Why is air tightness in a passive house important?
Air tightness is as essential as thermal insulation in a passive house — and represents one of the fundamental principles of the Passivhaus standard.
A building is subject to infiltration and air leakage:
- Infiltrations increase heating needs, reducing energy efficiency.
- Leaks, on the other hand, can damage building components, as warm, humid air escaping condenses on cold surfaces, causing moisture and structural degradation.
Main causes of air leaks:
- Poorly executed connections
- Ventilation system
- Heating/cooling installations
- Openings in the exterior envelope (doors, windows, chimneys)
Benefits of good air tightness:
- Improvement of indoor air quality
- Reduction of pollutants (such as radon)
- Prevention of moisture and mold
- Lower heat loss due to infiltration
- Better sound insulation
- Essential condition for the efficient operation of mechanical ventilation
- Increased thermal comfort
- Protection of structural elements against degradation
Rules for air tightness:
- Apply a continuous layer of airtightness, usually on the warm side of the insulation;
- Use wide, continuous surfaces;
- Properly join the different airtight layers;
- Avoid penetrations or perforations in the airtight barrier;
- Use suitable materials, such as membranes, OSB boards, plaster or concrete.
A space of only 1 mm in the airtight layer can cause the loss of 360 g of water per day per meter, due to the passage of warm, humid air — highlighting the critical importance of this layer.
How to test air tightness?
The most commonly used method is the Blower Door Test (BDT):
- A fan setup is installed in an opening (usually a door) to pressurize and depressurize the building (at 50 Pa).
- The airflow speed through the envelope is measured, with all exterior openings closed and interior doors open.
- Air leaks can be detected with a flow anemometer or a thermal camera.
In passive houses, the air exchange rate (n50) — that is, the volume of air that escapes through leaks divided by the total volume of the building — should not exceed 0.6 exchanges per hour.