Green is Not Black or White

Posted September 9, 2008

When looking at environmentally friendly products there is always a compromise involved.  For example in the production of Oriented Strand Board (OSB) they can use small weed tree species for the wood, but they have to use a lot of the phenol formaldehyde resin, which when traced back to its components is usually made from fossil fuels (natural gas and other hydrocarbons).  Plywood on the other had uses larger trees, but the entire tree is used and there is a lot less resin used.  So on one side, when you use plywood you have to use more wood, but it is a renewable resourse.  On the other hand when you use OSB, you use smaller trees, but you have to use more fossil fuels in its production. 

There is always a compromise when selecting a building material.  Some will be made from more renewable resources, such as wood, but will be causing the cutting of forests.  Others will use non-renewable resources, such as concrete, but will not rot and if properly built and maintained, could last for thousands of years.  A decision must be made that will take into account the environmental costs of a material versus the appropriateness and the longevity of the product. 

An example from my own house is the siding.  The options that were considered were natural wood, vinyl and fibre cement board.  The advantage of wood was that it is a renewable resource.  The disadvantages are that it requires the cutting of fairly large trees, and it requires regular refinishing, and will eventually rot.  The second option was vinyl which has the advantage of being inexpensive.  In terms of the environment, vinyl is a disaster both in the manufacture and disposal (watch Blue Vinyl).  During the manufacture of vinyl there are a number of toxic chemicals released into the environment, and after manufacture, if it is burned, it produces dioxins and a number of other toxic fumes.  Fibre cement board is made of concrete and fibre, so it has a fairly high embodied energy, but it does not require refinishing as often as wood and will never rot, so should last as long as the house.  Fibre cement board has the added advantage of being fireproof, and so will increase the saftey and possibly the longevity of the building.

When looking at a material for building, all the environmental factors need to be taken into account such as the production of the material, how far it is transported, the longevity of the material and if the material can be recycled at the end of its useful life.  Only once you have considered all the factors in the lifespan of a material can you make a valid choice as to which product is the right product for your situation.  There will always be a compromise involved in chosing a material, and sometimes what appears to be the best choice on the surface can be shown to be less than optimal when all the factors are taken into account.

Posted under Concepts

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

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.

Posted under Concepts