The air permeability test is a standardised laboratory (or field) procedure that measures the volume of air passing through a closed window or curtain wall assembly under a controlled pressure differential. The result directly quantifies gasket and seal performance — the primary barrier against air infiltration.
Air infiltration is the silent power consumer in buildings. Unlike visible water leaks, air leakage through degraded gaskets and poorly detailed joints is continuous, undetected, and cumulative over the building's lifetime. Research attributes 30–40% of HVAC energy loss in commercial buildings to facade air infiltration.
| Standard | Scope | Application |
|---|---|---|
| ASTM E283 | Laboratory air infiltration | Windows, curtain walls, storefronts — new specimens |
| ASTM E783 | Field air infiltration | Installed fenestration on existing buildings |
| ASTM E1424 | Building envelope leakage | Whole-facade testing, large-scale assemblies |
| AAMA 502 | Field testing protocol | Acceptance criteria for installed products |
| IS 3660 (India) | Air permeability — Indian BIS standard | Tested at 600 Pa; Class A3 limits; ECBC mandatory |
The specimen — a fully assembled window or curtain wall unit including frame, sash, glazing, gaskets, and all hardware — is mounted in a rigid test chamber. All perimeter edges are masked with impermeable tape. This masking is critical: it isolates product leakage (through joints, gaskets, and hardware interfaces) from frame-to-wall leakage, ensuring the test measures only the product itself.
The blower induces a controlled pressure difference across the specimen. Testing is conducted in both the positive direction (pressure inward — simulating wind push) and negative direction (suction inward — simulating leeward facade suction). Each level is held for a stabilisation period before the reading is recorded.
| Pressure | psf equiv. | Hold Period | Purpose / Notes |
|---|---|---|---|
| 75 Pa | 1.57 psf | 10 min | Baseline low-pressure check |
| 150 Pa | 3.13 psf | 10 min | Standard design pressure — residential |
| 300 Pa | 6.24 psf | 10 min | Moderate wind condition |
| 600 Pa | 12.5 psf | 10 min | High-wind / IS 3660 Class A3 test pressure |
| 900 Pa | 18.8 psf | 10 min | High-rise or wind-exposed facade specification |
| 1200 Pa | 25.0 psf | 10 min | Curtain wall special performance projects |
The flow meter records the total air volume passing through the specimen at each pressure level. The result is normalised in two ways, depending on fenestration type:
Units: L/s·m² or cfm/ft²
Where Q = measured flow rate (L/s) and A = overall frame area (m²). Standard reporting method for curtain walls.
Units: L/s·m or cfm/ft
Where L = total operable sash perimeter (m). Isolates gasket compression performance around the sash.
| Class | Max Leakage | Application | Gasket Requirement |
|---|---|---|---|
| Class A | ≤0.3 L/s·m² @ 75 Pa | High-performance commercial facades | EPDM 70° Shore A, continuous at corners |
| Class B | ≤1.5 L/s·m² @ 75 Pa | Standard residential windows | EPDM or TPE, sash perimeter, inspectable |
| Class C | ≤3.0 L/s·m² @ 75 Pa | Light commercial / budget residential | Minimum spec — not recommended for facades |
| IS 3660 A3 | ≤1.0 m³/h·m² @ 600 Pa | Indian BIS standard — ECBC compliant | Mandatory for energy-code buildings |
Mount the fully assembled specimen in the test chamber. Apply impermeable masking tape to all perimeter joints between the specimen frame and the chamber surround. Close and latch all operable sashes at all hardware points. Verify gasket continuity visually at all four corners before proceeding.
Zero the differential pressure manometer with the chamber sealed and blower off. Verify flow meter calibration is current (within 12 months per ASTM requirement). Set data logger sampling interval to 1 second minimum.
Apply 75 Pa differential and hold for 10 minutes. This pre-conditions the gaskets and seals, ensuring all components have reached their operational compression state before measurements begin. Record flow rate at end of period.
Increment to 150 Pa, 300 Pa, and 600 Pa (or higher per project specification). At each level, hold for the full stabilisation period and record the steady-state flow rate. Test both positive and negative directions independently.
For each pressure level, compute: Leakage = Q / A (per area) or Q / L (per crack length). Apply the correction factor for ambient temperature and barometric pressure as specified in ASTM E283 Section 10.
For installed windows, a portable test chamber is clamped over the exterior face of the unit. Results are compared against the laboratory test certificate. Field acceptance criteria (AAMA 502) allows a 50% increase over the laboratory limit to account for installation variation.
Report leakage values at each pressure differential, direction, specimen dimensions, test date, ambient conditions, masking details, and pass/fail status against the specified acceptance class.
The air permeability test is conducted on a new specimen under laboratory conditions. It does not replicate the degradation that increases leakage over time in service. Key mechanisms:
EPDM and silicone gaskets expand and contract with temperature. Repeated cycling causes permanent compression set — the gasket loses elastic memory and fails to maintain sealing pressure. A gasket that passes Class A on day 1 may exceed Class C limits after 5 years without UV stabilisers.
Urban pollutants (NOx, SO₂) cause chemical degradation. UV radiation embrittles EPDM within 5–8 years without carbon black and UV stabilisers. Surface cracking precedes bulk failure and is often undetected until the next permeability test.
Casement and sliding window gaskets are abraded at every operation. Sash rattle from loose hardware causes accelerated wear at contact zones. Without a maintenance schedule, failures go undetected for years.
Mitred corners with discontinuous gaskets are the single largest source of air leakage at fabrication stage. The corner joint must be vulcanised or moulded — field-cut butt joints degrade rapidly and fail the E283 test at 300 Pa or above.
Corroded friction stays increase opening torque — users force the sash, deforming the frame and breaking gasket seating. Corroded anchor fasteners loosen the frame, creating a perimeter gap that bypasses the gasket entirely.
| Component | Specified Material | Why |
|---|---|---|
| Gaskets | EPDM 70° Shore A + UV stabiliser + carbon black | Resists UV, ozone, thermal cycling. Carbon black = UV screen. |
| Hardware body | SS 316 / PA66-GF30 polyamide | No bi-metallic corrosion with aluminium frame |
| Fasteners | A4-70 stainless (marine grade) | No galvanic pair with aluminium; resists salt spray |
| Sealants | Neutral-cure silicone (not acetic acid cure) | Acetic acid cure corrodes aluminium and hardware |
| Friction stays | SS with PTFE-coated bearing surface | PTFE reduces friction, prevents sash forcing and gasket damage |
| Anchor fasteners | SS 316 expansion anchor or SS threaded rod | Eliminates frame loosening from fastener corrosion |
| Hardware class | Class 4 per IS 16225 / EN 1670 | 240h salt spray — minimum for coastal / urban facades |