Air Permeability Test Procedure

Air permeability measures the rate at which air passes through a material—whether a textile fabric or a building envelope—under a specified pressure difference applied perpendicularly across a defined test area. This article focuses on practical, step-by-step test procedures rather than abstract theory, covering both fabric testing in laboratory settings and whole-building air tightness testing using blower door equipment. Getting the procedure right matters for repeatability, regulatory compliance with standards like ISO 9237:1995 for textiles or ATTMA/Part L for buildings, and maintaining consistent quality control. For instance, a fabric tested at 100 Pa and 20 cm² might show 500 mm/s air permeability, directly impacting its suitability for outdoor jackets.

Key purposes of air permeability testing:

• Assessing breathability and comfort properties in textile fabrics
• Verifying wind resistance for outdoor clothing and protective garments
• Confirming filtration efficiency for medical products and industrial filters
• Measuring air leakage rate through building envelopes for energy performance certification
• Ensuring proper ventilation design by quantifying envelope air tightness

Key Standards and Test Methods for Air Permeability

Test methods vary slightly depending on the governing standard, but the core steps—conditioning samples, applying a controlled pressure differential, and measuring air flow—remain fundamentally similar across all protocols.

Major textile standards:

• ISO 9237:1995 – Determines air permeability of fabrics using 100–2500 Pa pressure and 5–100 cm² test areas
• ASTM D737-18 – Standard test method for air permeability of textile fabrics; default 125 Pa pressure and 20 cm² test area
• BS 5636 – British standard for air permeability testing of fabrics
• GB/T 5453 – Chinese equivalent largely interchangeable with ISO 9237
• JIS L1096 – Japanese industrial standard covering airflow through textiles

Building-related guidance:

• ATTMA TSL1 – Technical Standard for measuring air permeability of building envelopes in dwellings (UK)
• ATTMA TSL2 – Covers non-dwellings and larger commercial structures
• Part L Building Regulations – UK regulatory framework referencing air tightness testing requirements for new dwellings

The procedural sections that follow primarily reference ISO 9237:1995 and ASTM D737-18 for fabric testing, alongside a generic blower door protocol aligned with ATTMA guidance for building envelopes.

Equipment Required for Air Permeability Tests

Accurate procedure depends on correctly specified and calibrated equipment. Whether you’re conducting textile testing in a controlled laboratory or measuring air leakage across an entire building fabric, the instrumentation must deliver reliable results within documented accuracy tolerances.

Textile Testing Equipment

An air permeability tester for fabrics typically includes:

• Test head with interchangeable orifices – Common sizes: 5, 20, 50, and 100 cm² to accommodate materials with different levels of openness
• Clamping ring with rubber gasket – Creates an airtight seal around the test sample to prevent air leakage around edges
• Differential pressure gauge – Measures pressure drop across the specimen; typical range 50–500 Pa with ≥2% accuracy
• Controlled vacuum or pressure source – Maintains stable pressure difference during measurement
• Airflow meter – Calibrated flow meters covering approximately 1–10,000 dm³/min with ±2% accuracy
• Digital display or data acquisition system – Outputs results directly in mm/s or L/(m²·s)

Modern instruments measure air flow rate automatically once the target differential pressure stabilizes, eliminating manual timing and calculation errors.

Building Testing Equipment (Blower Door Kit)

For whole-building air tightness testing, the essential kit comprises:

• Calibrated fan – Variable-speed fan capable of generating airflow sufficient to achieve 10–60 Pa pressure difference across the envelope
• Temporary door panel – Adjustable frame fitting standard external doorways; mounts the fan assembly
• Pressure hoses and taps – Connect internal and external reference points to the manometer
• Digital manometer – Measures pressure differential typically in the 10–1000 Pa range with resolution to 0.1 Pa
• Test software – Controls fan speed, logs data points, and calculates air permeability at reference pressures
• Leakage detection tools – Smoke pencils or infrared cameras help identify major leakage paths

Typical building test systems measure air permeability in the range of 0.01–20 m³/(h·m²) at 50 Pa, covering everything from highly sealed passive houses to older, leakier construction.

Sample Preparation and Conditioning

Correct specimen preparation is critical to obtaining a valid permeability test. Poorly prepared samples introduce variability that masks true material performance and can lead to inconsistent results between laboratories or production batches.

Cutting and Selecting Specimens

• Cut at least 5 specimens of approximately 100 mm × 100 mm (or as specified by the test standard)
• Take samples from different areas across the roll width and along its length to capture structural variability
• Avoid areas with visible defects, folds, or contamination
• For asymmetric fabrics, mark the technical face and record which side will face the low pressure side during testing

Conditioning Requirements

Environmental conditions significantly affect air permeability measurements because humidity influences fiber swelling and fabric structure:

• Pre-relax samples if they were folded, rolled, or compressed during transport
• Condition specimens in a standard atmosphere—typically 20 ± 2°C and 65 ± 4% RH per ISO 139—for at least 24 hours
• Maintain conditioning until testing; avoid handling specimens with bare hands after equilibration

Special Considerations for Coated or Multilayer Fabrics

Fabric Type	Orientation Requirement	Notes
Single-layer woven	Technical face down (toward low pressure)	Follow standard or customer specification
Coated fabrics	Coating facing low pressure side	Prevents coating damage from clamping pressure
Multilayer composites	Specified layer sequence	Document orientation in test report
Nonwovens	Either side unless specified	Some nonwovens show 10–50% directional difference

Step-by-Step Air Permeability Test Procedure for Fabrics

This section provides a practical, chronological procedure based on ISO 9237 and ASTM D737. Following these steps systematically will help you measure air permeability consistently and generate accurate data for quality decisions.

Step 1 – Instrument Setup

• Connect power supply and compressed air or vacuum source
• Verify that test heads and rubber gaskets are clean and undamaged
• Check the calibration certificate date; recalibrate if overdue
• Zero the pressure gauge and flow meter with no specimen installed
• Confirm the instrument displays stable zero readings before proceeding

Step 2 – Selecting Test Parameters

Choose test conditions based on your governing standard or customer specification:

Parameter	Typical Value	Notes
Test area	20 cm²	Default for apparel fabrics; use 5 cm² for tight weaves, 100 cm² for open structures
Pressure drop	100 Pa	Common for clothing textiles; 200 Pa for industrial fabrics
Number of readings	≥5 per specimen	More readings improve statistical confidence

Record the selected parameters before beginning measurements.

Step 3 – Mounting the Specimen

• Place the conditioned specimen flat on the test head, technical face oriented as required
• Center the test area over the orifice, ensuring no creases cross the measurement zone
• Lower the clamping ring smoothly and tighten to compress the gasket
• Inspect the seal visually—there should be no visible gap where air could bypass the fabric
• If the specimen slips or bunches, reposition and reclamp

Step 4 – Running the Test

• Initiate airflow so the instrument adjusts automatically toward the target pressure difference
• Wait until both pressure and flow readings stabilize—typically 5–10 seconds
• Record the displayed air flow rate or direct air permeability value
• Note any fluctuations; unstable readings may indicate edge leakage or instrument issues

Step 5 – Repetition

• Test each specimen in at least 5 distinct, non-overlapping areas
• Calculate the mean value and standard deviation for the specimen
• Repeat across all 5+ specimens to generate a sample-level average
• A coefficient of variation below 10% indicates good repeatability; higher values suggest material variability or procedural issues

Step 6 – Calculations

Modern instruments typically output results directly, but the underlying formula is:

Air Permeability = Volume Flow Rate ÷ Test Area

For example, if 400 cm³ of air passes through a 20 cm² specimen each second at 100 Pa:

• Air Permeability = 400 cm³/s ÷ 20 cm² = 20 cm³/(cm²·s) or equivalently 200 mm/s

Convert units as required by your reporting standard (mm/s, L/m²/s, CFM/ft²).

Documentation Checklist

Always record the following to make the procedure auditable:

• Test area and pressure differential used
• Atmospheric conditions during testing
• Sample identification and orientation
• Individual readings and calculated statistics
• Instrument model and calibration status
• Test date and operator name

Air Permeability Test Procedure for Building Envelopes (Blower Door)

For whole buildings, air permeability is measured as the air leakage rate through the envelope at a reference pressure—typically 50 Pa—normalized by envelope area or floor area. This air testing procedure quantifies how easily air can pass through the building fabric, directly impacting energy performance and indoor air quality.

Preparation

• Schedule the test after major envelope work is complete (windows, doors, seals installed)
• Document which intentional openings will be sealed: trickle vents, extract fans, flues, letterboxes
• Seal openings temporarily using appropriate materials (tape, inflatable bungs, sheet plastic)
• Record the building’s floor area and envelope area for normalization calculations
• Ensure all internal doors are open to create a single pressure zone

Equipment Setup

• Fit the adjustable blower door frame into the main external doorway
• Mount the calibrated fan in the temporary panel, ensuring an airtight seal around the frame
• Connect inside pressure hose to a location away from direct airflow (at least 1.5 m from the fan)
• Connect outside pressure hose to a sheltered external point, protected from wind effects
• Link hoses to the digital manometer and verify communication with test software
• Confirm baseline pressure reading is near 0 Pa with the fan off

Test Sequence

The test creates a series of pressure points to characterize the envelope’s flow–pressure relationship:

• Start fan at low speed and gradually increase to achieve approximately 10 Pa difference
• Record airflow rate at 10 Pa, then increment pressure in 5–10 Pa steps up to 60 Pa
• Allow readings to stabilize at each point before logging
• Perform pressurization run (pushing air into building) first
• Repeat with depressurization run (pulling air out) if required by your test standard
• Aim for at least 5 data points across the pressure range for curve fitting

Data Handling

Test software fits the measured data to a power-law relationship:

Q = C × ΔP^n

Where Q is airflow rate, C is a flow coefficient, ΔP is pressure difference, and n is the flow exponent (typically 0.5–1.0).

From this relationship, the software calculates:

Metric	Typical Units	Description
Air permeability at 50 Pa	m³/(h·m²)	Airflow per square metre of envelope area
Air changes per hour at 50 Pa (ACH50)	h⁻¹	Airflow divided by building volume
Equivalent leakage area	cm²	Hypothetical hole size causing same leakage

Quality Checks

Before accepting results:

• Verify wind speed remained below 6 m/s during testing (check local weather station)
• Confirm indoor–outdoor temperature difference was manageable (ideally <15°C)
• Review the flow–pressure curve for smooth progression; erratic points suggest doors opening or gusty conditions
• Use smoke pencils or infrared imaging to identify major leakage paths during or after the pressure test
• If results seem anomalous, check that all temporary seals remained intact

Test Certificate Requirements

The final certificate should document:

• Building address, description, and floor/envelope areas
• Test date and weather conditions
• Standards followed (e.g., ATTMA TSL1:2021)
• Openings sealed and method of sealing
• Measured air permeability result with units
• Pass/fail determination against applicable building regulations

Recommended Test Conditions and Typical Settings

Consistent environmental conditions and test conditions are essential for achieving comparable results between laboratories, test dates, and different operators. Small variations in temperature, humidity, or pressure settings can cause varying results that mask true material or building performance.

Fabric Testing Conditions

• Conditioning climate: 20 ± 2°C, 65 ± 4% RH (per ISO 139)
• Equilibration time: Minimum 24 hours; some dense materials may need 48 hours
• Test pressure drops:
  • 50 Pa – Very open structures, coarse nonwovens
  • 100 Pa – General apparel fabrics, sports textiles
  • 200 Pa – Tighter weaves, industrial fabrics
  • 500 Pa – Coated fabrics, very low permeability materials
• Test areas:
  • 5 cm² – Extremely tight weaves where flow would be unmeasurable at larger areas
  • 20 cm² – Default for most apparel and home textiles
  • 50–100 cm² – Open nonwovens, very high air permeability materials

Building Testing Conditions

• Wind speed: Below 6 m/s (ideally calm conditions)
• Temperature difference: Indoor–outdoor ΔT < 15°C preferred
• Baseline pressure: Near 0 Pa with fan off; natural stack effect should be minimal
• Pressure range: 10–60 Pa in 5–10 Pa increments

Comparative Example

Consider two fabrics tested under the same conditions (100 Pa, 20 cm² test area):

Fabric Type	Air Permeability	Typical Application
Lightweight polyester mesh	800 mm/s	Sports T-shirt, high breathability required
Tight-woven nylon with DWR finish	35 mm/s	Windproof outdoor jacket shell

The mesh allows approximately 23 times more air flow, explaining why it feels cooler but offers no wind protection—demonstrating how test results translate to wearer’s comfort and functional performance.

Interpreting Results and Common Acceptance Criteria

Raw air permeability numbers must be interpreted against product requirements, comfort targets, or regulatory limits. A value that represents a pass for one application may be a clear failure for another.

Textile Acceptance Criteria

Higher permeability improves breathability but reduces wind resistance. Typical target ranges include:

Application	Target Range	Rationale
Windproof outerwear	< 50 mm/s	Prevent air leakage; maintain thermal insulation
Softshell jackets	50–150 mm/s	Balance breathability and wind protection
Sports T-shirts	200–800 mm/s	Maximize airflow for cooling during activity
Filtration media	Varies by efficiency class	Must balance airflow rate against particle capture
Parachute fabric	300–500 cm³/(cm²·min)	Uniform permeability ensures reliable deployment

When results fall outside specification, consider adjusting fabric structure, finishing treatments, or coating weight.

Building Acceptance Criteria

UK and European building regulations typically specify maximum air permeability at 50 Pa:

Building Type	Typical Limit	Notes
New dwellings (Part L 2021)	8 m³/(h·m²) @ 50 Pa	Some schemes require 5 m³/(h·m²)
Low-energy dwellings	3 m³/(h·m²) @ 50 Pa	Requires MVHR for proper ventilation
Passivhaus standard	≤ 0.6 ACH50	Extremely airtight; mechanical ventilation essential
Commercial buildings	Varies by use class	May follow ATTMA TSL2 guidance

Lower air leakage rates improve energy efficient operation but may require mechanical ventilation systems to maintain indoor air quality. Buildings failing their target must undergo leak detection and remediation before retesting.

Worked Pass/Fail Examples

Example 1 – Outdoor Jacket Fabric:

• Measured: 42 mm/s at 100 Pa
• Specification: < 50 mm/s
• Result: PASS – Suitable for windproof outdoor clothing

Example 2 – New Dwelling:

• Measured: 6.2 m³/(h·m²) @ 50 Pa
• Target: 5 m³/(h·m²) @ 50 Pa
• Result: FAIL – Must identify and seal leakage paths, then retest

Best Practices, Troubleshooting, and Retesting

Robust procedure includes proactive checks for errors and clear guidance on handling inconsistent results. Catching problems early prevents wasted testing time and incorrect quality decisions.

Common Issues in Fabric Testing

Problem	Likely Cause	Effect on Results	Solution
Artificially high permeability	Edge leakage around clamp	Bypass air inflates readings 10–25%	Check gasket condition; increase clamping pressure
Unstable readings	Insufficient stabilization time	Varying results between readings	Wait longer; verify pressure source stability
Unexpectedly low values	Sample over-clamped or folded	Compressed structure reduces flow	Reposition specimen; ensure flat mounting
Poor repeatability	Unconditioned samples	Moisture variability affects structure	Extend conditioning time; verify atmosphere
Zero or minimal flow	Clogged instrument filters	Restricts airflow path	Clean or replace filters per maintenance schedule

Common Issues in Building Testing

Problem	Likely Cause	Effect on Results	Solution
Fluctuating pressure readings	Wind gusts >6 m/s	Unstable baseline; erratic curve	Reschedule for calmer conditions
Higher than expected leakage	Unsealed intentional openings	Inflated air permeability value	Re-check all vents, flues, and penetrations
Results outside calibrated range	Building too leaky or too tight	Inaccurate extrapolation to 50 Pa	Use different fan ring; verify equipment range
Pressure won’t stabilize	Door opened during test	Massive uncontrolled air path	Secure building; restart test sequence

Retesting Protocols

• Single measurement outliers: If one reading deviates by more than 20% from others, investigate cause before discarding
• Systematic deviation: If all readings trend high or low, check calibration status and environmental factors
• Post-remediation retests: After sealing leaks in buildings or adjusting fabric processes, allow sufficient time before retesting (fabrics: recondition; buildings: sealants must cure)
• Full retest triggers: Equipment malfunction, specimen contamination, or protocol deviation discovered mid-test

Documentation Recommendations

• Develop and maintain a written Standard Operating Procedure (SOP) referencing ISO 9237, ASTM D737, ATTMA, or other standards as applicable
• Log all calibration dates, maintenance activities, and equipment serial numbers
• Archive raw data alongside calculated results for traceability
• Review procedures annually and update for new developments in standards or equipment

Conclusion

A reliable air permeability test procedure combines adherence to recognized standards, correctly specified and calibrated equipment, rigorous sample preparation, and disciplined step-by-step execution. Whether you’re testing textile fabrics for breathability and wind resistance or measuring building envelope air tightness for energy efficiency and regulatory compliance, the fundamentals remain consistent: control environmental conditions, apply the correct pressure differential, measure air flow accurately, and document everything for traceability.

Controlling and measuring air permeability is essential for both material performance—affecting wearer’s comfort, filtration efficiency, and durability—and building performance, where it directly impacts heat loss, moisture control, and energy consumption. Organizations conducting this testing should develop or refine their internal procedures based on the structure outlined here, ensuring alignment with standards like ISO 9237, ASTM D737, or ATTMA guidance. With a well-documented SOP, calibrated instruments, and trained operators, you can generate accurate data that supports confident quality decisions and regulatory compliance.