Exceptional airtightness results achieved But it’s not an end in itself and it’s not only the builder

19 March 2023 by Jason Quinn

We’re aware builders are competing to get the lowest air leakage results (well below the 0.6 ACHn50 Passive House requirement). It’s good fun: we have some great Passive House builders with considerable experience and commitment and they deserve the spotlight. So who has achieved the lowest blower door test results in New Zealand?

There are currently three certified Passive House projects that belong to what we could call the 0.1ACHn50 club*. (Not catchy. Let’s go with the 0.1 Club.)

Kowhai House 0.13 ACHn50 (case study) Stevenson & Williams Ltd

Holmes House 0.13 ACHn50 (case study) Central Otago Builders Ltd

Chingford Park Passive House 0.10 ACHn50 (case study in progress) WD Homes 

I’ve shown the result to two decimal places—but to be honest it’s unlikely the measurement is terribly accurate at that level. The result relies on the accuracy of the blower door testing equipment and the calculation of the building internal volume.

We’ve singled out builders above and their skill and attention to detail is something to applaud. But keep in mind some construction methods make it easier to achieve greater airtightness: for instance SIP compared to timber framing with membranes. Also the architect/designer has a big part in this too because a simple building form helps a lot.

Understand there is a law of diminishing returns. Passive House standards are all aimed at a sweet spot re cost and benefit. 0.6 ACH is not negotiable if you want to meet certification standards. Minimising uncontrolled air leakage results in energy savings and reduces interstitial moisture risks due to air leakage. The difference between 0.6ACH and a standard new build (or worse, a 1960s build) is massive. But 0.6 and 0.2ACH? The improvement at that level has much less impact. 

Driving the airtightness result so low is really only important once the building is quite close to Passive House upper limits. If the project is more airtight, it reduces heating/cooling load and demand. But what about meeting the certification targets by energy modelling the design on the basis of achieving 0.3ACHn50 or even lower? You could save on insulation costs and maybe even windows. There have been several projects planned in the South Island that require less than 0.6 ACHn50 in order to meet certification standards. This adds risk, but as long as the client is aware and understands the trade-off, I think it’s fine.

*PHI rounds the ACH result to one decimal place. Those projects named above achieved airtight results of 0.14 or less. There are other outstanding projects just fractionally higher. Note this also means that an air leakage test result of 0.649ACHn50 will meet the Passive House requirement.

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