Not long after I began design and building large scale commercial Passivhaus projects it became evident that the 0.60 ACH50 air tightness requirement for Passivhaus certification was not appropriate for larger buildings. This metric is a pressurization and depressurization test of the air tightness of the building envelope. It is based on net interior volume, so up to 0.60 of the net interior volume of building is allowed to “leak” through the envelope per hour under the test conditions. This requirement has two consequences: first is the significant energy savings through such a tight envelope; second is the enhanced durability of the envelope when designed correctly, as moisture carried by air is reduced.
Many of the original Passivhaus protocols are based on smaller residential and commercial projects. But as one starts to push a building to larger and larger sizes, both the analysis of the building and the testing protocols to ensure the intent of Passivhaus need to be fine-tuned. In general, as a building gets larger there is less skin and more enclosed volume proportionally. Sticking to the original, volume-based air tightness requirement for larger buildings does make it easier to achieve the requirement, but has the potential unintended consequence of allowing more air leakage per sq. ft. of actual envelope and thus reducing durability.
For smaller Passivhaus projects we find that the 0.60 ACH50 air tightness requirement equates to an air leakage rate of 0.03 – 0.05 CFM50 per sq ft of envelope. This rate of air leakage is very small. By comparison, the current Army Corps Air Leakage Protocol is 0.25 CFM75 per sq ft of envelope. As we examined the large commercial projects we were working on, we realized that using the Passivhaus 0.60 ACH50 air tightness requirement resulted in skin leakage rates that approached levels 5-10 times higher than smaller projects.
Two years ago we rewrote the testing specification for our projects to require the lessor of the following: 0.05 CFM50 per sq ft of envelope or 0.60 ACH50 based on volume. We have found this requirement to be easily attained when working under the guidance and direction of our Air Sealing Specialists. If you would like more information on this protocol, please do hesitate to reach out to us.