2.  HOUSING CALCULATIONS

Housing requires materials and energy for construction (the embodied energy) as well as energy for electricity, heating, and cooling.

Fossil energy land

The calculation of fossil fuel land for housing is similar to that of food, except that fossil fuels are required for both the construction and maintenance of housing.

Land area
=
Carbon sequestration ratio
*
Energy intensity ratio
*
Consumption quantity in metric or US standard
*
Metric conversion factor, if needed
*
Waste factor, if needed
m2/yr
=
m2
*
Gj
*
m2 house
*
*
Gj
m2 house/yr

The life-cycle embodied energy of a standard Canadian house with 350 square meters of living space adds up to 1,310 Gj (Canadian Mortgage and Housing Corporation, OPTIMIZE, 1991, researched by Sheltair). Because the life-expectancy of a brick building may be 70 years, the embodied energy is calculated as 1310 Gj/350 m2/ 70yr = .0534 Gj/m2/yr. The average life expectancy of a wood house is 40 years.

Hotel energy costs are estimated from the average resource use of households. Wackernagel et al. (2000) assume an average wooden house in the US (with the land) would cost 150,000 dollars and has 2000 square feet. This corresponds to a monthly mortgage cost of 1000 dollars. In addition, each square foot may use the equivalent of 36 Mj of energy per year, including hot water and electricity, or 3 per month times 2000 square feet = 6000 Mj/month, or 6 Mj per dollar. Apart from the energy aspect, if you enter 1000 dollars a month, you should get the same result as a 2,000 square foot house. If you delete the second term in the energy column (which corresponds to the 6 Mj per dollar operational energy), the energy column also should be the same.

Direct energy usage for a house can be used to calculate the fossil energy footprint (cell G54).

Land area
=
Carbon sequestration ratio
*
Energy intensity ratio
*
Consumption quantity in metric or US standard
*
Metric conversion factor, if needed
÷
Waste factor, if needed
m2/yr
=
m2
*
Gj
*
kWh
*
÷
kWh reaching house
Gj
kWh
kWh sent to house

In this calculation, 3.6 = energy intensity of production (Gj energy produced per kWh consumed), and 0.3 = amount of energy transfer due to energy loss in conversion from the primary energy source to electricity (in generating and delivering electricity, 70% of the energy is lost as heat due to the electrical resistance of power lines). You can see that dividing the quantity on the right by a 0.3 waste factor causes the ecological footprint to more than triple because of this wasted energy. Cell D54 = percent of total electricity generated from thermal sources. Note that a greater fraction of renewable energy would reduce the fossil fuel land footprint.

Additional heating may be provided by natural gas furnaces or oil furnaces (cells G63-65). For the carbon sequestration ratio, world average growing forests can absorb per hectare the carbon of 93 Gj/yr of gas. In Seattle, one hundred cubic feet (CCF) contains on average 1.1 Therms. One Therm contains 100,000 btus. This corresponds to 29.3 kWh (and cost in Seattle about 50 cents in 1999). Therefore, in this calculation, 10000/93000 gives the carbon absorption capacity of world average forests in [m2*yr/Mj], 1.1 gives Therms per CCF, 29.3 gives kWh per Therm, 3.6 gives Mj/kWh, and 1/0.3048^3/100 translates m3 into CCF. For home oil, the average fossil fuel footprint is 71 Gj per 10,000 m2 a year. The energy intensity ratio is 35 Mj/kg to produce oil. The energy intensity of coal is 35 Mj/kg (cell G66). Firewood requires fossil land (cell G67) to absorb CO2 as well as forest land (see below).

Note also the fossil energy land needed to support consumption of water (for sewage), appliances, and computers (cells G71-74). Note how the energy intensity ratio changes from 5 MJ/pound for construction wood to 200 MJj/pound for computer equipment. This shows how energy intensive the production of electronics is.

Arable land

With the exception of straw bale insulation (not considered in this spreadsheet), little arable land is required for building or providing energy for housing. Here, straw is considered as a fuel source for heating. Straw is calculated at the rate the removed biomass can be replaced (to make sure there is no nutrient loss on the field where the straw is grown). As a first approximation, cereals biomass productivity is used as a proxy of bioproductivity potential. On world average land, straw productivity is approximately double that of its cereal harvest. Hence, the productivity is 2 * 2904 kg per 10,000 m2 a year (cell H69).

Pasture

Little pasture land is required for building or providing energy for housing. Wackernagel and Rees (1994) assume that flooding of land for hydroelectric dams comes from this land use type. They estimate that 10000 m2 of flooded pasture land can produce 15,000,000 Mj (or 15,000 Gj) per year (cell I56). To convert kWh to Mj, they use the conversion factor 3.6 = Mj/kWh.

Forest

Forest land contributes to the production, heating, and furniture of housing. The following calculation shows the amount of forest land required for the construction of wooden homes.

Land area
=
Roundwood productivity
*
material intensity ratio
*
Consumption quantity in metric or US standard
*
Metric conversion factor, if needed
*
Waste factor, if needed
m2/yr
=
m2 land
*
m3 wood used in construction per house
*
m2 area of your house
*
*
m3 wood harvested
m3 wood harvested
m2 area per average house lasting 40 years
m3 wood used in construction

An average Canadian house uses 23.6 m3 of wood and may last 40 years (Government of Canada, 1991. The State of Canada's Environment. Ministry of Environment, Ottawa). The house may contain 150 m2 of living space. For every 10,000 m2, 1.99m3 of timber can be grown per year (roundwood productivity); 2.5 is the ratio of roundwood needed per unit of construction wood.

Fuelwood land area is calculated using the following formula:

Land area
=
Roundwood productivity
*
Wood density
*
Consumption quantity in metric or US standard
*
Metric conversion factor, if needed
*
Waste factor, if needed
m2/yr
=
m2 land
*
m3 wood
*
kg wood burned
*
*
kg wood harvested
m3 wood harvested
kg wood burned
kg wood used

This calculation assumes a world average forest yield is 1.99 m3 per 10,000 m2 a year; 600 kg/m3 is the average wood density; 0.53 is the waste factor for fire wood. It means that for each kg of firewood one needs 0.53 kg of roundwood. In this category, the waste factor is significantly smaller than 1 since about twice as much firewood than roundwood can be produced per m2 a year.

The same formula can be used to calculate the amount of wood required for furniture. In this case wood is consumed into a product instead of being burned. The waste factor for furniture is considerably higher (2.5), reflecting the fact that it takes 2.5 kg raw roundwood to produce 1 kg of furniture.

Built-up land

Every home requires built-up land for the lot, the roads and sidewalks to access the lot, and for the facilities to produce electricity for the home. Wackernagel et al. multiply the yard space by 1.5 to include the access areas (roads and streets outside of the property, but still in service of the property) (cell K50). For the built-up area of hydroelectric dams, Wackernagel and Rees (1994) estimate that 10,000m2 of dam facilities are required to generate 200,000 Mj of electricity (cell K55). Mj are converted to kWh using the formula 3.6 = Mj/kWh. For photovoltaic electricity, they assume that a home requires 24 m2 of solar panels to produce 3000 kWh, and they divide this land area by 0.75 to account for embodied energy of making the solar panels (cells K58-59).