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Maximizing thermal mass in passive solar high desert home

ztrain727 | Posted in Energy Efficiency and Durability on

Hello –

My partner and I are building a small (600 sq ft) house in climate zone 5B (NM/CO border). When we lived in Colorado, we had friends who built commercial passive solar greenhouses using south-facing glazing and water tanks for thermal mass. Using their formula, we calculated that we would need approximately 1500 gallons of water to meet their design specs for a self regulating high desert (7k ft elevation) house of this size.

The design specifics of storing that much water in an already very limited living space is an obvious practical constraint. As such, we were considering a couple of strategies, including:

1. A dark colored in-ground hot tub in the living room floor with a reinforced plexiglass cover. This could yield ~500 gallons of water thermal mass.

2. A dark 4″ stamped concrete slab floor over 3″ rigid foam. Since water is 4x as efficient for thermal mass, call this 1200 gallon concrete slab equivalent to an additional 300 gallons of water.

That brings the total up to 800 gallons, so to hit the target we would need an additional 700 gallons of water storage. Some potential options are:

a. Call it good enough, supplement with solar thermal radiant floor heat and a heat pump and propane water heater. I’d like to avoid the expense of a boiler. We plan to do this regardless.

b. Add another in-ground tub for rainwater storage, same design as above but with a fixed cover.

c. Add a covered rainwater cistern below the slab, possibly tied into solar thermal panels, either outdoor or built into the floor. This could easily start to get elaborate and expensive.

Another major consideration for all of the above are building code considerations. While the thermal mass is desirable, we would prefer not to jeopardize our ability to pass inspection.

We are still in the planning phase and fully open to suggestions. Any advise is very much appreciated.


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  1. Expert Member
    Dana Dorsett | | #1

    It's not clear that water thermal mass + hydronic heating/cooling would be the best way to spend the money here.

    Use BeOpt to simulate the house performance to tweak in the optimal glazing size & type, and rely on the thermal mass of the slab, walls, etc. On the slab, 3" of EPS is probably NOT the optimal amount of foam if you're using it as the thermal storage medium- it'll probably need 4" or more.

    With the building envelope fully optimized for passive heating & cooling, the remaining loads can usually be more cheaply & optimally managed with cold-climate ductless air source heat pumps (mini-splits) than with propane. Most of the time at 7K' a nighttime ventilation strategy will work for cooling on a "normal" house, but with a lot of glazing the summertime gains (even without direct sun) might warrant a bit of mechanical cooling, which the heat pump could provide.

  2. GBA Editor
    Martin Holladay | | #2

    You are approaching your house design with ideas from the early 1970s, back when Steve Baer was still removing the tops of cars in a junkyard and using the sheet metal to build zomes. Times have changed.

    "Lots of south-facing glass plus 1500 gallons of water" is an approach that has been obsolete since 1978. It's time for you to go to the library and find some more recent books.

  3. ztrain727 | | #3

    Haha thanks guys!

    Dana - I will look into BeOpt and reconsider the foam under the slab. I know a lot of passiv haus designs sit on very thick foam, so I suppose as much as ultimately fits the budget. I don't think cooling will pose much of an issue so long as the summer solar gain is managed (with overhangs, blinds or curtains) but an air - air heat pump certainly does create some flexibility in that regard.

    Martin - Can I please ask what a good starting point might be in terms of books?

    No doubt things have progressed substantially, but a part of me still feels that what worked then works now. The greenhouses our friends build may be "dated", but they grow tomatoes year round at 8k ft with zero and can be built very economically.

    I would guess the modern approach would be tons of insulation, minimal triple pane argon filled glazing, and a PV driven heat pump. Perhaps that makes more sense from an engineering perspective, but my goal on this project is to meet a healthy compromise of aesthetics, cost and comfort. The budget constraint essentially eliminates super insulated windows, yet I am unwilling to sacrifice the nice views and ample light offered by the glazing.

    So with that said, I don't want to come off as stubborn or inflexible, but would love to "modernize" and improve the design as much as possible without impinging too heavily on the other considerations.

    Thanks so much for your help!


  4. walta100 | | #4

    Those were cool ideas back then in there day. The truth is they never quite worked out, if they had the design would gone main stream and would now dominate the market.

    If money is no object then a “passive house” today hot item.

    If want to be cool then build a “net zero home” with solar electric panels.

    I you are about return on investment then a “pretty good house” is for you.

    Today’s model code home is really a very good home just be aware the local code likely to be way behind the model.

    I know the past is comfortable place but we need learn from our mistakes and live in the present.


  5. GBA Editor
    Martin Holladay | | #5

    For information to get you started, see these GBA articles:

    Green Building for Beginners

    Building a Low-Cost Zero-Energy Home

    The Pretty Good House

    The Pretty Good House, Part 2

    Martin’s Pretty Good House Manifesto

    Here is a link to an article that discusses the disadvantages of the Earthship approach. Earthships resemble the type of passive solar greenhouse that has captured your imagination:

    Earthship Hype and Earthship Reality

    You wrote, "I would guess the modern approach would be tons of insulation, minimal triple pane argon filled glazing, and a PV driven heat pump."

    If your budget is tight, you don't need "tons of insulation." Using the minimum levels of insulation required by code is fine, as long as you pay attention to airtightness. Air sealing is cheap. Air sealing is the step that commercial builders usually skip. It's the key to a comfortable house.

    Triple pane windows are rarely a cost-effective upgrade except in the coldest climates. Ordinary double-pane low-e windows can be fine, especially if you have a tight budget.

    An air-source heat pump is a good way to heat a house -- and it's inexpensive -- but if you live in a rural area, you might be happy with a wood stove. That's what I have at my house. A PV array is optional, although in many areas of the country, it's an excellent investment that will save you money.

  6. ztrain727 | | #6

    Martin -

    Thank you SO much for all those articles! I really look forward to further educating myself here. This is the kind of constructive feedback that makes resources like this Q&A so valuable.

    Good to know about the glass. My uncle built a passive house for a client and used german argon-filled triple pane windows. They were pretty neat. but the client probably spent as much on windows as we have budgeted for the whole house.

    I definitely hear you on the tight envelope. That is something echoed again and again, and for a DIY house this small, I have no problem whatsoever taking the extra time to tape/caulk/insulate every cavity and crack as thoroughly as possible. My father in law was in the energy efficiency industry and has a blower and thermal imaging camera, so we plan to put those to use.

    Anyway... I guess my main hangup is really on thermal mass. I get the issues with over glazing and heat loss at night, but I don't get why one wouldn't maximize the thermal mass, especially in a desert climate where temperatures fluctuate wildly. If thermal mass, either as masonry or water, can be added economically to an already tight envelope, is there a glaring disadvantage? Does it boil down to cost/convenience/pay off or am I missing something else?

    Thanks so much for the goldmine of information. I have some reading to do! Also starting playing with BeOpt - very cool stuff!

  7. Expert Member
    Dana Dorsett | | #7

    In a dry zone 5B climate the sweet spot on windows is usually a double low-E double pane, with a low-E coating on surface #2 (the interior side of the exerior pane) and second low-E coating on surface #4- the surface that is in contact with the room air. The coating on #4 reflects heat back into the room, and reflects mammalian body heat back at the mammal, it's a real comfort upgrade. Since the heat is being reflected rather than absorbed or passed through to the great outdoors, the coating surface #4 lowers the absolute temperature a bit, which in colder climates than yours can yield copious window condensation and loss of performance during the coldest hours of the year. But in a 5B climate it's triple-pane type performance in a double-pane window.

    Most double low-E glass runs about U0.20-U0.23, but the SHGC can vary by quite a bit depending on what the surface #2 coating material happens to be. For instance:

    A Cardinal Glass unit with LoĒ -180 on #2 with i89 on #4 runs U0.20 (argon fill) with an SHGC of 0.62, which is twice the heat gain of a typical code-min U0.32-ish single low-E.

    However, with their LoĒ-340 on #2 with i89 #4 it's also U0.20, but with an SHGC of 0.17, about half that of a typical code-min window.

    For passive solar the LoĒ 180 + i89 would be appropriate for south facing windows, and would deliver the same gain at about half the total window area compared to a typical code min window, at LESS than half the heat loss (about 30% lower U, about half the total area.) But for west facing windows that can't be shaded with overhangs to kill the late afternoon gains in summer something lower gain would be more appropriate.

    At 7K' of altitude there are pressure issues for insulated glass units to be aware of & specified when ordering glass.

    More thermal mass doesn't hurt, but beyond a certain point it's not worth paying for. Building the exterior walls with high thermal mass materials inside the insulation layer makes sense, as does an insulated slab floor in rooms that would be getting direct sun. Building partition walls that would be getting direct sun with masonry can also be useful. But tanks full of water to hit very high thermal mass for crazy-high thermal mass numbers is just silly, especially if you're optimizing the glazing specs.

  8. GBA Editor
    Martin Holladay | | #8

    Here is a link to an article about thermal mass: All About Thermal Mass.

  9. Expert Member
    Michael Maines | | #9

    Zane, don't automatically assume that you can't afford triple glazed windows. While it's true that their contribution to energy savings is sometimes exaggerated, they do save energy, and can greatly improve comfort in some situations. European-style UPVC triple glazed windows can be found for not much more money than double-glazed Marvin Integrity windows. Granted, Integrity is not the cheapest windows on the market, but pretty reasonable for the quality. Marvin Integrities are available with triple glazing as well; our local place estimates a $20/sf upcharge, but other places I've heard basically double the price. So do some research on what's available locally before discounting triple glazing altogether. But like Martin said, also don't feel like you have to have triple glazing to have an energy efficient house.

    Besides that, I agree with the other advice. The 1970's experiments were super cool. I'm starting design on a renovation to one now. But it turns out that insulation and air sealing really does work better than ducts running through thick layers of gravel under an uninsulated, raised slab...

  10. ztrain727 | | #10

    Thanks for all the help everyone! I have decided to take your advice and ditch the water storage in favor of a simple, tight envelope and a brick/slab floor.

    Sorry I dropped off for a while. I’ve been traveling for an extended period and have finally settled again. I did get a quote on triple glazed windows and they came in at $12k which unfortunately for a house this small blows our budget. Still glad I investigated.

    I have a question regarding framing. We are planning a shed roof and because of the dimensions, we must either plan for standard wall heights (8’, 10’, 12’) or standard pitch (2/12, 3/12, 4/12) and non standard wall heights (9’, 13, etc.) It seems to me the lost time from not using stands pitched on the speed square would quickly be recouped by not having to trim all studs/osb/drywall/lathe to customs dimensions. We are fairly far along in our floor plan and would prefer not to revamp the dimensions.

    Another question is if we do a brick floor, would it be appropriate to have a sandwich of 2” eps, a layer of rammed earth, then sand and bricks to gain some thermal mass. Is this asking for trouble regarding mold/moisture retention for negligiable gain?

    Many thanks for all the excellent resources. They have been very good reading!

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