REMARK: This Q&A was part of the AIVC special COVID-19 newsletter published in February 2021. To subscribe to the newsletter please click here.

Air infiltration in a building occurs as a result of the presence of air leakages in the building envelope, and of the existence of pressure differences inbetween inside and outside environment. The airtightness level of a building (air flow rate for a pressure difference at 50 Pa or another pressure difference, measured during a pressurization test) gives an indication of the overall air leakage of the building.

There is no straight relation between the airtightness level and the air flow rates by infiltration in real conditions.

These infiltration air flow rates are influenced by:

• the leakage distribution in the building envelope
• the internal partitions
• the pressure distribution over the building envelope (due to wind, outdoor and indoor temperatures and voluntary airflows due to systems operation: ventilation systems, kitchen hoods, etc.)

whereby these air flow rates also vary as function of time due to variations in wind (direction, speed), temperature difference between inside and outside, and in operating systems airflows.

However, the airtightness level allows to have a very rough estimation of the order of magnitude of the time and building averaged air change rate. In AIVC TN 23 [1], the following rule of thumb for the average air change rate is given:

The “basic” air change, or infiltration, rate of a building, in the absence of window opening, may be derived from the air leakage at 50 Pa using the simple “rule of thumb” below:

Qv(inf)= Qv(50)/K

Where Qv(inf) is the basic air change rate,  Qv(50) is the air leakage at 50 Pa and  is a constant which has a value between 10 and 30 (a value of 20 may be regarded as typical). A guide to appropriate choice of K is given below:

10 < K < 20: if a combination of at least two of the following characteristics is found:

• high rise buildings
• exposed situation
• average winter meteorological wind speed greater than 4 m/s
• uniformly distributed leakage area

20 < K < 30: if a combination of at least two of the following characteristics is found:

• Individual terraced houses
• Sheltered situation
• average winter meteorological wind speed greater than 4 m/s
• leakage area mainly situated at high level

Example:

• Free standing individual classroom of 200 m3, n50 =15 h-1 (old rather leaky building) (corresponding with q50 =15*200 = 3.000 m3/h):
• Average air renewal due to infiltration varies between 100 (q50/30) and 300 (q50/10) m3/h, hence an air change rate of 0,5 to 1.5 h-1
• This air renewal might be sufficient in case of low occupation of the classroom but probably for a nominal occupancy often not sufficient and the air renewal will be surely not sufficient during periods of low temperature difference between inside and outside and low wind velocities. This means that a ventilation system must be operated/installed or windows must be opened.
• Free standing recent building of 300 m3 with an n50-value of 1 h-1 (building with good airtightness) and q50 = 300 m3/h
• Average air renewal due to infiltration varies between 10 (q50/30) and 30 (q50/10) m3/h, hence an air change rate of 0,033 to 0,1 h-1
• These infiltration air flow rates are clearly much too low for achieving a reasonable indoor air quality in general and certainly too low in the COVID-19 context.

Conclusion:

• The airtightness level of a building allows to have a rough estimation of the seasonal air infiltration rate, whereby this air infiltration rate varies substantially over time due to wind and stack effects.
• It is impossible to assume that a poor airtightness level can guarantee during most circumstances a sufficient ventilation rate for normal occupancy.
• In case of most modern buildings with typically a rather good to very good airtightness level i.e. n50 of the order of 3 h-1 or better, there will be insufficient air infiltration during most of the time for covering the ventilation needs under normal occupation and certainly in COVID-19 context.

Author

Peter Wouters, INIVE

References