An Explanation of Thermal Mass

Posted September 8, 2008

I discussed thermal mass briefly in the post about passive solar.  Thermal mass is the ability of a material to hold heat and slowly release it back into the environment giving a flywheel effect.  All materials have a thermal mass,  everything from air to concrete.  The thermal mass of a building will store heat that is generated by burning fuel, or is collected from the sun.  The thermal mass can either be exposed in the building, such as a mass wall found in a passive solar structure, or can be hidden and the heat is carried to it in an active solar system, such as the hot water tank in a solar hot water collector.  The ability to store heat varies from material to material and is known as the specific heat capacity.  The following table shows the heat capacity of common building materials along with the density and the heat storage per volume

Heat Capacity
Heat per volume
Water 4.18 1000 4.18
Gypsum 1.09 1602 1.746
Air 1.0035 1.204 0.0012
Concrete 0.88 2371 2.086
Brick 0.84 2301 2.018
Limestone 0.84 2611 2.193
Basalt 0.84 3011 2.529
Sand (dry) 0.835 1602 1.337
Soil 0.80 1522 1.217
Granite 0.79 2691 2.125
Wood 0.42 550 0.231

For a material to be used in a building for thermal mass, you want a good combination of heat capacity and density.  As you can see, air has a higher heat capacity than concrete, but due to the low density of air and the high density of concrete, concrete can hold nearly 2000 times as much heat as air.   Water has the best heat capacity per volume which is why some passive solar installations have tubes or barrels of water in the building.  The problem is that water is a liquid and has a tendency to leak when you don’t want it to.  Of the other common materials, concrete has amongst the best heat capacity per volume, is inexpensive and easy to work with.  This is why concrete is commonly used as the thermal mass in passive solar buildings. 

In a passive solar design, it is preferable to have the thermal mass directly exposed to the sun in order to capture the most heat.  A good example of this is to use concrete for a floor or to build a concrete or stone wall close to the windows (generally less than 10 ft) so that it can act as a heat absorber.  A common way to do this is is to build a stone fireplace surround or feature wall.  A way to add thermal mass to a frame building is to use a double layer of drywall on walls that are exposed to the sun.

One thing to be cautious of when building with thermal mass is to not have too much thermal mass.  In some of the early passive solar buildings, large amounts of thermal mass were used in the form of stone and concrete.  During the operation of the homes, it was found that the thermal mass would continue to absorb heat all winter, only to release it in the summer.  It has also been found that only a portion of the thermal mass is absorbs and released heat during the day, for example with concrete only about the first 4 inches are active so very thick concrete walls can be counter productive.  Also remember that if the thermal mass is exposed to the exterior of the house, it should be insulated on the exterior.

A note on the units.  J=Joules, K=Kelvin.  1kilowatt-hour of electricty is equivilent to 3.6MJ of energy.

Posted under Concepts

19 Comments so far

  1. Martine September 29, 2008 9:51 am

    Thanks for a good discussion of thermal mass. It’s a concept we tried to incorporate in preparing for our woodstove install. If you’re interested in reading about it, we’d love to have your comments/suggestions. See posts dated 2 and 22 Sept 08.


  2. Suzanne January 4, 2009 6:31 pm

    Is there an easy way to figure out how much thermal mass one needs based on the size of south facing windows? The formula I heard was 5.5 sq. ft. for “lit” mass and 40 sq ft for indirectly lit mass (reflected light or in the same room with lit mass). However how does one factor in the tracking of the sun throughout the day, and the difference in lengths of days as a factor?

  3. Ward Edwards January 5, 2009 12:58 pm

    It would depend on the type of mass, as different materials can absorb different amounts of heat, as well as a number of other variables, such as the window type, the placement of the windows, sun angle, etc. I have a 3.5″ concrete slab floor and I find that in general that it provides sufficient mass. You could do a calculation where you calculate the heat input from the windows using solar insulation data and use that to calculate the mass required.

  4. Annie January 31, 2009 8:21 pm

    This site is so amazing. I’m just starting to read the posts & discussions. I’ll need a lot more time to catch up w/ all these info. What a great place to start learning about green energy. Thanks! (to everyone)

  5. Kai Korinth February 16, 2009 5:17 pm

    Can thermal mass actually be used to store the sun’s heat in the summer and recover it in the fall and winter?
    Can an in-house cistern be used effectively to store that thermal energy?

  6. Ward Edwards February 17, 2009 10:39 am

    There have been some attempts to store summer heat for the winter, mostly using earth tubes or heat pumps. In fact that is essentially what a ground source heat pump does.
    An indoor cistern can also be used, but are usually used with active solar thermal systems. One passive solar system that uses water has barrels of water in the area exposed to the sun during the day. Water is a very good heat storage medium, as it has one of the highest heat capacities of any commonly available materials.

  7. Darren March 5, 2009 3:24 pm

    Dear Mr. Edwards,
    I am contemplating building a passive solar collector to augment heating in Winter. The emphasis will be on simplicity (no fluids except air). Is there a formula that allows one to calculate the needed amount of a collector material based on a given insolation, a given thermal conductivity and a given output air temperature? The emphasis here is to use just enough Al or Ag as a collector and to harvest the transmitted heat as soon as possible. A high heat capacity and too large mass may waste a good portion of a day in heating the mass to a useable temperature. Is there a formula for the amount of potential peak insolation that a system should be designed to harvest? Simply using the peak capacity of maximum insolation may waste time and materials, ultimately affecting payback time.

  8. Ward Edwards March 5, 2009 5:06 pm

    For a solar thermal hot air collector, you want the thermal mass to be as small as possible. This is so that the unit can respond rapidly to changes in sunlight and heat up rapidly to a high temperature. For a hot air collector, you want the collector to get as hot as possible in order to increase the heat transfer to the air (the higher the temperature difference between the collector and the incoming air, the more heat will be transferred). You might want to do a google search on low mass sunspaces, an effective hot air collector.

  9. Sue Richard March 8, 2009 9:54 pm

    My husband John and I are building a new home. We bought and built to lock-up, a kit cottage through Home Depot. We were very impressed with the ease this home went up, with just two days help from the neighbours, and two four hour rentals of a Hi-ab unit. We began the foundation in June and even with numerous set-backs, had it to lock-up by Dec 2009. We would like to continue our build as eco friendly as possible. Do you know where we can get more imfo on low cost (running) heat systems, we would like a wood burning unit for when we are home, however, we travel a fare bit, so we also need a self supported alternative as well for back-up. Eco friendly materials for our interior finishes, an added full 32 foot front porch, (south facing), a hot-tub gazebo and efficient but eco friendly insulation. Would you also know if there is such a thing as recycled plumbing and/or electrical materials.We definately want to build a solar panel/wind power system also. We are on the grid now, and our build is in B.C., so we’ve been told that the energy companies there will pay you for energy put into their system from a home owners system. Any other ideas you may have would be much appreciated. This is a build for our retirement. We have been mortgage free since Sept 2007 and plan on staying that way. Both my husband and I will be 50 in June and have worked towards this goal for twenty years. Thanks for all your help.

    Sincerely, Sue & John Richard

  10. Darren March 9, 2009 1:34 pm

    Mr. Edwards,
    Thank you for your previous answer. Can thermal mass in a hot air collector be too small to be efficient?

  11. Ward Edwards March 9, 2009 9:05 pm

    For a low cost backup heating system, considering the raising cost of fossil fuel (it’s cheap right now, but don’t expect that to last) is to use electric baseboard heat. Electricity prices are not expected to raise as fast as other fuel sources, and it has the advantage of not having to ship it in like propane or fuel oil. I believe that BC uses generates most of it’s electricity from hydroelectric stations, so that would also be the greenest. For your other questions, I suggest you join the greenbuilding mailing list at That group has a lot of good answers. For renewable energy, I have found Home Power Magazine to be a great resource

  12. Ward Edwards March 9, 2009 9:08 pm

    If you build it with conventional materials (wood box, aluminum heat collector, etc) you shouldn’t have to worry about not having enough mass. Also the heat is not stored in the unit at all, but is transferred to the interior of the building almost as quickly as it is collected, so the only problem you might have with thermal mass is to have too much of it.

  13. bob mitchell March 2, 2010 11:16 am

    i need to know the thermal mass properties of 8″ & 10″ wide clay masonry units core filled with spray in place uratane
    Bob Mitchell
    manager london

  14. mike crawford April 13, 2010 6:17 pm

    If your thermal mass is storing heat through the summer and winter for a year, doesn’t it become saturated at one point and steadilly give out heat for a year ?


  15. Ward Edwards April 15, 2010 1:54 pm

    Thermal mass doesn’t generally work on that long a term. If the air is hotter than the thermal mass, it will heat up, if it is cooler, the thermal mass will heat the air. This way you can even out the daily temperature changes and use some of the extra heat that can be captured during the day and use it at night. Also the thermal mass cannot become saturated, it will just heat up to the temperature of what is heating it (sunlight, air, water, etc), once that heatsource is removed, or drops down to a temperature that is lower than the thermal mass, the thermal mass will start radiating.

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  17. Alex Cicelsky September 9, 2011 4:25 am

    There are many websites which contradict each other on the issue of ‘how much thermal mass’. This is the first I’ve found where the contradiction is in the same paragraph:
    a. “One thing to be cautious of when building with thermal mass is to not have too much thermal mass…it was found that the thermal mass would continue to absorb heat all winter, only to release it in the summer.”
    b. “It has also been found that only a portion of the thermal mass is absorbs and released heat during the day, for example with concrete only about the first 4 inches are active so very thick concrete walls can be counter productive.”

    Is the second case conditional on exterior insulation? Where is this information coming from (it is universally copied but may pertain to a specific climate and house design)?

  18. Ward Edwards September 9, 2011 8:57 am

    I probably should have been more accurate with the wording. Case (a) is for a long term, over the course of a year, case (b) is daily. You will get the heating and cooling of only the surface 4 inches or so during a day, so the heating during the day that is released during the night (the property that we are most interested in for the majority of buildings) is restricted to that zone. In the longer run, the deeper part of the mass will accumulate or dissipate heat at a much slower rate, but will affect the surface 4 inches that is active during a daily cycle. The source of my information for this is from a talk that was given at a green energy fair a number of years ago. The speaker had returned to a number of passive solar homes that had been featured in a Canadian magazine called Harrowsmith, and found that a number of the homes that had been built with an excessive amount of thermal mass were performing poorly, due to the long term heating and cooling of the mass.

  19. Garry Brisbois February 1, 2012 8:26 am

    As designers of residential hvac systems, how does one figure in the amount of heat (btu’s) being generated by a thermal wall to balance the heat load calculations?


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