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

Building Green

Posted September 3, 2008

Building an environmentally friendly, energy efficient, occupant friendly, affordable house is an art of compromises.  The most environmentally friendly house is no house at all, since if it doesn’t exist, it doesn’t use any energy or materials, and it doesn’t impact the land, but it isn’t very occupant friendly, since people generally want some form of shelter.  You can also build a home that doesn’t use any fossil fuels and generates all its own power, but it will be more expensive than most people can afford.  The goal is to create a house that makes a balance between the needs of the occupants, the needs of the environment and the budget of the owner.

Almost all houses can be made more efficient, and with the more money you have available, the more efficient it can be made.  For those on a very limited budget, there are design methods that give a very high return for virtually no cost.  An example of this is passive solar design which can be done for free, just by proper orientation of the house and the placement of windows. For only slightly more cost, the use of larger, energy efficient, south facing windows and the placement of thermal mass, the heating costs of the house can be lowered by at least 1/4.

Further upgrades can be items such as more insulation in the walls and ceiling, which will further reduce the heating and cooling costs of the building. For increasing the insulation alternative building techniques, such as strawbale, double stud wall, Structured Insulated Panels (SIPs) and Insulated Concrete Forms (ICFs) are some common examples. These techniques can be used to either increase the amount of insulation in the walls, or to reduce the amount of air infiltration through the walls.

If the budget is larger, or as savings accumulate from energy savings, other systems can be added to reduce the enviromental footprint. An example would be adding a solar hot water heater to the home to generate hot water. A solar hot water heater can supply up to 100% of the hot water for a home, particularly in the summer, and even in winter it can still significantly reduce the energy use. For those with a bigger budget, you can add a solar electric panels to the home to generate a portion, or even all the electricity used in the home. With newer equipment, the solar panels can be connected directly to the power grid, and effectively store the excess energy generated during the day for use at night. In some areas, the electric utility will even pay the homeowner a premium for the excess solar power. If the site is suitable, electricity can also be generated using the wind or flowing water.

Even those with an existing home and no budget can reduce their energy usage by using conservation techniques, such as turning off lights when leaving a room and turning down the thermostat. So there is no excuse for anyone to not reduce their energy usage.

Posted under Concepts