This post is a continuation of the story of CSV’s engagement with the Passive House standard. This time we’ll look at verification of one component of building performance: air tightness testing of the building enclosure.
What are we testing for?
Passive House (PH) emphasizes reduced heating, increased insulation, and reduced air infiltration and exfiltration through the enclosure. The energy impact of uncontrolled airflow through a building can be large, but, as a component of the heating requirement, it is commonly invisible. For Passive House compliance the finished building may not leak more than 0.6 air changes per hour at a pressure difference of 50 Pascals (ACH50).
As the PH concept is applied to commercial buildings or multi-unit residential buildings they enter new territory. Typical practice in commercial building includes using the heating, ventilation, and air conditioning (HVAC) system to pressurize the building, preventing cold air from getting in, but squeezing a lot of heated air out of cracks and corners. This approach can cover up a variety of problems, at the cost of creating new problems.
In contrast to conventional practice, the PH standard requires reducing the airflow in the building to no more than the volume required to provide good air quality. Compared to the amount of air that typically flows to heat or cool a space, the breathing air volume is very small. It’s so small that standard testing equipment for large buildings can’t effectively measure it.
In a large building, small airflows can be very hard to control. The Passive House standard requires ventilation air to be ducted directly to each occupied space. It further requires supply and exhaust air in the space must be balanced within 10%. To achieve this balance, air can't be allowed to leak out of the enclosure.
Why are we testing?
While the principal objective of air tightness testing is to provide good building performance, it also communicates the value of enclosure commissioning. The combination of heavily insulated assemblies and the preliminary air tightness test provides good visible demonstration of ongoing quality assurance.
Where does the air go?
Triple sealed PH certified windows and doors are very good for controlling air leakage, but they still allow some airflow. Building assemblies rarely leak in the middle of the assembly. They commonly leak at the edges and at joints with other assemblies. What becomes important is ensuring that the connections between systems: walls, roofs, floors, and openings, are airtight.
To help implement air tight connections PH product suppliers have brought forward new membranes and tapes. The tapes are longer lasting than gunned sealants, and easier to apply effectively. Even with improved products available, the question remains; how does one know that the shell going to be airtight?
The answer is “get it tested”. Air tightness testing with a blower door has been used in the industry since the R-2000 Program was developed but it may not be familiar because it’s being applied to commercial buildings rather than detached houses. With few exceptions, it hasn't become a regular part of the commissioning process.
Blower door testing for a single floor in a large building can be quite complex. If one is testing one floor air can escape in 6 directions. For low and midrise buildings testing a whole building is a simpler exercise. There is not much detail to compare, but CSV has now done blower door tests on a variety of low-rise buildings with good results. The practice usually involves a preliminary test, as soon as the building enclosure is complete, with the final test done once the building is near completion.
Testing a building that intentionally addresses air tightness is remarkably simple. In our first example, a 3½ storey stacked townhouse with a volume of 3,228 m3 was depressurized to -50 Pa in less than 20 seconds. Openings were sealed with polyethylene and tape, and the exterior enclosure sealed with taped OSB sheathing plus an exterior air/weather barrier stapled and taped to the sheathing. The preliminary results achieved 0.42 ACH50 under depressurization and 0.54 ACH50 under pressurization. In our second example, a four-storey ICF building with a volume of 10,848 m3 was depressurized to -50 Pa in less than a minute. That building similarly had openings sealed with polyethylene and tape. The preliminary results achieved 0.36 ACH50 under depressurization and 0.39 ACH50 under pressurization. These two example projects are candidates for PH certification and as such, designers and constructors paid a lot of attention to air tightness.
More remarkable is in the case of a recent large addition to a municipal recreation centre. The building was a candidate for LEED Silver certification, not Passive House, however the architectural specification carried a requirement for an air tightness test that referenced the PH threshold of 0.6 ACH50. The details and membranes were selected thoughtfully, and the client hired a third party to review building envelope installation. The contractor was skeptical, but the blower door test went ahead. The result of the test was 0.86 ACH50, not PH level, but still quite respectable. Translating that figure to the terms of the NECB 2017 requirement gives a measured air tightness of 0.18 L/s/m2 of above ground wall and roof area which is 0.07 L/s/m2 better than the OBC requirement.
The moral of these stories is that what is considered a very strict level of air tightness is readily at hand. For a small investment in time and attention a significant degree of control over the indoor environment is available. Get it tested.