Window air infiltration testing is used to determine whether the window or curtain wall is tight enough against air infiltration.
Why is window air infiltration testing needed?
If air infiltration is excessive, building occupants can experience discomfort through a feeling of draftiness. Further, condensation and even frost can form around crack in windows and curtain walls where air infiltration occurs. Finally, energy loss from air infiltration can bring significant costs to property owners.
How is window air infiltration tested?
On the interior, a test chamber is built and sealed to the test window. To it is attached a variable blower, a manometer and a flow meter. The variable blower sucks air out of the chamber to create a negative pressure on the interior of the window. The manometer measures the negative pressure so that the proper negative pressure is achieved. The flow meter measures how much air is being removed from the chamber in order to maintain the proper negative pressure. The flow meter reading is recorded at two points: the “tare” measurement and the gross measurement.
At the start of the window air infiltration test, a plastic sheet is taped to the window and sealed around the edges. This is called the tare sheet. When the variable blower is turned on and chamber pressure is achieved, it is not uncommon to see this sheet suck against the window. This is because air infiltration that is being drawn out of the chamber is also being drawn through the window, causing the tare sheet to form closely onto the face of the window. When the chamber pressure is stabilized at the proper level, the flow meter reading is recorded. This recording represents all air infiltration in the system (from the chamber, the hoses and possibly the surrounding condition) exception for air infiltration at the windows.
The next step of the window air infiltration test is the removal of the tare sheet. The blower is typically kept running during this step, however the manometer (chamber pressure) reading will usually drop because now there is additional air flowing through the window. So the blower must be cranked higher to get the chamber pressure back to the proper level. When the manometer shows that the chamber pressure is correct and stable, the second flow meter reading is recorded. It is called the “gross” reading because in includes the window and all the extraneous sources of air.
In the next step of the window air infiltration test, the tare reading is subtracted from the gross reading. This is called the “net” air infiltration. It is the air infiltration that is attributed to the window alone.
Usually, window air infiltration performance is measured in cfm (cubic feet per minute) of airflow per square foot of window. So the net air infiltration reading must be divided by the area of the window (measured in square feet). If the result is at or below the allowable, as defined by the Project Specification or the window’s performance rating, then the test is a PASS. If not, it is time to investigate the problem.
How to investigate excessive air infiltration
It is very handy to have the chamber and blower still in place for this phase. The chamber is typically made of lumber with clear plastic stretched over it, so you can see the window through the chamber. This comes in handy during the diagnostic phase.
We turn on the blower to a low pressure, then apply either smoke or dry ice smoke onto the various parts of the outside of the window. In real time this is viewed from the inside. When the smoke hits the gap in the window system, it is easy to see the smoke come through. Then someone (either the manufacturer or testing agency) should determine the remedial work. If the work is done right away, the test rig can be used to verify performance.
Window Air Infiltration Tests we perform
ASTM E783 (Field version)Standard Test Method for Field Measurement of Air Leakage Through Installed Exterior Windows and Doors
ASTM E283 (Factory version)
Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen
Typical air infiltration test chamber pressures
Test pressures are determined by either the Project Specification or the window performance class, which is a rating it achieves through comprehensive product testing per AAMA (American Architectural Manufacturers Association) specifications. The two most typical pressures are 1.6 psf (pounds per square foot) or 6.2 psf.
These pressures have equivalencies in metric units, in inches of water column and also in wind speed as follows:
Typical chamber pressures for air infiltration tests
|Pressure (psf)||Pressure (Pa)||Water column (in)||Equivalent wind speed (mph)|
Typical allowable air infiltration
The allowable (passing the test) air infiltration is determined also through the Project Specification or the product performance class. Depending upon the window type, these are the most common allowable air infiltration rates: Typical allowable air infiltration rates
|Air Flow (cfm/ sf)||Air flow (L/ s•m²)|
Same or different performance in lab vs field?
A common argument that takes place during field testing is whether the window should be required to perform at the same level in the field as it did in the lab during certification. As the argument goes, once the window is transported and installed, more variables affect it that may change air infiltration performance. This is why some architects allow (and some do not allow) a lesser performance in field tests compared with certification tests. The source for this performance “discount” is AAMA 502, Voluntary Specification for Field Testing of Newly Installed Fenestration Products, unless rejected by the specifier, allowable air infiltration rates can be increased to 1.5x the values achieved during certification testing. Download excerpt editions of various versions of NAFS NAFS-North American Fenestration Standard/ Specification for windows, doors, and skylights
- AAMA/ WDMA/ CSA 101/ I.S.2/A440-05 (2005)
- AAMA/ WDMA/ CSA 101/ I.S.2/A440-08 (2008)
- AAMA/ WDMA/ CSA 101/ I.S.2/A440-11 (2011)
- Comparison between NAFS 1997, 2002, 2005, 2008, 2011