Airtightness & Air Pressure Testing: Where Leaks Happen & How to Seal Them

Summary

Airtightness is one of the most impactful but least understood aspects of energy-efficient building. Every gap or hole in the building envelope allows warm air to escape in winter and cold air to infiltrate — driven by wind pressure and the stack effect (warm air rising through the building). Air leakage accounts for a significant proportion of heat loss in typical UK homes and is much harder to retrofit than insulation. Getting it right during construction or refurbishment requires systematic planning, not just a final-stage seal-up.

The blower door test (or fan pressurisation test) is the standard method for measuring air permeability. A variable-speed fan mounted in an external doorframe pressurises or depressurises the building; the airflow required to maintain a specific pressure differential (50 Pascals is the standard test pressure) is measured and divided by the building envelope area to give the air permeability result in m³/h/m² @ 50 Pa. This result is used in SAP calculations to determine the energy rating.

A common misconception is that airtight buildings are unhealthy or stuffy. In fact, airtight buildings require controlled ventilation — typically Mechanical Ventilation with Heat Recovery (MVHR) for high-performance buildings or decentralised mechanical extract ventilation (dMEV) for good-practice homes. The key principle is that ventilation should be controlled and measured (via MVHR, trickle ventilators, or mechanical extract), not uncontrolled via leaks. Controlled ventilation brings fresh air where needed, in the right quantity, without wasting heat.

Key Facts

• Air permeability target — Approved Document L 2021: design target set by SAP; maximum permitted 10 m³/h/m² @ 50 Pa; good practice is 3–5; Passivhaus requires ≤0.6 ACH @ 50 Pa
• ATTMA TS L1 — UK standard for air pressure testing of dwellings; specifies testing methodology, equipment, and reporting requirements
• Blower door test — fan pressurisation; tests whole building; measures flow rate at 50 Pa; divides by envelope area
• ACH at 50 Pa — air changes per hour at 50 Pa; the Passivhaus metric (different from m³/h/m² used in UK Building Regs); roughly 1 ACH = 2 m³/h/m² [approximate — varies by building geometry]
• Air barrier vs vapour control layer — often combined in the same membrane; air barrier prevents air movement; vapour control layer (VCL) limits moisture diffusion into the structure
• Intelligent membranes — vapour-variable membranes (Pro Clima Intello, Siga Majrex) allow moisture to pass outward in summer while restricting vapour ingress in winter; preferred in high-performance construction
• Continuity of air barrier — the air barrier must form a continuous, connected layer around the entire thermal envelope; any gap or penetration breaks continuity
• Airtightness layer in masonry — typically the plaster coat on the inner face of masonry walls; or a service cavity membrane; or a proprietary airtight plaster (Knauf Airless, Parex Parageste)
• Service cavity — a service cavity inside the air barrier allows pipes and cables to be run without penetrating the air barrier; strongly recommended in high-performance builds
• Tape quality — airtight tapes (Pro Clima Tescon Vana, Siga Fentrim) have much better long-term adhesion and airtightness than standard builders' tape; use the correct tape for the specific material being sealed
• Testing frequency — mandatory for new dwellings in England; timing is at practical completion before commissioning; some projects test twice (intermediate and final)
• Post-occupancy testing — airtightness can degrade over time if joints are not properly taped or if services penetrations are made after testing; plan for robust, permanent sealing

Quick Reference Table

Detailed Guidance

The Four Zones of Air Leakage

Air leakage occurs at any discontinuity in the air barrier. The four main zones to focus on:

1. Wall-to-floor junction (ground floor) The junction between the inner leaf of masonry/timber frame and the ground floor slab or suspended floor is a common gap, particularly where the floor screed doesn't fully seal against the wall. Fix: apply a continuous bead of airtight sealant (Soudal Fix All Flex or equivalent) at the junction before screeding; or use a proprietary airtight skirting system.

2. Wall-to-ceiling/roof junction At eaves level in rooms-in-roof and loft conversions: the membrane must wrap from the roof plane across the junction and connect to the wall membrane. This junction is responsible for a large proportion of air leakage in poorly detailed loft conversions.

3. Service penetrations Every pipe and cable that passes through the air barrier creates a potential leakage path. Fix:

• Electrical cables through masonry: back-fill with fire-rated foam and silicone
• Soil pipes through floors: use a proprietary pipe collar or mould foam around and tape membrane to pipe
• Gas/water pipes through external walls: use expanding foam rated for purpose; seal with silicone; tape membrane to pipe with a collar
• Cable entries in meter cupboards: seal thoroughly — meter cupboards are often poorly sealed and directly connected to outside

4. Windows and doors The interface between the window frame and the surrounding structure is a common weak point. Fix: use an airtight tape (Siga Fentrim or Pro Clima Tescon Vana) to seal the frame to the structural opening; apply on the warm side (inside). Expanding foam alone is not adequate — tape over the foam with an airtight tape.

Airtight Membranes — How to Install Them Correctly

In timber frame or SIPs construction, a continuous membrane on the warm side of the insulation forms the air barrier:

1. Start with a plan — draw out the air barrier layer on the drawings before construction. Know where the membrane goes, where it laps onto other elements, and how service penetrations will be treated.

2. Overlap and tape all joints — membrane sheets must overlap by at least 100mm (some products specify 150mm) and all joints must be taped with a compatible airtight tape. Do not staple overlaps without taping.

3. Service cavity — fit a service cavity batten (typically 38x50mm or 50x50mm) inside the membrane, before boarding, to allow services to be run without penetrating the membrane. This is the single most effective strategy for maintaining airtightness integrity in timber frame.

4. Around structural elements — where timber posts or steel columns penetrate the membrane, cut neatly and tape with a preformed corner piece or a flexible tape capable of following the contour.

5. Test early — if possible, carry out an interim airtightness test before boarding out (when penetrations are still accessible). Fix issues found before they are hidden.

Recessed Downlighters — The Airtightness Enemy

Recessed downlighters in ceilings are one of the most damaging airtightness details in UK housing. A single unprotected recessed light can lose more air than all the window frame junctions combined. Solutions:

• Avoid recessed lights in insulated ceilings — use surface-mounted or suspended fittings
• IP65 rated fire-rated fittings — some recessed fittings are sealed internally and can be installed through a sealed ceiling; check the manufacturer's airtightness certification
• Airtight enclosures — purpose-made rigid enclosures sit above the ceiling, around the light fitting, and are taped to the membrane; the enclosure prevents air movement while the fitting remains accessible from below
• LED panel fittings — a surface-mounted LED panel or a recessed LED plaster-in fitting is a better alternative to a GU10 downlighter in a ceiling that forms part of the thermal envelope

Smoke Testing to Locate Leaks

Smoke testing (or "fog testing") is used to locate specific air leakage paths rather than measure overall air permeability. A theatrical smoke machine is used inside a pressurised or depressurised building; smoke is visible as it is sucked through gaps. This technique is particularly useful for locating service penetrations that are difficult to see from outside.

Procedure:

1. Set up a blower door to depressurise the building to 50 Pa.
2. Introduce smoke inside the building using a smoke machine.
3. Watch around window frames, skirting boards, light fittings, service penetrations, and loft hatches for smoke being pulled out.
4. Mark each identified leakage point for sealing.

Frequently Asked Questions

Do I have to carry out an air pressure test on a new house?

In England and Wales: yes, for most new dwellings. The Approved Document L requirement for air testing applies to all new dwellings unless a default air permeability value is used in the SAP calculation (which penalises the energy rating significantly). Check with building control and the SAP assessor at design stage.

Can I improve the airtightness of an existing house?

Yes — while it is much harder than achieving airtightness during construction, retrofit airtightness improvements are possible and cost-effective. Focus on the biggest leakage paths first: loft hatch (sealing and insulating), chimney breast (removing unused chimneys or fitting a chimney balloon), skirting boards (sealing behind them), and service penetrations.

Won't an airtight house cause condensation and damp?

Not if correctly ventilated. An airtight house should have controlled ventilation — trickle vents in windows, MVHR, or dMEV — providing fresh air at the correct rate. An uncontrolled, leaky house may actually have more condensation risk because warm moist air can reach cold surfaces via unpredictable paths and condense. Controlled, continuous ventilation manages humidity more effectively than background leakage.

Regulations & Standards

• Building Regulations Approved Document L (2021) — Conservation of fuel and power: air permeability requirements for new dwellings

• ATTMA Technical Standard L1 — Measuring air permeability of building envelopes: UK testing methodology standard

• BS EN ISO 9972:2015 — Thermal performance of buildings: determination of air permeability of buildings (fan pressurisation method)

• Passivhaus Institute — Criteria for Passivhaus certification including ≤0.6 ACH @ 50 Pa requirement

• ATTMA — Air Tightness Testing & Measurement Association — ATTMA TS L1 standard and certified tester directory

• Pro Clima UK — Airtightness Products and Guidance — Membrane selection, taping guidance, and technical data

• Energy Saving Trust — Airtightness Guidance — Overview of airtightness in UK homes

• NHBC Technical — Airtightness Details — Construction detail guidance for new build housing

• ventilation strategy — Ventilation strategy: MVHR, dMEV, and Part F requirements

• loft insulation — Loft insulation and the importance of sealing the loft hatch

• thermal bridging — Thermal bridging and its interaction with airtightness

• part l energy — Part L energy efficiency requirements in full