Air Tightness Testing for London Extensions and New Build: What Is It, When Is It Required, and What Results to Expect

**What a blower door pressure test measures**:

A blower door test (also called an air permeability test or air leakage test) is a standardised method (BS EN ISO 9972:2015) of measuring the total rate of air leakage through the building fabric at a standardised pressure differential. The test is conducted by: 1. Temporarily sealing all intentional openings (extractor fans; trickle vents; letter boxes; etc.) that would be sealed in normal occupied use 2. Mounting a calibrated fan (the 'blower door') in an external doorway 3. Using the fan to pressurize (or depressurise) the building to 50 Pascals (Pa) of pressure difference relative to outside 4. Measuring the fan air flow rate required to maintain 50 Pa — this flow rate represents the total air leakage of the building fabric at 50 Pa 5. Dividing the fan flow rate (m³/hour) by the total building envelope area (m²) to give the air permeability in m³/h/m² @ 50 Pa

*Alternatively*: the test result can be expressed in Air Changes per Hour at 50 Pa (ACH₅₀) — the total volume of air leaked per hour divided by the internal volume of the building. The Passivhaus standard uses ACH₅₀ as its performance metric.

• *Converting between metrics*: approximately, for a typical 3-bedroom London terrace:
• •5 m³/h/m² @ 50 Pa (Part L 2021 target) ≈ 5–8 ACH₅₀ depending on the ratio of envelope area to internal volume
• •Passivhaus target: 0.6 ACH₅₀ is approximately 0.5–0.8 m³/h/m² @ 50 Pa

**When is a pressure test required?**

*New-build dwellinghouses (England)*: a blower door pressure test at completion is mandatory for all new-build dwellinghouses under Part L 2021. The target air permeability is 5 m³/h/m² @ 50 Pa. A new-build dwelling that fails this test (i.e., exceeds 5 m³/h/m²) does not have a valid SAP energy certificate, which is required before an Energy Performance Certificate (EPC) can be issued, which is required to sell or rent the property.

*Extensions to existing dwellinghouses*: Part L 2021 does **not** require a mandatory pressure test for extensions. The extension must meet the Part L 2021 fabric U-value targets (walls; roof; floor; windows) and the insulation/construction must be completed to a reasonable standard — but there is no mandatory air permeability test.

• *When a voluntary pressure test is worthwhile for a London extension*:
• •The extension incorporates MVHR — MVHR ventilation systems depend on the building being reasonably airtight to function correctly. An MVHR system in a very leaky extension will waste most of its heat recovery benefit because large quantities of unconditioned air enter through gaps in the fabric, bypassing the MVHR heat exchanger. A pressure test confirms that the MVHR installation is appropriate for the fabric performance
• •The extension is a high-specification energy upgrade (Passivhaus EnerPHit retrofit standard; high-fabric-performance specification) — where the client wants to demonstrate performance
• •The client is applying for a Green Homes Grant voucher or similar incentive that requires demonstrated energy performance
• •The architect or designer has specified an airtight construction approach and wants to confirm the construction team has achieved the design intent

**Typical air tightness values for London buildings**:

| Building type | Typical air permeability | |---|---| | Unimproved Victorian terrace (solid brick walls; original sash windows; no draught-proofing) | 15–30 m³/h/m² @ 50 Pa | | 1970s–1990s cavity wall house (some draft-proofing; modern windows) | 10–20 m³/h/m² @ 50 Pa | | New-build to Building Regs 2010–2021 standard | 5–10 m³/h/m² @ 50 Pa | | Good-practice new build (above Part L minimum) | 3–5 m³/h/m² @ 50 Pa | | Very good practice / BREEAM Excellent | 1–3 m³/h/m² @ 50 Pa | | Passivhaus EnerPHit (retrofit) | < 1.0 m³/h/m² @ 50 Pa | | Passivhaus new build | ≤ 0.6 ACH₅₀ (≈ 0.5–0.8 m³/h/m²) |

*Implication for London extensions*: a well-built London rear extension, with careful attention to air tightness at junctions and penetrations, can realistically achieve 3–6 m³/h/m² — significantly better than the existing Victorian house. The junction between the new extension and the existing house is typically the weakest air tightness point.

**The continuous air barrier principle**:

The key principle of air tightness construction is that there must be a continuous, uninterrupted air barrier layer around the entire building envelope — including the junction between the extension and the existing house. A single gap in the air barrier (even a 5mm gap around a pipe penetration) can account for a significant proportion of the total air leakage through the building.

• The air barrier layer is typically one of:
• •The inner face of the external wall (rendered blockwork or plastered masonry wall)
• •A dedicated airtight membrane (VCL — vapour control layer — taped as an air barrier, used in timber-frame and SIPs construction)
• •Rigid foam board insulation (PIR; EPS) with taped joints
• •Wet plaster finish on a masonry wall (wet plaster provides a better air seal than dry-lined plasterboard on battens — a significant reason why wet plaster is still preferred in air tightness-conscious construction)

**The ten most common air leakage points in London extensions**:

*1. Wall/floor junction (where the extension wall meets the ground floor slab or subfloor)*: This is typically the most significant air leakage point in a ground-floor extension. The structural wall and the concrete slab rarely form a fully airtight junction without deliberate detailing. Solution: airtight mastic sealant (good quality polyurethane sealant; butyl mastic) at the wall/slab junction; ensure any insulation in the wall/floor junction is fully continuous.

*2. Wall/roof junction (where the extension wall meets the flat roof structure)*: The junction between the extension masonry wall and the flat roof structure (typically a timber flat roof deck) is another high-risk air leakage point. Solution: ensure the wall plate (top of wall) is sealed to the flat roof build-up with a continuous airtight mastic or tape. In a timber-frame flat roof, use an airtight membrane at the wall/roof junction, taped to both the wall air barrier and the roof air barrier.

*3. Window frames (perimeter of the window frame to the surrounding masonry)*: The gap between a window frame and the surrounding masonry opening is a significant air leakage path in all window types. In Victorian properties, this gap is often filled only with mortar (which cracks over time) or with expanding polyurethane foam (which also deteriorates). Solution: apply a continuous bead of airtight mastic (polyurethane sealant or butyl mastic) on the inner face of the junction between the window frame and the wall reveal, before finishing plaster is applied — this ensures a continuous air seal at the window perimeter.

*4. Pipe and cable penetrations through the external wall or floor*: Every pipe (boiler flue; soil pipe; gas pipe; drainage; UFH supply/return pipes) and cable (electrical conduit; data cables; TV aerial) that passes through the external wall or ground floor slab is a potential air leakage point. Solution: fire-rated intumescent collars or firestop compounds around pipe/cable penetrations (which also provide Part B cavity barrier compliance); acoustically sealed and airtight grommets for cable penetrations.

*5. Loft hatch (where the extension ceiling meets the loft space)*: A standard loft hatch in an extension ceiling is a major air leakage point. The lightweight plywood hatch lifts on air pressure and allows direct exchange between the heated space and the cold loft. Solution: insulated, draught-stripped loft hatch (e.g., Manthorpe Energy-Saving Loft Hatch) with a good-quality compression seal; or eliminate the loft hatch from the extension roof by making the extension a fully sealed warm roof (insulation above the structural deck).

*6. Recessed downlights in the extension ceiling*: Recessed LED downlights (GU10 spot fittings) in extension ceilings cut through the ceiling plasterboard and create direct air paths from the warm room below to the cold roof void above. Solution: use airtight downlight covers (plastic covers that clip over each downlight fitting above the ceiling) — these are a quick, cost-effective retrofit for downlights already installed; or specify surface-mounted or integrated ceiling lights in airtight extensions rather than recessed downlights.

*7. Junction between the extension and the existing house (the most critical junction)*: The point where the new extension connects to the existing house is almost always the most difficult air tightness junction to achieve. The existing house fabric (Victorian solid brick; 1930s cavity brick) is inherently leaky — and the junction between the new extension's airtight construction and the existing leaky wall is a significant weak point. Solution: ensure that the junction is sealed on the inside with a continuous airtight mastic and a bridging tape (airtight tape applied across the junction); in masonry-to-masonry junctions, a bead of polyurethane sealant at the internal face before plaster is applied.

*8. Extractor fan apertures in external walls*: Extractor fan outlets cut through the external wall and, if not fitted with a self-closing back-draught shutter, allow air to flow freely when the fan is not running. Solution: specify extractor fans with automatic back-draught shutters (a spring-loaded or gravity damper that closes when the fan stops running); or install a dedicated recirculating extractor fan (no external duct needed in some kitchen configurations).

*9. Velux or rooflight frame perimeter (in flat or pitched roof)*: Rooflight frames penetrate the roof structure and, at the frame/structure junction, can be significant air leakage points — particularly if the installation relies on expanding foam alone at the perimeter. Solution: seal the rooflight frame to the surrounding roof structure with a dedicated airtight flashing tape (e.g., Illbruck airtight tape; Pro Clima Tescon tape); ensure that the rooflight manufacturer's flashing kit is used correctly at the upstand/curb junction.

*10. Service ceiling void (in extensions with a service void above the plasterboard ceiling)*: A common construction detail in extensions is a service void (a small gap between the plasterboard ceiling and the structural flat roof) for running electrical cables, MVHR ducts, and other services. If the service void is connected to the loft or to external air, it undermines the air barrier. Solution: ensure the service void is fully sealed at its perimeter (all junctions with external walls and the roof structure), and that all penetrations into the void from the room below are airtight.

**Why air tightness and ventilation must be designed together**:

There is a common misunderstanding that building airtight is dangerous because it prevents fresh air from entering the building. This is wrong: the purpose of air tightness construction is not to eliminate air exchange, but to make air exchange *controlled* — that is, all air entering and leaving the building passes through the designed ventilation system (MVHR; trickle vents; extract fans) rather than through random gaps in the fabric.

• In an uncontrolled (leaky) Victorian terrace:
• •Air leaks through gaps in sash window frames; gaps at floor/wall junctions; around pipes; under skirting boards — wherever the wind creates a pressure differential
• •In winter, this uncontrolled leakage brings in cold outdoor air that must be heated by the central heating — a significant energy cost
• •The leakage rate is unpredictable and not correlated with the actual ventilation need of the occupants
• •Moisture from cooking and showering can be driven into the wall structure by the random air pressure — contributing to interstitial condensation

• In an airtight extension with MVHR:
• •The only significant air exchange is through the MVHR system
• •The MVHR system recovers 85–95% of the heat from the extracted air and transfers it to the incoming fresh air — dramatically reducing the heating energy cost of ventilation
• •The MVHR continuously maintains indoor air quality without the energy penalty of uncontrolled infiltration
• •Indoor humidity is managed: the MVHR removes moisture continuously, preventing the humidity spikes that cause surface condensation

**The air tightness target for MVHR-designed extensions**:

For MVHR to function correctly and provide its designed heat recovery benefit, the building should have an air permeability of approximately 3–5 m³/h/m² @ 50 Pa or better. At air permeabilities above 5–7 m³/h/m², the benefit of MVHR heat recovery is significantly undermined by uncontrolled infiltration bypassing the system. The tighter the building, the more effective the MVHR.

*Passivhaus EnerPHit standard for retrofit*: the Passivhaus Institute's EnerPHit standard for retrofit projects (as opposed to new-build Passivhaus) accepts a higher air permeability than new-build Passivhaus (1.0 m³/h/m² @ 50 Pa for EnerPHit vs 0.6 ACH₅₀ for new-build). Achieving EnerPHit air permeability in a retrofit London extension is challenging but achievable with careful attention to all the leakage points listed above.

**Practical advice for improving air tightness in London extensions**:

• *During construction — the right stage for air tightness work*:
• •Air tightness is best addressed during construction, not after. Retroactively sealing a completed extension is significantly harder and less effective than building it right from the start
• •The critical airtight layer should be identified on the architectural drawings: in a masonry extension, the inner face of the blockwork/masonry (which receives the plaster) is the air barrier layer; in a timber-frame extension, the airtight membrane on the inner face of the frame is the air barrier layer
• •All trades must understand where the air barrier is and must seal any penetrations they make through it. A plumber running a pipe through the external wall without sealing the penetration undermines the air barrier at that point

• *Wet plaster vs dry-lining for airtight masonry walls*:
• •Wet plaster (sand:cement or gypsum plaster on masonry) provides a significantly better air seal on masonry walls than dry-lining (plasterboard on battens with a VCL). This is because wet plaster forms a continuous seal over the masonry face, filling all small gaps and voids. Dry-lined walls have a service void between the plasterboard and the masonry — and unless the VCL is meticulously taped and sealed, this void connects to the external air at junctions and penetrations
• •For airtight masonry extension construction: specify wet plaster finish on all external masonry walls in preference to dry-lining where possible
• •Where dry-lining is used (for thermal performance, to keep wall thickness down, or for MVHR service void): use a dedicated airtight VCL (e.g., Intello Plus; Vario KM Duplex) taped at all joints and sealed at all penetrations with appropriate airtight tape (Pro Clima Tescon; Contega Solido tape system)

**Air tightness and Building Control for London extensions**:

• Building Control inspectors do not typically test air tightness in extensions (no mandatory test required for extensions under Part L 2021). However, the Building Control officer will check:
• •That the insulation is installed correctly and continuously (no gaps; correct thickness; correct material) at the inspection stages (typically at first-fix stage, before the walls/roof are closed up)
• •That cavity barriers are installed at all junctions (this is both a fire safety and an air tightness issue)
• •That the roof structure and wall structure are consistent with the approved drawings

For designers and contractors who want to validate the air tightness of an extension voluntarily: a mid-construction test (a 'design stage pressure test' before the finishing plaster is applied) can be very valuable — it allows air leakage points to be identified and sealed before they are covered by plasterwork. This approach is standard in Passivhaus and high-performance construction.