What We Found When We Tested a Bank Branch (ASTM E779 Case Study)

The building was a single-story retail branch, slab-on-grade, with a masonry exterior and a storefront glazing system across the front. The commissioning agent had listed a test result target tied to the owner’s spec. Our job was clear. Prove the envelope met the required air leakage rate through thorough bank-branch envelope testing, and flag anything that could pull the number the wrong way.

We reviewed the plans first. Storefront glazing, a drive-through window, and rooftop mechanical penetrations all stood out as spots worth watching. That planning shaped where we’d look once the fans were running.

Setting Up the Test

ASTM E779 uses fan pressurization to assess overall building performance. Setup takes discipline, and shortcuts show up in the data.

• We confirmed the building envelope was complete, with the air barrier in place and finishes not yet hiding key details.
• We sealed intentional openings such as exhaust fans, rooftop units, and floor drains, so we measured the envelope rather than the mechanical systems.
• We mounted the fan on an exterior door and set up manometers to measure pressure differences across the envelope.
• We checked that all interior doors were open, so the building acted as a single connected volume.

Then we brought the space to test pressure and recorded flow at several steps, both under pressurization and depressurization.

Sampling and Judgment Calls

Even on a compact branch, a clean test needs a few judgment calls. Wind was light that morning, which helped, since gusts can smear readings and inflate the air leak you record. We ran the test early to stay ahead of afternoon weather.

We also watched the baseline pressure before starting. A stable baseline tells us the numbers we capture reflect the envelope, not swings from outside. On larger commercial jobs, sampling and setup choices matter even more, which is why correct method execution separates a real test from a rushed one.

Where the Leakage Showed Up

The overall number came in under the target. That’s the good news. But depressurization is where the story got useful.

With the building under negative pressure, we traced airflow at two spots the drawings never flagged:

• The drive-through window assembly: The seal around the transaction drawer let air pull through at a rate we could feel by hand. It was a detail the trades had treated as a finish rather than an envelope penetration.
• The storefront-to-slab transition: At one corner, the joint between the glazing frame and the slab showed a steady draft, a sign the air barrier hadn’t been tied in cleanly at that edge.

Neither leak was large enough to cause the branch to fail. Both were easy fixes while access stayed open. Caught after finishes, they’d have meant tear-out. That’s the practical value of running the test at the right moment.

Reporting Into the Cx and Owner Workflow

A number alone doesn’t help a commissioning agent. Context does. So our report did three things.

• It stated the measured result against the spec target, with the raw flow and pressure data behind it.
• It documented the two leak paths with location notes, so the GC knew exactly where to direct the sealing crew.
• It arrived in the format the Cx agent’s file expected, ready to fold into the commissioning record and the owner’s closeout package with no rework.

The GC handled both fixes within days. We didn’t need a full retest on such minor corrections, and the commissioning team closed the item with a clear paper trail. That’s how commercial bank branch air leakage testing should end: a defensible number, a fixed envelope, and a report the whole team can act on.