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The Trash Bag Method

The trash bag method is a simplified and reliable method for measuring supply register airflow during short-term testing at a field test site. It requires a trash bag, small metal or wood frame, tape, stopwatch, and a properly sized piece of cardboard (or similar) [1].

A large and sturdy trash bag, such as a 30- or 50-gallon bag, commonly used for garage or job-site waste, is attached to a wooden frame with tape. The frame should have an opening larger than the register. Fully deflated, the bag is placed over a supply register with the cardboard blocking the airflow. The cardboard is removed quickly and at the same time, the stopwatch is started. The frame should be held tightly to the floor, wall or ceiling during this time to avoid leakage. When the bag reaches capacity, the stopwatch time is recorded. Trash bag volume and time can then be used to calculate the volumetric flow rate:

$CFM=8.02\cdot \frac{V_{bag}}{t}$

Where CFM is register flow rate in cubic feet per minute, Vbag is the trash bag volume in gallons, and t is the fill time in seconds. This equation accounts for the bias error described in the literature, so gives a CV of ±11%.


In certain situations, the trash bag method represents the only reliable measurement for register flows. Examples include a register crammed between a cabinet and a wall, and a ceiling register next to a light fixture – in those cases, a flow hood cannot be placed around the register for an accurate measurement.


Literature shows the following uncertainty estimates using this method: bias error of -5% and RMS differences of ±11% on flow rate. It is recommended that practitioners record several measurements to verify repeatability in their practice. Uncertainty is expected to increase as airflow drops – at low flows (below 50CFM), the leakage between the frame and floor or wall can become significant and another method may be considered.


1. Walker, I.S. (2003). Garbage Bags and Laundry Baskets. Home Energy Magazine, Vol. 20 No.6, November/December 2003. Energy Auditor and Retrofitter, Berkeley, CA.