Updated: Mar 8
As I sit here, all huddled up in a blanket on a cold night at home, I made a quick basic heat loss calculation for my house. If I make it as airtight as a passive house, leaving insulation as it is (only in the ceiling currently), how much money on heating could I save? The answer is above 10% savings on heating cost, at an outdoor temperature of minus 3 degrees Celsius. What a shockingly high number! That would look nice in my bank account.
So, what is airtightness and why does it make such a significant difference to our heating bill? In this article, we will explore and explain questions you might have regarding airtightness.
What is air tightness, and how is it measured?
What are the benefits and drawbacks of airtight buildings?
How airtight should I build?
How can I make my project more airtight?
What are the available air barrier options?
What should you look out for in your air barrier?
Is it costly to construct airtight buildings?
1, What is airtightness, and how is it measured?
Airtightness is a fancy term for measuring the cracks and gaps in our buildings that let warm air escape to the outside. These gaps and cracks can be around windows, doors or joints of different building materials, or air travelling through the building materials themselves.
Generally, cold outdoor air enters our buildings due to:
Air pressure. Wind creates pressure on a building. If the outdoor wind pressure is higher than indoor air pressure, it could push the outdoor cold air through the building gaps. Once inside, the wind pressure would further drive the warm indoor air out through the opposite walls of where it enters. Air pressure doesn’t need wind; as long as the air pressure on the outside of the house is stronger than the inside, air infiltration can occur.
Temperature differences. Nature prefers balance. The warm indoor air tends to escape and draw in cooler outdoor air. Drought is a typical result of this air movement. The higher the temperature difference between the indoor and outdoor, the more droughts created.
Building materials. Many of our building materials let air travel through them usually. We might think of our plasterboards stopping air from moving into walls, but this is not the case—air flows right through them.
Airtightness is measured by the number of air changes per hour using a “blower door test”. The higher the number of air changes in an hour, the lower the building’s airtightness.
In a “blower door test”, all windows and doors except the building’s front door are shut. A blower door which is a fabric door with a fan, a manometer and calibrated plates, is attached to the front door. The house will be pressurised and depressurised with around 50 Pascal. Once the pressure is stabilised, we could understand how much airflow is moving through the fan per minute using the manometer. We could then divide the airflow number with the house’s volume to determine how many air changes per hour.