It sometimes seems like GBA is a place where perfectionist homeowners come to bemoan the shortage of builders who want to do things right, and perfectionist builders and architects come to bemoan the shortage of clients who want to do things right. If we weren't so geographically widespread, there would be more happy matches made!

  9. SwitchgrassFarmer | | #9

    Mark, I pasted in a few SIP installation pictures. (Some day I am go to categorize all of my project pictures...)

    The first pix shows one of our roof panels being lifted in place. If you look carefully you can see that we actually have metal flashing inside the house, between the ridge beam and the SIP. This is a Lstiburek suggested detail to minimize the potential for ridge rot. Some of the previously installed roof panels have been foamed between seams too.

    The next picture shows our west wall. Observe the metal strapping that our structural engineer specified to prevent interior sheetrock cracks at the windows, and the strapping from the ICF foundation to the SIP panels. Note also the significant amount of structural lumber still used, and that had to be isolated with exterior insulation to prevent thermal bridging.

    The last picture shows some of the interior seam sealing that was done. First a bonding primer is painted on the intersection of the seams, and then the tape is applied. You can also glimpse the aforementioned interior ridge flashing too.

    What you haven't see so far is that we also have full rain screen siding and cold roof systems. Those techniques were used so that the SIP assemblies can breath and not rot, critical because, as Charlie pointed out, these panels are structural in nature.

    So yes, you can build the shell relatively quickly with SIPs, but all the stuff afterwards is what eats up time and money. (And, you also spend a lot of money upfront for coordination that with stick framing can probably be done on the fly.)

  10. marcussax | | #10

    Hello Again,

    I would like to express gratitude to Andrew, Charlie, Martin, Richard and GBA for this site and the Q&A. I've learned that if I stay as open minded as possible, there is something to learn from every exchange. While I may not agree or like everything I hear or read, bigger perspective is better. It may change my mind or reinforce a present idea. I have been my own worst enemy in my pursuit if perfection which is an illusion. What I'm getting to, often, is excellence when I pay attention to detail. My evolving role seems to be a supportive and motivational design / build team leader. The ability to find and work with reputable professionals with an abundance of integrity and a central goal is an ongoing challenge. There is no substitute for experience and I need several more lifetimes for all the experiences I'd like to have. Continually growing my education is a big part of my journey to building reputation, self respect and the respect of others. GBA is a great place to come for that.

    Again, many thanks to all.


  11. marcussax | | #11


    Thanks for the SIPs pictures. An idea that's gelling in my head is having a crew that sets panels, experts because that's all they do. Then a crew that are the "detailers" who attend to everything with sealing. I'm going to make myself a manual of all the tips and details I can find around SIP sealing. I've taken the SIPA course, am noting your details from your last message and will comb the articles you've suggested. I ordered Lstiburek's building science book for mixed-humid regions. Are the straps for the drywall cracks addressing seismic issues or temperature / humidity concerns?
    I'm still getting a handle on managing air quality with respect to smart ventilation. Whether it be ERVs and / or something else.

    One of the more progressive design / build firms here in Winston Salem put up 3 new modern design residential homes. I asked one of the principals about SIPs and he said they went with 6" studs and spray foam because there were too many headaches working around the lack of awareness in the trades community here.

    Warm Regards,


  12. marcussax | | #12


    Would you clarify "Multi Aqua from S C"? My internet search yielded nothing useful. I like your point about water versus refrigerant.



  13. Expert Member
    MALCOLM TAYLOR | | #13

    Looks like a really nice house. Any chance of a pic of how it turned out?
    I've (unfortunately) had quite a bit of experience with carpenter ant infestations, and like you haven't found any evidence of the widely held belief that the building materials have to be damp to attract them.

  14. user-2890856 | | #14

    Here you go mark.

  15. SwitchgrassFarmer | | #15

    Malcolm, it's continuing to turn out...this has been a multi-year journey.

    Our 2015 project was the deck, associated retaining and support walls, and the bluestone front steps. (The bluestone steps required jackhammering out a slab that wasn't quite right the first time.)

    What you see in this November '15 picture is the naked ICF foundation; we had to tie into it for the deck. The ICF blocks were covered with Permabase board in December and this spring '16 we will be doing a white Senergy stucco covering over them and the CMU walls. So long term the house will look pretty much the same visually. (I could go on and on about ICFs, but that question wasn't asked here...)

    Also have attached a picture showing how rough our cats have it, late afternoon the other day. (Inside was complete 4/1/15.)

  16. SwitchgrassFarmer | | #16

    I would starve as a travel agent, because I wasn't able to convince you to take a cruise versus building a SIP home.

    The strapping etc on our house is due to the high wind exposure for our site. We take a pounding up here on the hill, even our pole barn was specially braced. I think we got those precautions largely right everywhere; the only thing I wish we had done differently was use the big (fender) washers on our SIP screws for the roof. They weren't a recommendation from our SIP supplier, and I didn't learn about them in time.

    Adding to your library, I think you might also want to purchase Lstiburek's "Builder's Guide to Structural Insulated Panels (SIPs) for all Climates".

    While true that you can do a lot of the seam sealing work post install, there is a need to do foam sealing throughout the install and subsequent construction. You almost need a foam gun on your belt more than a hammer.

    The integration with other trades is an issue with SIPs, but primarily for your electrician, because it's generally good practice to avoid plumbing and HVAC in exterior walls. While SIPs usually have chases for electric runs, doors and windows complicate that task let alone any ceiling fixtures on the underside of a SIP roof. BTW those chases, ants love them, they are like a super highway throughout your home. I have had to do a lot of perimeter spraying and spreading of diatomaceous earth; now the ants fall out of the outlets dead. (Yes, we have looked for sources of moisture, haven't found any. I think it's just the pressure from living adjacent to an unhealthy woods. I have been pondering whether it would be possible to retro fill those chases with insect resistant spray foam:

    One good aspect of SIPs, some things are easier, your drywall guy will have a breeze. Can't go wrong hanging a heavy picture on an outside wall either.

    Switching gears, if you like the notion of a water based heating and cooling system, start to read articles by John Siegenthaler, He is "the industry expert" on radiant, has written application papers and manuals for a number of the geo system manufacturers, etc. (John designed the geothermal and radiant HVAC system for my home.)

  17. Expert Member
    MALCOLM TAYLOR | | #17

    it looks great.

  18. marcussax | | #18


    Shale Acres looks great! Do you have a PV array?

    Since you mentioned it, go on and on about ICFs ... please.


  19. SwitchgrassFarmer | | #19

    Malcolm and Mark, thanks for your kind comments.

    No, we do not have a PV array. Thinking about it someday, but our electric bill isn't such (yet) that it makes sense to pull the trigger. One nice thing is that we could likely be Net Zero if we did go with a PV system. (Of late we have been reading up on the Zero Energy Ready certification.)

    Wind, we could definitely do wind up here, but that's kind of like building a SIP home. Very cool but not the best cost/benefit ratio. *smile*

    Mark, I suspect Martin would prefer you pose your ICF question(s) in a newly titled thread or annex on to another existing one.

    I will leave you with one thought though: ICFs are like painting. It's all in the prep. And it's when you can get the pumper truck. If you can get the pumper truck two days from now, or two weeks from now, pick two weeks from now so you are absolutely sure that the ICF prep is correct.

  20. marcussax | | #20


    I promise this will be my last post on this thread. Let me know if I get your message about SIPs correctly.

    1. If you were building another home, it would not use SIPs.
    2. There are other ways to get the same performance which would be less costly.
    3. Any savings on wall building labor will be consumed by sealing prep and application and other moisture mitigation and insect control measures.

    If the above is true, what would you do differently having the Shale Acres experience?

    Best Regards,


  21. SwitchgrassFarmer | | #21

    Mark, you have accurately captured my sentiment about SIPs. I would recommend looking at some of the double wall techniques instead, maybe still with a SIP roof.

    I can't give you a straightforward answer as to "what would you do differently". Frankly, if I had a book of coupons for a time machine, I wouldn't use one for this house.

    My choice of SIPs has a lot to do with my personality; I love to do things that are unusual, challenging, etc. I tend to thrive in complex, bewildering, and yes even disastrous situations. We had all that here at Shale Acres, particularly with the SIP to ICF interface. The good news is that out of that mess we ended up with a project team that works quite well together, and, believe it or not, an end product that is significantly better than if things had gone well.

    The problem is that the aforementioned morass takes time and money, and, most importantly, an unbelievably patient spouse if you have one. In the end you have to ask yourself, what is important, the journey or the destination? I like the journey.

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