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Light-weight thermal mass option for passive solar?

boulder_89 | Posted in PassivHaus on

Hi, I am trying to design a single 40′ long high cube storage container to be converted into a living space. It will be completely off-grid, insulated very well with closed cell insulation. I want to take advantage of as much passive solar heating as I can. The container will have great un-obstructed southern exposure. I want to keep construction weight down as much as I can.

I am wondering if I could have a recess in the floor. Would be roughly 4″ deep, maybe 2 1/2′ wide, and long enough to cover the kitchen, dining and living space (maybe 10′-15′, or wherever I have window space to heat it). The recess would be then filled with 3″-4″ rocks, and covered with a piece of glass.

Would be an effective thermal mass to collect energy? I dont know if the glass over the rocks would be detrimental to the energy transfer, or if the rocks are even a good mass? My idea is that I could take the rocks out to keep weight down, for ease of future transportation. Also could be very appealing to look at.

Other things to consider:
-could also fill in gaps between rocks with water?
-if container is on piers what r value would you suggest for floor insulation? (closed cell spray foam)
-walls will have around an R-35 (closed cell foam), ceiling will have around R-60 (closed cell foam)
-windows will be a quality triple pane window with high SHGC

Please let me know your thoughts, any help would be appreciated

Thank you,

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  1. morganparis | | #1

    Mass is mass so lightweight thermal mass is self-contradictory. The rocks under glass would work but would not be light in weight and removing all that for transport would a lot of work. But in any case will it be useful? Thermal mass solar is not suitable for all locations and occupancy conditions. What are yours?

  2. boulder_89 | | #2

    It would be my main residence with one or two people living in year round and located in souteastern Minnesota (zone6). Lightweight mass was not the best term.... removable mass. I would not plan on moving the mass, although I would like to know I could if I wanted to relocate.


  3. STEPHEN SHEEHY | | #3

    Making the recess would be a lot of work for probably little benefit. Either you need to cut a hole in the floor and put in the recess with a new, lower floor or you'd need to put a new floor with the recess built into it.
    Do the economics of using a container even work? The box isn't the expensive part. Windows, doors, appliances, cabinets, insulation, plumbing and wiring, etc. are, if anything, probably more difficult to install in a steel box. It might not be any cheaper than just building a box. Plus, then, you wouldn't be stuck with the 40 x 8 foot footprint that you need to design around.

    How will you insulate it? Can't really install studs on the inside, because you don't have the space to spare.

    Are you planning to locate it someplace where a crane and 40' trailer can have easy access?

  4. charlie_sullivan | | #4

    Given that you are asking on a "green building" site, we are obligated to mention that closed cell spray foam is made with blowing agents that have an extremely high global warming effect, >1000 X the effect of CO2. It's also expensive, adding to the cost concerns that Stephen mentioned.

    Rocks work as thermal mass. Water is even better. Rocks have the advantage that you can blow air between them, an advantage that goes away if you fill the space between them with water. Water has the advantage that it's easy to remove if you want to later, and the disadvantage that it can leak, cause rust, and increase humidity in the space if it's not in air-tight containers.

    Thermally, a glass floor would behave the same as a plywood floor. Neither is transparent to thermal radiation. It would be more expensive than plywood, but would let you look at the rocks.

    If you tell us why you are thinking about a shipping container, we might have suggestions about better ways to achieve that objective.

  5. Expert Member
    Dana Dorsett | | #5

    Lightweigh thermal mass is NOT self contradictory, because thermal mass is not identical to mass.

    Materials have very different specific heats- a pound of water ( BTU per lb per degree F) has about 9x the thermal mass of a pound of iron (0.11 BTU per lb per degree F.) Pound or pound glass has about 1/6 the thermal mass of water, granite has about 1/5 the thermal mass of water, soapstone about 1/4 the thermal mass of water.

    The apparent thermal mass of waxy phase change materials can be VERY high per pound when operating around the phase change temperature, an order of magnitude or two higher than the thermal mass of water, but only across a narrow band of temperature. Finishing out the interior with high-thermal mass phase change material wallboard can be useful in passive solar designs when operating around it's phase change temperature, eg:

    But for storing large amounts of heat, water (mixed with antifreeze if necessary) is likely to be your best bet. Solar thermal panels with photovoltaic driven pumps, and large insulated tanks works off grid, but that's not exactly passive solar.

    Going high-R with closed cell foam is practically a crime against the planet due to the high polymer per R, and the very high global warming potential of the HFC245fa blowing agent (~1000x CO2 for the 100 year warming potential.) If going high-R, use dense packed cellulose 3.5lbs per cubic foot or higher, which is very low impact, and has a substantial amount of thermal mass. It's considerably cheaper than closed cell foam too. This will eat up about 50% more volume than closed cell foam, but it's really a greener way to go.

    If fiber insulation isn't going to cut it, go with rigid EPS, which is blown with pentane (about 7x CO2), most of which is recovered at the manufacturing plant and burned for process heat (converting it to CO2 and water). Avoid XPS, which uses even more damaging blowing agents.

  6. boulder_89 | | #6

    I would put a new raised floor in the container. The hi cube container is 9'6" tall, so after 8-9" of foam on the roof that leaves 9" of height to play with the floor to give you a finished ceiling height of 8'. As far as walls they would be framed with studs (probably 2x3) set off a few inches off of the container, to prevent thermal bridging. Finished width of container im shooting for is 7'. I am okay with the small width.

    I have gone back and forth on the idea of the shipping container. In my mind I dont have to side, or install a roof. I also like the idea that I am able to move it if need be.
    I know I still have to frame it all in and I think between the roof and siding the cost probably offsets from purchasing the container. If I knew it was going to stay in the same place I would look at another construction method.
    I know the spray foam is not great and is expensive. I would be willing to look at other methods but I would be worried about condensation in the wall with the metal container. Also with limited space that is going to give me the best insulation value.

    Thank you all for brainstorming with me, I am looking for ideas and concerns. I am just trying to build a very efficient space.

  7. boulder_89 | | #7

    I could do a hybrid insulation system with spray(2") to get a vapor barrier and finish with dense pack cellulose

  8. morganparis | | #8

    Dana, thanks for clarifying the difference between mass and specific heat. I'd still point out that none of the thermal mass materials you mention are lightweight. Cameron, you might want to use the GBA search box to review the several useful discussions of shipping containers as construction modules that have been conducted here in the past few years.

  9. GBA Editor
    Martin Holladay | | #9

    You've gotten some good advice here from some smart people. Your project raises all kinds of tangential issues that are worth discussing, but it would take a long time to discuss them thoroughly. If you have time on your hands, use the search box to research these issues on GBA.


    1. Transforming a shipping container into an insulated building envelope is a misguided, expensive approach. It costs more than starting with new framing lumber and OSB (or framing lumber and plywood).

    2. In Minnesota, installing extra interior thermal mass is unnecessary and often counterproductive.

    3. Passive solar design principles from the 1970s are obsolete.

    Here are a few links to get you started:

    All About Thermal Mass

    Reassessing Passive Solar Design Principles

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