As the Passive House standard continues to make waves across New York City and the U.S., an entirely new design process has evolved to respond to the challenges of higher insulation levels, balanced mechanical ventilation, and perhaps the most difficult hurdle – an air tightness level that most would think is impossible. For the recently certified Cornell Tech building on Roosevelt Island, the tallest Passive House in the world, a several year-long coordinated effort was required to achieve such a feat. So what is the requirement, how is it measured, and what are the strategies and considerations required to achieve it?
How to measure air tightness?
First, how does air tightness actually get measured in buildings? In general, evaluating the air tightness of a building’s exterior envelope will always include some form of blower door testing, where large fans are used to create a pressure difference inside a building while simultaneously measuring the volumetric flow of air required to maintain that pressure difference. What gets confusing is how this flow is converted to a metric that can be compared across buildings. There is an array of metrics used by different standards making it difficult to compare, such as ACH50 (Air Changes per Hour) or CFM75 (Cubic Feet per Minute), but ultimately these are all normalizing that volumetric flow measurement based on the specific building’s geometry. To begin to understand a particular standard, there are two main questions you need to answer: is it based on “net volume” or “square footage of envelope,” and at what pressure difference is it being tested?
Passive House Requirement – International
The requirement most commonly known to those familiar with Passive House is the international standard of “0.6 ACH50”. Let’s start by breaking that down a bit. ACH50 refers to the air changes per hour at a pressure differential of 50 Pascals (Pa). This is approximately equivalent to a 20 mile an hour wind on all sides of the building. And air changes per hour is just what it sounds like: the number of times that the interior volume of air leaves or enters the building through cracks and holes in the facade within one hour. This is highly dependent on how you measure the interior volume of air. The measurement method outlined in the European standard EN13829 starts by measuring from drywall to drywall and finished floor to finished ceiling, but excludes interior walls, floors, and spaces above drop ceilings. This can be a painstaking calculation, but the accuracy of this number is critical for testing.
Passive House requirement – U.S.
The new requirement set out by the Passive House Institute U.S. (PHIUS) does not use volume but instead uses envelope square footage for their threshold. For smaller buildings (less than 5 stories) the requirement is 0.05 CFM50/ft2 while for larger buildings built of non-combustible materials it is 0.08 CFM50/ft2. This standard no longer cares about the volume – the critical calculation now is the area of the exterior envelope of the building. The volumetric flow (in cubic feet per minute or CFM) measured during the test at 50 Pa divided by this envelope area is the final number. See the following equations for how to calculate both metrics.
What’s the Big Deal?
The difference in how this leakage is measured is very important. Air tightness based on volume (or ACH) is disproportionately easy for large buildings to achieve because they have lower surface area to volume ratios (SA/V). The following image compares the geometry of Cornell Tech to a 3-Story Home, illustrating the effect of SA/V on these two air tightness measurements. Achieving 0.6 ACH50 on a building of Cornell Tech’s size would translate to 0.15 CFM50/ft2, not meeting the PHIUS standard of 0.08 CFM50/ft2. For a 3-story home however, 0.6 ACH50 translates to 0.01 CFM50/ft2, well below the PHIUS requirement. For this very reason, the Passive House Institute in Germany has a highly recommended air tightness threshold for large buildings over 140,000 ft3 of 0.033 CFM50/ft2.
Credit: Steven Winter Associates – Comparing Cornell Tech to a 3-Story Home: a lower surface area to volume ratio is typical in large buildings, making 0.08 CFM50/sf more difficult to achieve than 0.6 ACH50
When most people think about Passive House, they think about reduced energy consumption. So when asked why these buildings must be so air tight, the typical response would be “because the more air tight a building, the less energy it will use.” While this is absolutely a benefit of building air tight, it is not the only reason for the requirement. Passive House is just as much about ensuring comfort and durability as it is about reducing energy consumption, and if there is any single factor that can increase the durability of a highly insulated envelope it is its air tightness through the prevention of condensation. A highly insulated envelope results in colder surfaces within wall cavities. When these surfaces come into contact with humid air, there is potential for that moisture to condense and cause damaging mold. An air tight wall drastically reduces the amount of moisture transported into a wall cavity, and therefore drastically reduces the potential for condensation and mold as illustrated in the image.
Credit: Building Science Corp. – Moisture vapor diffusion due to pressure differences vs. moisture vapor transport due to air leakage.
In addition to this, reducing uncomfortable drafts and increasing the efficiency of the envelope insulation make this requirement absolutely necessary for a high performance building.
High-rise buildings and air tightness – what does it take?
The recently finished Cornell Tech project on Roosevelt Island is the tallest Passive House in the world and passed the air tightness requirement with flying colors. Achieving such a strict level of air tightness in a high-rise building required a substantial amount of planning from the very beginning of design until the end of construction. The following is an overview of the process that was required for this project to be so successful.
- Air sealing strategy is created.
- What will be the air barrier for each type of envelope construction and how will it make critical transitions?
- “Red-line” test – all building plans, sections, and section details are reviewed and the continuity of the air barrier is confirmed with an unbroken “red line.”
- Any unique air sealing conditions are called out and the materials used are labeled in the drawings
- Coordination between all parties to confirm understanding of air sealing details
- Blower door test plan created by the testing body
- What parts of the building must be sealed prior to the test?
- Where will the fans be set up?
- Drawings for a testing mockup are developed
- Should include a typical window installation
- Useful for educating sub-contractors on correct use of materials
- Inspections and intermittent testing schedule developed
- Contractor training
- Inspections of air barrier installation throughout construction
- Mockup testing
- Intermittent façade testing
- Whole floor guarded blower door tests
End of Construction:
- Final blower door test
Testing, testing 1, 2, 3…..4, 5, 6, 7, 8, 9, 10
The name of the game is test, test, and re-test. While visual verification is an important part of ensuring that the quality of install is high, the only way to know a strategy is working is to test it. For the Cornell project, this meant verifying the first few windows and doors were installed correctly by pressurizing a shroud taped to the wall surrounding the install, measuring the leakage through the component, and blowing in smoke to see where the leaks were occurring. For most projects this often results in finding weak points in the installation to be corrected before testing again.
Credit: Steven Winter Associates – To test the air leakage of individual windows, a shroud is taped to the wall surrounding the opening and a duct blaster is taped to a hole in the shroud. Once pressurized, smoke is blown into the shroud and the leakage locations can be observed from the exterior.
Additionally, a whole floor guarded blower door test should be conducted to find any issues in the air barrier in one fell swoop. During this test, three stacked floors are pressurized to 50 Pascals so that the middle floor can be inspected. The reason for pressurizing the floors above and below is to neutralize any leakage between floors so that only leakage to the exterior can be observed. During this test, smoke pens or thermal cameras can be used to identify leaks through the exterior walls. This way any issues can be corrected before continuing up the building.
Put your tapes away, it is time for the final exam
All of this preparation comes down to one final evaluation – the whole building blower door test. For a high-rise building, there is often not much that can be done after failing a blower door test. Due to the sequencing of the project, it is often impossible to test the entire building before covering up the air barrier with drywall which is why intermittent testing is so vital. The goal is that by the time final testing comes around, so much of the envelope has been verified that it should not be a concern. For the Cornell project, while there is always a degree of anxiety on final testing day (just like with any final exam…) the team had a high degree of confidence because of the test results throughout construction.
Blower door test – more like blower doorS testS
On a project of this size and complexity, running a blower door test is not as simple as putting a fan in the front door and closing the windows. It requires a day of preparation, an empty construction site (or almost empty), an entire team of experts, and multiple fans in different locations connected to one master computer. The software controlling these fans coordinates their fan speeds to create the pressure difference required. Although the test used for Passive House is a 50 Pascal test, the testing procedure requires testing at 10 Pascal intervals in both pressurization and depressurization to ensure accuracy. If the results of all these tests don’t follow a typical curve then retesting is required after some investigation. This typically means something blew open – such as a window or the seal on an exhaust vent – and finding this on a 26 story building often requires expert detective work or simply a lot of people.
Credit: Steven Winter Associates – Left: a blower door set up for testing at Cornell Tech. Right: a computer controlling the fan speeds of all blower door fans.
Remember what makes the dream work? Oh yea, teamwork.
In the end, what matters the most when constructing a 26-story balloon complete with functional windows, doors, and mechanical systems is summed up during most middle school basketball team huddles: teamwork. It must be a part of the culture of the project from the beginning that the air barrier is sacred and anyone who walks on site is responsible for looking out for its integrity. One sub-contractor can easily nullify the efforts of the rest of the team by overlooking their mistakes or covering up sloppy work. A strong sense of teamwork along with a highly detailed strategy planned and executed consistently over the course of 1-3 years, and you have a recipe for success. So get out there and start taping!
Written by Chris Hamm, Building Systems Engineer