Another Take on Passive Solar Design

Posted September 11, 2008

The home built by John Kosmer in New York State (I know it’s not Canadian, but the climate is similar) has an interesting method of using thermal mass. Instead of just having the sun heat the thermal mass directly, his thermal storage consists of a 12″ slab of concrete with air ducts running through it. Hot air is collected at the top of the building and then forced down and through the concrete. This has the advantages of heating the concrete from both the top (from the sun) and the bottom (from the air ducts) allowing for a larger amount of heat to be stored.

The house is also completely surrounded with 4″ of rigid polyurethane foam, giving a R27 of insulation on not only the walls but also under the floor and on top of the roof. Using the foam outside the building envelope gives the advantage of having the dew point (the temperature at which the moisture in air starts to condense) in the foam insulation which is waterproof. Due to the dew point being outside the stud wall fibreglass batts within the wall cavity will never have a problem with moisture condensing within the fibreglass. When fibreglass gets damp the insulation value plummets and quite often sags, permanently reducing the effectiveness of the insulation. The other advantage to the foam is that it provides a very good air seal, reducing air infiltration through the walls and roof.

The windows that he has used are energy star rated casements, which as discussed before have the advantage of having amongst the best seal against air infiltration. In a typical passive solar design, as you put more insulation in the walls and ceiling you reduce the amount of windows on the south side of the house in order to reduce the possibility of overheating. With his design, the window expanse is quite large relative to the floor space, but overheating is avoided by having the large thermal mass that is heated from both the top and bottom, essentially creating a larger heat battery. There is also a four foot overhang on the south side of the roof to shade the windows during the summer and so reduce the cooling load on the house.

The passive solar heating is very effective. On a January day when the outside temperature ranged from between -23C to -13C, the interior temperature of the house rose from 20C in the morning to 24C by about 2PM. The total heating costs for the 4000 sq ft home are estimated to be between $900 and $1200 with oil at $100/barrel.

The home has a number of other green features, such as the use of tiles made from recycled material, bamboo flooring, 50 year shingles and James Hardie Hardieplank fibre-cement siding (also with a 50yr warrantee). Auxillary heating is provided by an energy-star Baxi propane fired boiler with a water to air heat exchanger, a Vermont Castings woodstove and a two panel active solar hot water heater.

Even though he included a lot of energy saving and collecting features into the home, the cost remained quite reasonable at around $125/sq ft, which is very comparable to typical new home construction in his area but has the advantage of having energy bills that are about 70% less than traditional energy-star homes

To see more on John Kosmer’s home, take a look at his website at

Posted under Projects

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