Performance

What Is Air Infiltration? Why Mail Slots Leak Air

Air infiltration is unwanted air movement through gaps and openings in a building's exterior. It is measured in specific units, tested under controlled pressure, and it applies directly to mail slots. Not every home has one, but in the homes that do, the opening itself is not an optional design choice. It exists because federal mail-delivery regulations required it.

What This Actually Means

Building professionals call the outer shell of a house, walls, windows, doors, roof, the building envelope. It's supposed to keep outside air out and inside air in. Air infiltration is what happens when it doesn't: air working its way through gaps, seams, and holes that were never meant to be there. Exfiltration is the same leak running in the opposite direction, with conditioned air escaping outward through those same gaps.

A mail slot is one of those gaps. It's a fixed rectangular hole, usually cut through the exterior door, or sometimes through an adjacent wall or a garage door. If the hardware installed in it doesn't actually seal, that hole just sits open in the envelope, letting air move through it in both directions, every day, for the life of the door.

How Air Infiltration Is Measured

Two units come up constantly in infiltration testing, and both appear on independent test reports:

• Pascals (Pa), a unit of pressure. Testing labs pressurize or depressurize a test chamber to a specified level, then measure how much air moves through the product being tested at that pressure.
• Cubic feet per minute per square foot (cfm/ft2), or the metric equivalent liters per second per square meter (L/s/m2), is a unit of airflow rate normalized to the size of the tested opening.

The recognized test method for exterior windows, skylights, curtain walls, and doors is ASTM E283/E283M-19,⁵ which specifies exactly how the product being tested, called the specimen in a test report, is mounted, pressurized, and measured so that results from different labs and different products can be compared on equal terms. Read the full ASTM E283 explainer for how the test itself works and what a passing result requires.

Why the Mail Slot Is a Common Leak Point

Mail slots exist in millions of American doors because of a specific piece of history: as of January 1, 1923, the United States Postal Service required all private dwellings to be equipped with a mail receptacle, preferably a slot in the door, as a condition of continued delivery.¹ Decades of homes built or retrofitted under that requirement means a fixed opening was cut into a huge number of exterior doors, most of them long before ASTM E283 testing, or any comparable performance standard, existed for the hardware installed in that opening.

Government data on the U.S. housing stock shows just how much of that older inventory is still in use today. In the Northeast census region alone, 7.15 million housing units were built before 1950, the largest single construction era cohort in that region, and 1.95 million single-family attached units were standing there as of the 2020 survey.² That is one region with detailed public data, not the whole picture nationally, but it illustrates the pattern: a large share of homes with a mail slot are older homes, and the hardware installed decades ago was rarely engineered, let alone tested, to today's air leakage standards.

That gap matters because a mail slot is not a small or incidental opening. It sits at a fixed, permanent height on the door, commonly chest or waist height, though some are set low near the bottom of the door, and it's typically on the front door, an exterior opening exposed to outdoor air and weather. An untested or poorly fitted flap can leak air at that spot, regardless of how well sealed the rest of the door and frame are.

Infiltration and Exfiltration at the Mail Slot Specifically

Independent testing on a mail slot assembly typically reports both directions:

• Infiltration measures outside air entering through the slot under positive pressure on the exterior face, simulating wind loading against the door.
• Exfiltration measures conditioned interior air escaping outward through the same assembly under the reverse pressure condition.

Reporting both directions gives a complete picture of how the hardware performs, rather than a single number that only tells half the story.

What Independent Testing Shows for the Brass-Seal

The Brass-Seal Forged Brass Mail Slot System is among the first mail slots independently tested to ASTM E283/E283M-19, with results verified by Intertek, an accredited third-party testing laboratory, at a pressure differential of 75 Pa. The interior standalone configuration returned an infiltration result well below the maximum allowable rate defined by the standard. Full numeric results, the test conditions, and the pass and fail thresholds are published on the ASTM E283 explainer page.

Where Air Infiltration Standards Show Up in Practice

For architects and specifiers, air infiltration performance connects to a few recognized frameworks worth knowing:

• LEED v4.1 includes a mandatory Indoor Environmental Quality prerequisite, Compartmentalization, for attached single-family projects.³ The prerequisite text names mail drops specifically among the required sealing locations, alongside utility chases, garbage chutes, and elevator shafts, and it sets a maximum leakage of 0.30 cfm per square foot at 50 Pa across all enclosure surfaces, verified by a whole building blower door test. That is a different measurement context from the component-level testing ASTM E283 performs on hardware alone, even though both use similar units.
• WELL v2 addresses envelope air infiltration management as part of its air quality requirements for commercial projects, focused on envelope commissioning and blower door verification rather than a single hardware component.⁴

Neither framework is mail slot-specific, and BOTA does not claim credit under either framework on behalf of its products. They are included here because they are the standards architects are most likely already working within, and because component-level test data, like ASTM E283 results, is the kind of documentation those frameworks typically ask a specifier to provide.

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