Green Building
There are a number of things you can do to reduce the energy consumption (and the associated greenhouse gas emissions) of your home. The following discussion is based on a 30 ft x 30 ft bungalow with a full basement in Lethbridge, Alberta.
Heat loss from a home has two main paths. The first is the heat lost through the walls, the ceiling, windows, doors and the basement walls and slab – this type of heat loss is called ‘conduction’. The second is the heat lost from the air passing through the home. This air movement can be uncontrolled (infiltration) or controlled with a system of fans and heat exchangers (ventilation). Another important factor in heat loss from infiltration or ventilation is humidification, where water is added to the air so it is not so dry in the winter.
The following discussions will compare an intervention to a standard home. Of course, the ‘standard’ home is not so standard, so we have listed the important factors in the base-case below:
Size of home: 30 ft x 30 ft x 10 foot height.
Basement: Unfinished concrete buried to 7 feet
Standard 2x6 construction - Walls: R20; Ceiling: R30; 16” stud spacing
Windows: Double-Paned, low-emissivity, 15% of wall envelope
Door: Steel, insulated
Air movement:
Infiltration = 1 air change per hour (ACH)
(Note: 1 ACH may be low in an older home in a windy region like Lethbridge … but it is bad enough)
Ventilation = 0
Humidification = Off
Indoor temperature maintained at 21 degrees C
Lethbridge Climate:
Using the average temperature for each month over the past ten years, a representative month of hourly data was chosen to build a model climate for Lethbridge.
And the following chart shows the heating-degree days (HDD) and cooling-degree days (CDD) for Lethbridge. This value represents the cumulative difference between the inside temperature and the outside temperature over the period of a month. The higher the number, the more energy you require to heat (or cool) the home to the set temperature.
Energy Prices (December 2023 in Lethbridge):
The energy prices also include extra charges which increase with the increase in consumption. We will use:
Natural Gas = $14.25/GJ / 85% efficient furnace = $16.76 / GJ
Electricity = $0.34/kWh
Basic Model Results:
With these basic design parameters, the home loses about 128 GJ of heat over the nine heating months from September to May.
Conductive Heat Loss = 95 GJ
Air Heat Loss (infiltration) = 33 GJ
The paths of the conductive heat loss are shown as follows:
One will notice that the heat loss through basement walls is enormous. Contrary to common belief, concrete is a poor insulator (though it has good thermal mass to absorb energy), and the soil surrounding the basement is a heat sink. This means that throughout the year, the earth is sucking energy from your home, which is why an uninsulated basement is always cool. One category that might be unfamiliar is ‘bridging’ which describes the heat loss through the studs in a wall – the heat loss bridges the insulated portions of the wall.
Insulate the Basement:
But this scenario isn’t too fair of a starting point, as most modern homes have insulated walls (with a minimum of a 2x4 construction). We will now add 3.5” of fiberglass insulation to the basement walls.
The conductive heat loss has dropped by almost 35 GJ each year (a third of the conductive heat loss). It is clear from the other paths for heat loss, that insulating your basement walls is the single most beneficial intervention you can do.
Savings: 35 GJ x $16.76/GJ = $585/year
Emission reduction: 35 GJ x 50 kg CO2 / GJ = 1.75 tonnes CO2 per year
You may wish to better insulate your basement, like using 2x6 construction. The resulting heat loss is:
The additional 2 inches of insulation reduced the heat loss by an addition 5 GJ. Each inch of insulation you add to the building envelope (walls and ceiling) has diminishing returns. This is to say that the first 3.5 inches reduced the heat loss by 35 GJ, while the next 2 inches reduced heat loss by only 5 GJ. At a certain point, you will be embedding more energy (the energy used to make insulation) than you save from heat loss over the life-cycle of the home. We will return to this concept in the roof insulation discussion.
For the additional 2” of wall insulation in the basement,
Savings: 5 GJ x $16.76/GJ = $85/year
Emission reduction: 5 GJ x 50 kg CO2 / GJ = 0.25 tonnes CO2 per year
Turn Down the Thermostat:
Perhaps the easiest thing to do is to turn down your thermostat when you are not home, or during the night when you are under the comforter.
The standard home is set at a constant temperature of 21C. If one were to reduce this to 20C, the heat loss would be reduced by 4 GJ. This costs you nothing.
If you bravely went to 18C at night, the heat loss would be reduced by another 10 GJ
Even if you left the home at 21C but were to drop the temperature of the basement to 16 C (close vents, add a door to the basement), the heat loss reduction would be about 5 GJ.
We will leave the home at 21C for the rest of the example, but consider that you might save a hundred or more dollars each year by turning down your thermostat. Wearing a sweater is always your cheapest approach (wearing a parka is better, but …)
Air Heat Loss:
Assuming we have now insulated the basement walls, the next largest contributor to heat loss is uncontrolled air movement through the home. This is of particular importance in a windy climate like Lethbridge, where the wind can literally push air through cracks and gaps in the envelope and suck air out of the down-wind side of the home.
The absolutely best way to tell how tight your home is built (preventing unwanted air movement) is with a Blower Door Test. This diagnostic test uses a blower installed at a door to measure the amount of air moving when the home is depressurized. It essentially tells you how many gaps and cracks and wiring penetrations there are, allowing unwanted air movement.
For this example, we will use an average 1 air-change per hour (ACH) of infiltration. This would be much higher for homes that are not built tight (or homes where people are constantly coming and going, opening and closing the doors). Modern homes use a wrap to seal the envelope of the home prior to the siding being installed. This wrap reduces unwanted air movement considerably, typically to about 0.5 ACH. Other sources of unwanted air movement include gaps around windows and doors, and the rim joist where your home sits on the concrete foundation.
Remember – you have paid to heat this air in the home!
Using our standard design, the energy lost from air movement is approximately 33 GJ. This is about a third of your total heat loss:
If you have a humidifier attached to your furnace, set at 60% humidity, there will be an additional heat load of 10 GJ, making infiltration roughly half of your heating bill.
Humidification is a personal choice. The typical range for home comfort is 30% to 60% humidification. Without humidification in Lethbridge, when you are heating very cold outside air to home temperatures, the humidity in your home could fall to values well below 10%, which may be uncomfortable for some people. But if you can do without it … that’s 10GJ saved ($170) each year.
We will leave the humidification set at 60% for the rest of this example.
Seal the Home:
After insulating your basement walls, the next best intervention is to seal your home. If you are replacing your siding – seal the home with a wrap. Otherwise, insulate the gaps around doors and windows, add weatherstripping to your doors, caulk/foam-insulate the rim joist where your home sits on the foundation, install a vapour barrier behind your plug-ins and light-switches located on the outside walls.
If one were able to reduce the obvious gaps and cracks where air can move into and out of the home, the infiltration value used in the model could be reduced to 0.5 ACH.
The heat loss due to infiltration would drop by half from 33 GJ to 16 GJ.
Savings: 16 GJ x $16.76/GJ = $270/year
Emission reduction: 5 GJ x 50 kg CO2 / GJ = 0.80 tonnes CO2 per year
Build it Tight, Ventilate Right:
Though sealing the home well will reduce heat loss, this might become ‘too much of a good thing’. It is important for indoor air quality and your health that the air in the home be replenished with fresh air. Of course, you can open a window – but in the winter this could be an expensive waste of energy. A better way is to install a Heat Recover Ventilator (HRV).
An HRV draws in fresh air to the furnace while capturing some of the heat from the air being exhausted from the home. A minimum desirable air exchange is about 1/3 ACH. This means that there is a complete change of air in your home every three hours. The HRV uses a heat exchanger to capture heat from the air being blown outside, which is about 60% efficient. This means that you are only using 40% of the energy to heat air in your home compared to infiltration or even a fresh-air intake directly to your furnace.
Imagine building a super-tight home (0.1 ACH) and adding an HRV (0.3 ACH):
The heat loss is reduced by another 10 GJ:
Savings: 6 GJ x $16.76/GJ = $100/year
Emission reduction: 6 GJ x 50 kg CO2 / GJ = 0.30 tonnes CO2 per year
An HRV costs about $1000 installed in conjunction with a furnace.
What About Windows?
In our standard home model, the windows account for a little over 10 GJ of heat loss. The windows cover approximately 15% of the home envelope and were double-paned with a low-e coating (to help keep heat in the home).
If these standard windows were to be replaced with a triple glazing, the heat loss would drop to about 3 GJ per year – a savings of roughly 7 GJ.
Savings: 7 GJ x $16.76/GJ = $120/year
Emission reduction: 7 GJ x 50 kg CO2 / GJ = 0.35 tonnes CO2 per year
It is estimated that a new triple-pane window will cost about $1200 installed – about 15% more than a double-pane. Assuming 10 windows and a differential of $200, the additional cost for replacing your windows will be about $2000 (but $12,000 overall). In addition to the cost savings from heat loss, good windows reduce drafts and provides greater home comfort. [Note: prices for different types of windows and regions can vary widely]
Solar Gains:
Windows are unique in that they can let a considerable amount of energy into the home when the sun is shining. You may have felt this warmth in your own home. The following chart shows that for a south-facing window in Lethbridge there is more energy entering the home (above the horizontal axis) than leaving the home (below the axis) over a period of a year for a triple-pane window.
This is encouraging, and might amount to a number of GJ of free heat in the winter. In the summer, however, this additional heat is unwanted and may require air conditioning to remove it (at an energy loss). Proper shading on south-facing windows can help reduce these unwanted gains in the summer.
Compare the above chart to double-pane windows, the solar gains are less than the heat losses for most of the winter. This is still free energy and makes south-facing windows better than windows facing other directions.
Now, returning to triple-pane windows, what does this chart look like for east/west facing windows?
Solar gains are highest in the summer (when you don’t want them) and lowest in the winter. It is also difficult to shade the east/west windows in the summer as the sun is rising and setting, but one can install reflective curtains to help keep unwanted heat out, or buy special coatings for these windows to help reduce solar gains (Low Solar Heat Gain Coefficient coatings). If you are designing a home, minimizing windows facing west and north will help the home perform better, or one might consider landscaping the west side of the home to shade west-facing windows. In Lethbridge, this also provides some shelter from the predominantly west winds.
Ceiling/Roof Insulation:
There is little one can do about the amount of insulation in the walls of your home after it has been built. It is possible to add some rigid insulation on the outside of the home if the siding is being replaced. Or, if you are building a new home, one might consider using double-walls with more insulation, and increasing the stud spacing to 24” to reduce bridging losses
Adding more insulation to the ceiling is typically an easier intervention to reduce heat loss. Our standard home started with R30 (or about 8” of cellulose insulation). Adding an additional 4” of insulation (over R40) will reduce heat loss by 2 GJ
Savings: 2 GJ x $16.76/GJ = $35/year
Emission reduction:2 GJ x 50 kg CO2 / GJ = 0.10 tonnes CO2 per year
We introduced, above, the fact that each inch of insulation added will have less effect in lowering heat loss. The chart below shows the heat loss through the roof for insulation thicknesses increasing from 8” to an enormous 24”. The heat loss curve levels off with each additional inch of insulation. At a certain point you will never recover the money you spent on the insulation and, worse, there is a point where you will have consumed more energy embodied in the insulation (the energy required to mine, manufacture and transport the insulation) than you will recover in the life in the home. In other words, you will have increased ghg emissions … not our intention. 
Summary:
Compared to our standard home, we have:
1. Insulated the basement walls 2x6 construction = 40 GJ
2. Sealed the home = 16 GJ
3. Added an HRV = 10 GJ
4. Replaced old double-pane with triple-pane windows = 7 GJ
5. Added insulation to the roof = 2 GJ
Initial Total Heat Loss = 128 GJ
Total Saved = 75 GJ
Greener Home Heat Loss = 53 GJ … a 58% reduction in heat loss (and emissions)
Savings: 75 GJ x $16.76/GJ = $1250/year
Emission reduction: 75 GJ x 50 kgCO2/GJ = 3.75 tonnes CO2 per year
A (very) rough estimate for this work is $11,200, which gives an emission reduction cost of about $100/tonne CO2 assuming a life-cycle of 30 years for these interventions.