Everyone loves to complain about how cold it gets in Canberra. And it’s true; a month of regular dips below freezing makes it the coldest capital city in Australia. Yes, even colder than Hobart! Among the solutions to combat the cold, many people often consider improving insulation in the walls, roof and windows. While this is certainly important, it can have very little effect if the house is not airtight. Even new buildings focusing on double glazing and high insulation R-values often don’t specify any airtightness target.

A house could have R6 insulation everywhere, with triple glazed windows and doors, but still be expensive to heat if it has not been built with airtightness in mind.

Here’s why:

From materials science we know that it takes 1000 joules of energy to heat up a kilogram of air by 1°C. To put that in real terms, a 1kW heater will heat up a cubic metre of air by 1°C every second it’s running. This is what your oven does with about 0.1 cubic metres of air every time you turn it on.

The average Australian house was tested by CSIRO to leak around 15 air changes per hour at 50 Pascals of pressure (15 ACH @ 50 Pa). This is quantified by a Blower Door test, the best science experiment you can do on your house by far. Now, 15 ACH @ 50 Pa results in around 0.5-1 ACH at normal atmospheric pressure, depending on wind and height of the building.

In a 200m² home, every hour, around 500m³ of warm air leaks out of the gaps in the plaster behind kitchen cabinetry, or windows and doors as well as through AC ducts and extraction fans. Conversely, 500m³ of cold air is leaking in as well, which means you have to pay to heat all that air back up again.

A quick look at some cold hard real-world numbers:

If it’s 0°C outside, and 20°C inside, there’s a 20°C difference in air temperature (ΔT).

ΔT x Air Volume/Second x Power = Energy Loss per Second (We’ve skipped some unit conversions here).

20°C x 0.14m³/Second x 1kW/°C/Second = 2.8kW of power loss every second, which will consume around 24kWh thermal energy every night from just air leakage.

To pay for these holes in your house, it will cost you around 60c/hr with a small electric heater, or around 40c/hr with a gas burner. This equates to $288 ‘extra’ over the winter quarter, which you are paying to heat air that is just leaking straight out of the house.

If you add the slightest bit of wind, or if it gets even colder overnight, then it just gets worse. So, no matter the level of insulation, air sealing and draught proofing remain the best solutions to keep you and your bank balance comfortable.

A little extra something for the building physics nerds...

A recently modelled building in the Passive House Planning Package would only require 16W/m² of heating at an airtightness of 0.6 ACH @ 50 Pa. In the event of air leakage at 1.5 ACH, it would rise by ~18% to 19W/m², or another ~20% if at 3 ACH.

The moral of the story: airtightness > insulation in terms of heating costs.