The nitty gritty on airtightness

By - , Build 156

A current BRANZ Building Research Levy project looking at airtightness trends of our houses needs your help.

Figure 1: Infiltration rates over a year on the BRANZ WAVE test building at Judgeford.

BRANZ’S Weathertightness, Air quality and Ventilation Engineering (WAVE) research project identified a clear trend of New Zealand residential stock being built more airtight than ever before.

This is great if you are seeking to keep as much energy inside the building as possible. However, extra focus needs to be paid on ventilating buildings effectively to avoid indoor moisture problems and the build-up of other contaminants over time. See New home, old habits on page 47 for a case study in a modern home.

Understanding blower door results

Airtightness is usually measured with a blower door, and gives an idea of the infiltration rate of a building. There is, however, some confusion about what a blower door can tell you.

A blower door gives an indirect measure of how leaky a construction is from the point of view of air. Typically, the results from a blower door test are reported as x ACH@50 Pa, where x will usually be a number in the range of 1–10 depending on the construction.

The result indicates the number of air changes a building is undergoing with a sustained pressure difference of 50 pascal across the envelope. The 50 Pa, however, is well in excess of typical in-service pressures that drive infiltration in a building. Therefore, it should be stressed that a house with an airtightness of 5 ACH@50 Pa will most definitely not experience an average of 5 air changes per hour in practice.

Rule of thumb to divide by 20

As a blower door result is reported at a reference pressure greater than typically experienced, how do we relate this to normal pressure levels? A rule of thumb to translate a blower door result to typical in-service infiltration is to divide the result by 20.

This does not take factors such as wind speed, direction, site topography and location of leakage openings into account. In reality, the situation is much more complicated, and this rule of thumb can only give an indication of long-term average infiltration.

These factors lead to the variability in infiltration rates being greater for the buildings that are less airtight, which can lead to comfort concerns or homes that prove more difficult to heat consistently. The following results from WAVE show this variability in a little more detail.

Modelling infiltration gave insights

In WAVE, infiltration rates were measured in the BRANZ test building at a range of airtightness levels (using tracer gases). These results were then used to calibrate computer models of the building. See Figure 1 for the output of one of these models for the WAVE test building over a calendar year.

The blue line is the modelled infiltration and the yellow line is the average infiltration calculated from the model. The red line is the recommended average ventilation level (taken from ASHRAE 62.2).

The plots show some key points:

  • The average infiltration (yellow line) is below the recommended ventilation rate in all these cases. This should be obvious, as we cannot expect uncontrolled infiltration to deal with all contaminants. It is interesting to note that, as a building is made progressively more airtight, infiltration reduces in a non-linear fashion (see yellow lines for each airtightness in Figure 1).
  • What is also clear is that the transients or noise in the blue line reduce as the building becomes more airtight. This should make designing supplementary ventilation easier in the tighter building, as the demands of such a system become more closely linked with the activities in the building. They also become more independent of the variation in the environment.
Figure 1: Infiltration rates over a year on the BRANZ WAVE test building at Judgeford.

So what does this all mean?

This raises some interesting questions, as we are now seeing plenty of new builds around 3 ACH@50 Pa without any deliberate attempts to make them airtight.

Some thought will need to go into how homeowners ventilate their homes, particularly with education, as old habits can prove to be incompatible with modern buildings.

The airtightness project aims to keep an eye on how our buildings progress, with the national airtightness database feeding into other research projects like energy-efficient ventilation and codes and standards development.

Note

To submit any data, email it to [email protected].

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Infiltration rates over a year on the BRANZ WAVE test building at Judgeford.

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