How to calculate heat loss in a Canadian home

What is the basic formula?

For each part of the envelope, heat loss equals the area, times the U-value, times the temperature difference between inside and outside. The U-value is the reciprocal of the thermal resistance. Canadian products are labelled in RSI (metric) or R (imperial), and one RSI is about 5.7 R, so a wall of R-20 is roughly RSI 3.5 and has a U-value near 0.29 watts per square metre per degree.

Air leakage and ventilation are added separately. The loss is roughly 0.33 watts per cubic metre of air per degree, multiplied by the house volume, the air changes per hour and the temperature difference. The result is summed over all parts and rooms.

What does a worked example look like?

The figures below are assumptions for a small, well-built house. Say the design day is 40 degrees C colder outside than inside, and ignore the basement.

• Walls. 100 m2 at U 0.29: 100 x 0.29 x 40 is about 1,160 W.
• Windows. 15 m2 at U 1.4: 15 x 1.4 x 40 is 840 W.
• Roof. 60 m2 at U 0.11: 60 x 0.11 x 40 is about 265 W.
• Air leakage. 400 m3 at 0.3 air changes an hour: 0.33 x 400 x 0.3 x 40 is about 1,580 W.

The total is near 3,850 W, or about 13,000 BTU per hour. Notice that air leakage and windows together account for more than the walls, which is why sealing and glazing matter. Compare this with a house at 0.6 air changes, where the leakage line doubles. A pressure test, covered in air tightness and the blower door test, gives the real number.

Which outdoor temperature should you use?

Designers use a design temperature for the location, a cold figure that is exceeded only on a few hours a year, not the all-time record low. Tables of design temperatures exist for Canadian cities, and the person doing the calculation should use the one for your location. Picking an extreme value oversizes the system, and picking a mild one leaves the house cold in the worst week.

Indoor setpoints matter too. A common assumption is about 21 degrees C, but a different comfort level changes the result.

Why does heat loss matter for sizing?

The calculation sets the capacity of the heating equipment. An oversized furnace short cycles, a point made in our furnace replacement guide. An undersized heat pump leaves the house cold unless backup heat helps, and cold-weather capacity is lower than the nameplate, as explained in heat pump sizing for cold climates. In Canada, a recognized standard (CSA F280) covers residential heating and cooling load calculations, and a contractor should be able to say which method they used.

Do not size by the old equipment or by square feet alone. Insulation, windows and sealing have usually changed, and the heat pump load calculation guide shows how a proper calculation differs from a rule of thumb.

How can you use the result?

The breakdown shows where upgrades pay off. In the example, cutting air leakage in half saves about 790 W, more than doubling the roof insulation would. A home energy assessment, covered in home energy audits, can add a rating and a list of fixes, and the EnerGuide rating explained shows how results are reported. The priority order is discussed in insulation upgrade priorities.

Heat loss work also underpinned Drake Landing Solar Community in Okotoks, Alberta. Its 52 R-2000 homes had low heating loads, which made 40 to 50 degree C district water practical, as the homes page explains. A tight envelope shrinks every number in the calculation.

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