WHAT IS ASTM E283 AND WHY IT MATTERS FOR MAIL SLOTS
ASTM E283 is the standard method for testing exterior windows and doors for air leakage. Mail slots are almost never submitted for independent testing under this standard. Here is what the test measures, what the results mean, and why the distinction matters.
The Brass-Seal™ system is among the first mail slots to be independently evaluated under this standard.
The Standard
ASTM E283 is the industry-recognized test method for determining the rate of air leakage through exterior fenestration and door assemblies under controlled pressure conditions. It is published by ASTM International, the organization responsible for voluntary consensus standards used across construction, manufacturing, and materials science.
The test is conducted by an accredited third-party laboratory under controlled conditions. A specimen is mounted in a test wall, pressure is applied across the assembly, and airflow through the specimen is measured. The result is reported in liters per second per square meter of specimen area, or in cubic feet per minute per square foot.
The standard applies to assemblies that form part of the building envelope: the boundary between conditioned interior space and the exterior environment. ASTM E283 was designed to evaluate windows, curtain walls, exterior doors, and skylights.
What 75 Pa Means
Test pressure is expressed in pascals, a unit of pressure. The standard test pressure used for code compliance and performance verification is 75 Pa.
A pressure difference of 75 Pa corresponds to wind conditions of approximately 25 mph acting on the exterior face of a building. This is not a worst-case scenario. Wind-driven conditions at this level occur on ordinary weather days in most climates. The 75 Pa test pressure is the threshold at which performance is verified, not an extreme event margin.
For context, the passive pressure differential between a heated interior and a cold exterior on a windy day can approach or exceed 75 Pa at the building envelope. An unsealed opening in the door, such as a standard mail slot, conducts air at a rate determined by the opening geometry and the pressure difference across it. Testing quantifies what that rate is.
Why Mail Slots Are Rarely Tested
Windows and exterior doors are routinely submitted for ASTM E283 testing because code compliance and product certifications require it. A window manufacturer cannot make performance claims without third-party verification. The market demands documentation.
Mail slots are not subject to the same requirement. They are classified as hardware, not as fenestration or door assemblies in most jurisdictions, which means they fall outside the scope of code requirements that would otherwise mandate independent testing. The result is a product category where performance claims, if made at all, are largely unverified.
Mail slots form a direct, unsealed path through the building envelope. The market has treated them as hardware. The building science does not.
This creates a gap between how mail slots are categorized and what they actually do. A mail slot is an opening in the building envelope. What passes through that opening, and at what rate, is a building performance question. Without testing, there is no data to answer it.
What Intertek Verified
Intertek Building and Construction, operating as Architectural Testing, Inc., conducted independent testing on the Brass-Seal™ Dual Forged Brass Mail Slot system under ASTM E283 at its facility in Fresno, California. Three configurations were evaluated: the interior unit alone, the exterior unit alone, and both units deployed together as a paired system.
| Configuration | Test | Result | Allowed Max |
|---|---|---|---|
| Interior Mail Slot | Air Infiltration | 0.6 L/s/m² (0.12 cfm/ft²) | 1.5 L/s/m² (0.3 cfm/ft²) |
| Interior Mail Slot | Air Exfiltration | 0.3 L/s/m² (0.06 cfm/ft²) | 1.5 L/s/m² (0.3 cfm/ft²) |
| Exterior Mail Slot | Air Infiltration | 1.0 L/s/m² (0.20 cfm/ft²) | 1.5 L/s/m² (0.3 cfm/ft²) |
| Exterior Mail Slot | Air Exfiltration | 0.6 L/s/m² (0.11 cfm/ft²) | 1.5 L/s/m² (0.3 cfm/ft²) |
| Paired System (Interior + Exterior) | Air Infiltration | 1.0 L/s/m² (0.20 cfm/ft²) | 1.5 L/s/m² (0.3 cfm/ft²) |
| Paired System (Interior + Exterior) | Air Exfiltration | 0.6 L/s/m² (0.11 cfm/ft²) | 1.5 L/s/m² (0.3 cfm/ft²) |
These results fall within the performance range typically required of exterior door assemblies, placing the closed mail slot in the same class of air leakage performance as the door itself.
All three configurations measured well within the maximum allowable threshold of 1.5 L/s/m² (0.3 cfm/ft²) established by the standard. Each unit functions as an independent system. At the door's interior face, the interior unit alone addresses the building envelope. The exterior unit alone addresses the building envelope at the exterior face. When deployed together, the two units create a dual-stage barrier spanning the full depth of the door assembly.
What the Results Do and Do Not Say
The report documents the tested values for each submitted configuration.
Each configuration, tested independently and in combination, produced measured air leakage rates within the threshold the standard defines as acceptable for exterior door assemblies. The data is in the report. The report number is T4376.01-301-44-R1.
For a product category where independent testing has historically not existed, a report with those numbers is a baseline. It is also a document that architects and specifiers can request and read directly.
Conclusion
A mail slot can now be evaluated rather than assumed. Performance can be specified, not guessed. The opening in the door can now be held to the same standard as the door itself.
For full ASTM E283 performance data and specification details, visit the Brass-Seal™ system page.
1. ASTM International. ASTM E283/E283M: Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Skylights, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen. West Conshohocken, PA: ASTM International. store.astm.org/e0283_e0283m-19.html
2. Intertek Building and Construction. Test Report T4376.01-301-44-R1: Air Leakage Testing of BOTA Brass-Seal™ Dual Forged Brass Mail Slot System per ASTM E283. Fresno, CA: Intertek B&C, 2026. Available on request at info@blockouttheair.com
3. North American Fenestration Standard (NAFS). Voluntary Specification for Windows, Doors, and Skylights. American Architectural Manufacturers Association (AAMA). Referenced for performance classification context.
The Brass-Seal™ Forged Brass Mail Slot System. Independently tested and proven under ASTM E283 by Intertek.