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Questions about Thermal Mass for Energy Storage
with replies by Thomas J. Elpel

Living Homes: Stone Masonry, Log, and Strawbale Construction.


Questions Policy
      To avoid re-writing my book Living Homes for every person that comes along, please read the book before you write to me. Then, if you have any questions beyond what is presented in the text, then yes, please do write and ask away! I may be a little slow to answer, since I have more than a few distractions, but I will get back to you in time, and I will answer your question to the best of my abilities. Please let me know if I can post your letter and name to the website. Thanks!

      Also, if you have a better answer to a question than I do, or additional useful information, then please send me a note through our E-mail Contact Page, and I'll add your commentary to the web page. Questions and answers on these pages will help guide revisions of future editions of Living Homes.

Does thermal mass work for heat storage where limited solar energy is available?

Living Homes: Stone Masonry, Log, and Strawbale Construction.

Hello, Tom:
      You may recall I wrote you last Fall regarding building a house outside of Helena. As building season approaches I have some fundamental design concerns upon which I would I highly value your opinion.

      We planned to incorporate a large amount of thermal mass in the form of concrete walls and slab. However, I have heard that such designs are not energy efficient where, as here in Western Montana, there can be very little solar energy available during the heating season. My main concern is whether to insulate the concrete walls on the inside or the outside. Some builders claim that insulating the outside would not be efficient because I would have to generate so much energy to heat the mass. What are your thoughts?

Thank you, Kurt

      Thermal mass always helps to store heat, although it definitely works best when there is incoming heat to store. In a cloudy place you wouldn't get much use out of thermal mass for storing sunshine, but it would be helpful in the form of a wood-burning stove to reduce temperature swings (too hot when the fire is going/too cold when the fire is out). Helena, however, is very sunny, though some winters are more cloudy than others. On average, Helena gets a tremendous amount of sunshine in winter, and it would be very worthwhile to capture all that you can in thermal mass.

      Even concrete walls without any solar exposure are useful as thermal mass, so yes, I would recommend putting the insulation on the outside of the wall. The walls will absorb heat from the air when it is available in surplus, and gradually discharge it when the air is cooler. A house without any notable thermal mass will only be able to store heat in the air, and that's not very much.

      As you may know from reading Living Homes, we heat our house with a masonry fireplace about once a week on average through the winter. The thermal mass radiates heat for about three days, and the house gradually cools off over the course of the week. On sunny days we pick up enough solar gain each day to offset the loss of heat during the same twenty-four hour period. Thus we light the fireplace less than once a week during sunny spells, but more than once a week during extended cloudy weather.

      I hope this helps!


Thomas J. Elpel

Would a stone house be more efficient with the insulation on the outside?

Mr. Elpel,
      I purchased your Living homes book and Slipform Stone Masonry DVD and am very interested in starting work on a new house using some of your methods. A lot of what you say makes sense and is quite thought out. I'm still in the planning and drafting stanges on my house (to be built in Missouri) and I came up with an interesting idea that I'd like your opinion on.
      I was reading an article on insulation and thermal mass arrangement, when the idea came to me. The research essentially concludes that the optimal configuration for energy efficiency is a large thermal mass surrounded by insulation instead of vise-versa. I do realize that much of your focus for your stone slipforming is on asthetics as well, but upon further reflection, using the SIP panels as the exterior wall and a concrete "slab" for interior walls has potential for some cost savings as well in addition to other plusses...

  • The need for gypsum board is eliminated.
  • There are fewer thermal bridges to the outside environment (possibly eliminated if you can insulate the footings).
  • Exterior siding or facia can be of any type (plus or a minus depending on perspective).
  • Reduced amount of concrete needed for same-sized house.
  • No need for rock gathering/placement as the concrete is not seen.
  • Optimal configuration for energy conservation.
  • Optimal configuration for interior moisture control. (.pdf file)
Minuses I can see are:
  • Need for exterior siding.
  • Need to be more attentive to outlet/service placement (conduit would be almost a necessity).
  • Difficulty of mounting interior cabinets/fixtures.
  • Increased amount of SIP insulation needed.
Do you have any thoughts on this idea?

Thank you for your time and any advice you could provide,



      Sounds like you have put some thought into this. You are correct that the thermal mass is most effective on the inside. The reason we put the stonework on the outside instead of the inside is only for aesthetics--a stone house doesn't look much like a stone house if you can't see the rocks!
      To build a stone house with the thermal mass on the inside, consider adding a greenhouse to the south side, such that the sun will warm the stone wall in the greenhouse. There is no need to insulate this stone wall. It will absorb heat from the sun and radiate it through to the house.
      A second option is to build part of the house with a double stone wall, using stone inside and outside, with insulation sandwiched in the middle, as shown in Living Homes.
      A third option is to build an attached garage on the north side, with the insulation in the garage, rather than in the house. All the other walls might have the mass on the outside, while this one wall would have the mass--and the stonework--on the inside.
      If the stonework isn't important to you, then consider using the beadboard sandwich panels, as mentioned in Living Homes. These panels have an insulated core with wire mesh on both sides for stucco. Although more costly, you could rapidly assemble a house and shotcrete the walls inside and out. If desired, you could even add some stonework on the outside for looks--maybe a couple feet up from the ground to give the appearance of a stone footing. I hope this is helpful. Let me know how it goes.


Thomas J. Elpel

Living Homes: Stone Masonry, Log, and Strawbale Construction.
Check out Living Homes: Stone Masonry, Log, and Strawbale Construction.

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