Early on the morning of Friday Nov 21, 2014, I set up to run a preliminary blower door test of the Orchards of Orenco Phase 1 multifamily building in Hillsboro, OR, with the help of Earth Advantage Technical Field Consultant, Sara Walker. This three-story L-shaped building had 56 units and was attempting to be the largest certified PHIUS+ (Passive House) building in North America.
When we first ran the test, we thought something must have been off. The 359 CFM50 score converted to a tiny 0.056 ACH50 number, less than a tenth of the already super tight 0.6 ACH50 requirement. So, we turned the whole system off, re-ran the wires and basically set up again—only to find the same results. We set up manometers at extreme ends of the large building to confirm that we were getting equal pressure across the entire air barrier and ultimately settled on the fact that we had just tested one of the tightest buildings on record! Members of the project team insisted something must be off, but Sara and I reassured them that we held an annual EA technical team meeting to recalibrate our diagnostic equipment and that our HERS provider also signed off on the results of this exercise.
When construction was nearly complete, we ran a final blower door test across a whole host of different pressures, both positive and negative, and settled on an average ACH50 blower door score of 0.13 ACH50—well below the PHIUS+ threshold of 0.6 ACH50 and easily certifiable under the EA Multifamily certification program, which it also achieved.
How could these results be possible when much smaller PHIUS+ single family projects I had tested struggled to hit 0.6 ACH50 and the best CFM50 scores would typically come in around 100? A big part of the success was the fact that Walsh Construction and the entire project team took the challenge very seriously, and met early on in the construction process to nail down every last detail of the air barrier. The Walsh team shut their construction site down for two entire days to conduct their own uninterrupted blower door testing, in advance of my tests, to diagnose leaky areas and improve.
Another reason for the unusual results can be attributed to the metric itself. The ACH50 metric, (Air Changes per Hour at 50 Pascals of pressure), favors large buildings due to the decreased surface to volume ratio as compared to a smaller building. The surface, being the exterior envelope being tested, and the volume, being the amount of air inside the exterior envelope. This also helps explain why it is so difficult to get a great ACH50 blower door test result on a small home, and even more difficult on an accessory dwelling unit (ADU).
As a refresher for those who haven't run a blower door test in a while, or explored the details, here's how the test works. The blower door fan depressurizes a building to -50 Pascals (Pa). This simulates a 20 mph wind on all sides of the building. At this pressure, the manometer measures how much air is flowing through the fan and at what rate in Cubic Feet per Minute (CFM). Since one unit of air out equals one unit of air in, the amount of air going out through the fan is the exact same amount as the make-up air infiltrating the air barrier of the building. To convert the CFM metric to ACH you multiply by 60 (minutes) to get Cubic Feet per Hour and then divide by the Volume of the building. Therefore, increasing the volume of air inside the building (the denominator in this equation) can have a large role in promoting a smaller ACH50 result.
Passive House Institute US (PHIUS) has addressed inconsistencies by moving to a new metric that levels the playing field for all building types and sizes: 0.05 CFM / sq. ft. of envelope area. The envelope area is the area of all of the exterior walls, ceilings, and floors, combined. This makes it slightly harder for large buildings and slightly easier for small buildings to meet the air sealing requirement than it would have been under the old standard. For example, this new metric meant that a roughly 1,000 sq. ft. PHIUS+ home I rated had an ACH50 blower door requirement equivalent to 1.1 and another larger PHIUS+ home an equivalent ACH50 of 0.89. Had the Orchards at Orenco Phase 1 building been subject to this requirement, the ACH50 threshold would have been 0.57 vs 0.6.
Since the current Earth Advantage standard for certification is 5 ACH50 or less and likely moving to 4 ACH50 in 2019, what if we updated our metric to a level playing field standard of 0.33 CFM / sq. ft. of envelope area? It's safe to assume this would create challenges for our builders to compare to past results, and it would create challenges for our technical team to submit results to Energy Trust of Oregon, among other challenges. Is it worth it to make this move? The Passive House Institute is hoping this new metric will level the playing field and provide a fairer metric.