Quick Answer

What Is Airtightness Testing in a London Renovation?

Q: Is airtightness testing legally required?

New builds: yes, ATTMA Level 1 test at completion mandatory for Part L compliance. Extensions and material renovations: not statutorily required (default 7.0 SAP value assumed) but commonly tested if claiming improved EPC. MVHR install: testing required to verify <3.0 minimum for MVHR effectiveness. EnerPHit/Passivhaus: certification requires verified test.

Q: How do I improve airtightness?

Continuous airtightness membrane (Pro Clima Intello Plus, Siga Majpell) — applied internally at insulation layer, tape all joints, seal service penetrations with airtight grommets. Airtight tape at window/door junctions, at wall-floor joints, at wall-roof. Replace draughty doors with EPDM-gasket airtight doors. Seal floorboards (tape at perimeter or full membrane). Block fireplaces (cap chimney if disused). Foam-fill service penetrations. Achievable improvement: 12 → 4 m³/h/m² with conscientious detailing.

Q: What does 50Pa pressure feel like?

Mild — equivalent to 30 km/h wind pressure outside. Test is conducted at this benchmark pressure for comparability. At normal indoor conditions (1–4Pa) leakage rates are 3–10× lower; the test deliberately exaggerates to detect issues. Occupants experience tighter dwellings as quieter, more stable temperature, less dust ingress — not specific pressure sensations.

Airtightness testing pressurises a dwelling to 50Pa using a calibrated fan ('blower door') and measures air leakage in m³/h/m² of envelope. Part L 2025 limit: ≤8.0 (new builds), aspirational 3–5 (good practice retrofit), Passivhaus 0.6, EnerPHit 1.0. Test cost £350–£950 per visit. Critical for MVHR effectiveness, energy compliance, and EPC. Verify at completion stage; intermediate test mid-build catches issues cheaply.

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What the test is

ATTMA (Air Tightness Testing and Measurement Association) Level 1 test: certified tester arrives with portable blower door (sealed temporary frame + calibrated fan + manometer). All windows + doors closed, intentional vents temporarily sealed, trickle vents masked. Fan pressurises dwelling to 50 Pascals (storm equivalent); measures air flow needed to maintain 50Pa = the leakage rate. Result in m³/h/m² of envelope surface area, or equivalent air changes per hour (ACH50). Lower number = tighter dwelling. Test typically 1–2 hours; tester provides PDF certificate.

Targets and benchmarks

Part L 2025 new build limit: ≤8.0 m³/h/m² @50Pa (relaxed from 10 in 2010). Existing UK housing stock typical: 10–25 (poor); 5–10 (average post-1990 build); 3–5 (good modern retrofit); 1.5–3.0 (excellent renovation with attention to detail); 1.0 (EnerPHit standard); 0.6 (Passivhaus). London 1900s solid-wall house pre-renovation typical: 12–22. Post-renovation with EWI + airtightness membrane + sealed services penetrations + airtight doors: achievable 3–5. EnerPHit retrofit: <1.0 verified by Builderr's Stanmore project (0.88 m³/h/m² @50Pa).

Why airtightness matters

(1) Energy: each ACH50 above 5.0 wastes 8–12% of heating load. (2) MVHR effectiveness: MVHR requires airtight envelope to recover heat — leaks bypass the heat exchanger, defeats purpose. MVHR mandatory in airtightness <3.0 m³/h/m² @50Pa otherwise indoor air quality fails. (3) Comfort: draughts at leakage points create cold spots and pressure-driven air movement. (4) Mould: warm humid air leaking out of dwelling through joints condenses in cold cavity — interstitial condensation rots fabric. (5) Acoustic: leaks transmit external noise. (6) Compliance: SAP calc requires airtightness assumption — default 7.0 (conservative); demonstrated test result improves EPC. Intermediate test at first-fix services + insulation stage catches leakage issues when membrane is still accessible — repair cheaply with tapes; retest at completion shows final figure.

Related questions answered.

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Is airtightness testing legally required?

New builds: yes, ATTMA Level 1 test at completion mandatory for Part L compliance. Extensions and material renovations: not statutorily required (default 7.0 SAP value assumed) but commonly tested if claiming improved EPC. MVHR install: testing required to verify <3.0 minimum for MVHR effectiveness. EnerPHit/Passivhaus: certification requires verified test.

How do I improve airtightness?

Continuous airtightness membrane (Pro Clima Intello Plus, Siga Majpell) — applied internally at insulation layer, tape all joints, seal service penetrations with airtight grommets. Airtight tape at window/door junctions, at wall-floor joints, at wall-roof. Replace draughty doors with EPDM-gasket airtight doors. Seal floorboards (tape at perimeter or full membrane). Block fireplaces (cap chimney if disused). Foam-fill service penetrations. Achievable improvement: 12 → 4 m³/h/m² with conscientious detailing.

What does 50Pa pressure feel like?

Mild — equivalent to 30 km/h wind pressure outside. Test is conducted at this benchmark pressure for comparability. At normal indoor conditions (1–4Pa) leakage rates are 3–10× lower; the test deliberately exaggerates to detect issues. Occupants experience tighter dwellings as quieter, more stable temperature, less dust ingress — not specific pressure sensations.

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