Airtightness trends

By and - , Build 166

BRANZ is researching the airtightness of new homes as part of its ‘warmer, drier and healthier buildings’ programme. This, the first of three articles, discusses airtightness and infiltration and dispels some common myths.

Figure 1: Houses are becoming more airtight over time.

AIRTIGHTNESS IS the measure of how impermeable a construction is to air and is a commonly referred to property of buildings. It is an indirect measure of infiltration, which is air entering the building through unintentional and uncontrolled openings.

Infiltration air can impact on energy use, thermal comfort, indoor air quality and durability of a home.

Confusion over airtightness

There is considerable confusion in the building industry as to what airtightness means and how to compare it to other performance factors like thermal resistance of insulation and the U-value of windows.

To help explain this, we start by looking at the airtightness typical new builds are achieving, discuss why you might want to have a reasonably airtight structure and point out what is required if your house is airtight.

Airtightness measured at pressure

In Build 127, Changing the air indoors on pages 48–49 details how to measure airtightness, but here is a recap.

A large fan or blower door is used to subject the building to a range of pressure differences while measuring the flow through the fan. Fitting a best-fit line to this data allows us to calculate the flow at a reference pressure of 50 Pa – the n50 number commonly quoted.

The important thing to note is that this pressure level is considerably more than average pressures across a building envelope. While the n50 value is a somewhat arbitrary number allowing comparisons of airtightness between different buildings, it is not the infiltration rate of external air into the building.

Infiltration rate rule of thumb outdated

Infiltration means the actual exchange of inside air with fresh outside air under normal climatic conditions.

In the past, a rule of thumb has been used to estimate the average infiltration from the blower door result, which was simply dividing the result by 20. This works reasonably well where the n50 is of the order of 10 air exchanges per hour (ach) or so.

As buildings get more airtight, the divisor tends to get larger, meaning the relationship is not linear. To give an idea of how non-linear, we have seen a divide by 100 factor on the BRANZ ventilation test building. This has an n50 of 1 ach, and we measured 0.01 ach of infiltration, instead of the theoretical 0.05 ach from the divide by 20 rule.

Diminishing returns from increasing airtightness

This indicates that the energy gains with increasing airtightness have diminishing returns, and this should be considered if we go down the path of considering a recommended airtightness level.

There is a lot to this, and we will get to it later in this series of articles.

New homes becoming even more airtight

In Build 127, we published results from WAVE research (repeated most recently in Build 165, Flat-out testing!). Since then, we’ve tested over 100 homes to fill in the gaps. Most of these were built post-2010, with the rest in the 2005–2010 time period.

The updated plot (Figure 1) shows how the trend to more airtight construction has continued. Nearly 75% of the post-2010 homes have an n50 value of under 5 ach.

Figure 1: Houses are becoming more airtight over time.

What does this mean?

Take a look at the graph in The nitty gritty on airtightness (Build 156, pages 86–87). One of the major benefits of a more airtight building is that the variability in the infiltration is under much better control as we get more airtight. This has two benefits:

  • Designing of ventilation provisions becomes easier – with a more predictable rate of infiltration, the right amount of ventilation is easier to calculate.
  • With less variability in infiltration, thermal comfort is typically better – less noticeable draughts.

Airtightness is good, but ventilation essential

Can a building be too airtight? The answer is no, with a caveat.

While the current state of new builds is a positive move for energy efficiency and thermal comfort, it does raise questions around how we provide the essential ventilation to remove moisture and contaminants.

Building Code clause G4 Ventilation allows for occupants opening windows for ventilation, but it also inherently assumes a degree of infiltration adding to the background ventilation of a dwelling. This may not be happening in the majority of new builds.

BRANZ occupant-behaviour research has shown that window opening is not as often and regular as we might like, so in the longer term, mechanised options will be needed. Not having enough ventilation in an airtight building can have serious ramifications for the health of the building and its occupants.

Keep an eye out for more in future Build issues where we will go into the energy impact of infiltration air, comparing it to conductive losses, and more.

For more

To read the Build articles mentioned here, see

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

Figure 1: Houses are becoming more airtight over time.