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An Explanation of Thermal Mass

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

Material
 Heat Capacity
(J/gK)
 Density    
(kg/m3 ) 
Heat per volume
(MJ/m3K)
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.

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Insulated Concrete Forms

You may have seen the Insulated Concrete Forms (ICFs) being used to build a house, or heard about them in discussions about green building. ICFs are Styrofoam blocks that are stacked to make walls and then they are filled with re-bar and concrete. Unlike a normal concrete wall the forms are left in place. There are different variations on this theme, with most ICFs consisting of two flat 2-3″ pieces of foam separated by a 6-8″ space and attached together by wire or plastic braces. Other ICFs have a waffle like structure on the inside of the form which reduces the amount of concrete used and increases the insulation.

If you read the literature presented by the ICF manufacturers, they come up with statements about the “effective” R value up into the 50 range. This is misleading advertising. Concrete has essentially no insulating value (.08/inch) and the foam has an insulating value between 4 and 5 per inch. An ICF with 4 inches of foam (fairly typical) would then have an insulating value of between R16 and R20, way shy of the R50 advertised. The way that an ICF make houses more energy efficient is that the shell of the house is truly air tight. If the window and door penetrations are properly sealed (spray in foam and caulked), an ICF house would be essentially air tight. As noted in an earlier post, most heat is lost through air infiltration.

There are several disadvantages to ICFs. The biggest is that they use a lot of concrete, and the manufacture of concrete is one of the larger contributors to greenhouse gases in the world. Second is that they are made of plastic, which comes from fossil fuels. Another thing I don’t like about ICFs is that they have plastic foam on the inside of the structure. I don’t like this for 2 reasons. The first is that in the case of a fire, it could possibly produce very toxic smoke. Secondly having the insulation on the inside reduced the effect of the thermal mass of the concrete.

The advantages of ICFs is that they are much more flexible in the way that concrete walls can be formed. With conventional forms, it is much more expensive to have walls taller than 8 feet, as the forms have to be stacked which is much more labour intensive, whereas the ICFs don’t have that limitation. Also, ICFs can be installed by a Do It Yourselfers with the help of a few friends, but be careful to follow the instructions and make sure the walls a thoroughly braced, as a blowout can make quite the mess. Also if you use the waffle type of ICF, you can create a very solid wall that uses less concrete than a conventionally poured wall.

For my house I decided to go with the conventional poured concrete wall for the foundation. I did this because I designed the house as a walkout with passive solar input and needed as much thermal mass exposed as possible. If the foundation was not a walkout, I would have strongly considered using waffle type ICFs for the foundation, but I would be reluctant to use it for the above ground walls due to the high greenhouse gas emissions from the manufacture of the concrete.