Diagnostics · South London
Blower Door Testing in South London
Standardised airtightness testing using a calibrated blower door fan to quantify how much air a building leaks at a reference pressure of 50 Pascals. Specialist building-performance diagnostics across South London — SE1-SE28 · SW (south) · CR (Croydon) · BR (Bromley) · KT (Kingston).
Blower Door Testing in South London
What we typically diagnose across South London.
Across South London we work on everything from the bay-windowed Victorian terraces of Clapham and Battersea through the Edwardian housing of Dulwich and Forest Hill to the modern apartments of Bermondsey and Bankside. The common thread is solid-wall construction and lightweight conversion flats needing a measured, building-science-led retrofit approach.
- Victorian terrace party walls in Clapham, Brixton, Camberwell and Peckham with no cavity to insulate
- Wandsworth / Battersea conversion flats with cold ground-floor suspended timber decks
- North-facing bedrooms in Brixton, Streatham and Tooting bay-window Victorian terraces
- Bathroom and kitchen mould in Wimbledon and Earlsfield conversion flats
- Suspended floor perimeters in SW4, SW8, SW11, SE5 Victorian terraces
Real blower door testing on site
Real photos from a recent blower door airtightness test.
Smoke tracing in action
Smoke escaping at the eaves during a real blower door test — air-leakage path made visible.
What this service is
Blower Door Testing — in plain English.
A blower door test fits a calibrated fan into an external door, pressurises (or depressurises) the building, and measures how much air leaks through the envelope. The result is an air change rate at 50 Pa (ACH₅₀) and a measured airflow (m³/h) — the same metric used in Passive House and Part L compliance.
Common problems we solve
The symptoms that bring people to this service.
- 01Cold draughts you can feel even with the heating on
- 02Sounds (and smells) bleeding through between flats
- 03New-build air-tightness failing Part L
- 04Energy bills 20–40% higher than expected
- 05Uncontrolled air-leakage undermining a brand-new MVHR system
- 06Hidden bypass paths in lofts, service penetrations and intermediate floors
How we investigate
Our diagnostic approach
- 01Pre-test walkthrough — seal intentional openings (vents, flues, MVHR)
- 02Pressurisation + depressurisation test (typically 25–70 Pa range, single-point and multi-point)
- 03Simultaneous smoke tracing to visualise the actual leakage paths
- 04Thermal imaging under depressurisation to confirm cold-air ingress
- 05Itemised leak inventory with photos, locations and remediation priorities
Diagnostic equipment used
What we bring on site
- Minneapolis-style calibrated blower door fan + frame + shroud
- Manometer / pressure transducer with software logging (TECTITE / TEC ducted air-flow analyser equivalent)
- Smoke tracer pen / theatrical fog machine for leak visualisation
- FLIR thermal camera for paired thermal-under-pressure imaging
- Anemometer for spot velocity readings at suspect leakage points
Building physics — why this works
The science behind the diagnosis.
Air movement through the envelope carries heat (convective loss), moisture (vapour) and noise. Pressurising the building amplifies pressure differences across every crack, gap and penetration, so even a 1 mm gap that's invisible under normal conditions becomes a measurable, locatable air-leakage path. That's how you separate symptom from cause.
What you get
Measured benefits — not vague promises.
- Measured airtightness in ACH₅₀ / m³/h — not assumed
- Locate every meaningful air-leakage path with smoke + thermal evidence
- Quantify the heating-energy penalty of uncontrolled infiltration
- Verify that MVHR will actually deliver controlled, balanced ventilation
- Provide compliance evidence for Part L, Passive House, AECB and Enerphit
Where it's used
Where it's used
- New-build airtightness verification
- Passive House and EnerPHit certification testing
- Pre-retrofit baseline measurement
- Post-retrofit verification of airtightness improvements
- Diagnosing draughts, cold spots and ventilation imbalance
What you receive
What you receive
- Measured air-change rate (ACH₅₀ or m³/h·m² at 50 Pa)
- Smoke tracing to locate dominant leakage paths
- Targeted recommendations to reduce air leakage
- Optional re-test to verify improvements
Typical findings
What we commonly discover during blower door testing investigations
- 01Loft hatch and ceiling penetrations leaking at >10× allowed rate
- 02Window and door reveals leaking around poorly sealed perimeters
- 03Service penetrations (waste pipes, cables) unsealed at external walls
- 04Suspended ground-floor perimeters open to ventilated subfloor voids
- 05Chimney flues and party-wall bypasses acting as constant leakage paths
- 06Smoke tracing visualises every leak pathway — quantified at 50 Pa
Findings reflect patterns observed across completed RetrofitIQ projects — every survey is interpreted in the building’s specific context.
Real project examples
See this service applied on real, completed projects
FAQs
Blower Door Testing — common questions
What is a blower door test?+
A blower door test is a controlled pressurisation/depressurisation test that measures how airtight a building is. A calibrated fan is fitted into an external doorway; the house is pressurised (or depressurised) to a known pressure difference against the outside (typically 50 Pascals); and the airflow required to hold that pressure is measured. The result tells you exactly how leaky your building is — in litres-per-second per square metre or air-changes-per-hour.How does a blower door test identify air leakage?+
Under depressurisation, every gap, crack, service penetration and unsealed junction in the building envelope becomes an air leakage path with outside air rushing in. We use a smoke pen at suspected points to make those leakage paths visible, and a thermal camera to show them as cold fingers of incoming air. The combined output is a measured leak inventory — a list of every meaningful air leakage path with its location and severity.Why is the test carried out at 50 Pascals?+
50 Pa is the industry-standard reference pressure used in ATTMA TSL1 (UK), the Passive House Institute, Approved Document L (England) and the international ISO 9972 / EN 13829 protocols. At 50 Pa the airflow signal is strong enough to drown out wind and stack-effect noise, but well below any pressure that could damage the building. It also enables direct comparison with other measured properties.Can a blower door test help reduce heating bills?+
Yes — uncontrolled air leakage is one of the single largest hidden heat losses in UK homes. A typical Victorian terrace can leak 10–20 air-changes per hour at 50 Pa; tightening that down to 3 ACH50 (a realistic retrofit target) reduces heating energy by 15–30 %. The blower door test quantifies your starting point and lets us prioritise the highest-impact air-sealing works.Can blower door testing help identify draughts?+
Yes. Cold draughts are uncontrolled air movement driven by pressure differences across the envelope. The blower door amplifies these pressure differences so every draught becomes locatable with a smoke pen — even the small ones you do not normally notice but which add up to 30–40 % of your heat loss. We document each one with photographs and a written remediation order.Can thermal imaging be combined with blower door testing?+
Yes — combining the two is the most diagnostic surveying technique available. With the blower door running depressurised, cold outside air is pulled through every leakage path; the thermal camera reveals those cold inflow plumes as bright cold fingers of air pushing into the warm interior. This pinpoints leaks that smoke alone cannot find, such as flanking air paths inside intermediate floor zones or stud walls.How long does a blower door test take?+
A standalone airtightness test takes 90 minutes to 2 hours including setup, temporary sealing of intentional openings, the calibrated test run, smoke-tracing and removal. A combined Home Health Diagnostic Survey (blower door + thermal + smoke + moisture) takes 3–4 hours on site plus 2–3 days for the written report.Do windows and doors need to be closed during the test?+
Yes — all external windows and doors are closed during the test (apart from the door holding the blower door fan). Internal doors are left open so the test measures the whole envelope as one volume. Intentional ventilation openings — trickle vents, extract fans, MVHR ducts, open flues — are temporarily sealed before the test and re-opened immediately afterwards.Should heating be on before the test?+
For airtightness measurement alone, heating does not need to be on. However, if the test is combined with thermal imaging (which we always recommend), we need at least 24 hours of stable heating beforehand so internal surface temperatures are settled. We will confirm the heating regime needed when we book the appointment.Can smoke testing be used during a blower door test?+
Yes — smoke is the single most useful add-on. Under depressurisation a smoke pen held near a suspected leak shows the air drawn through the gap; under pressurisation the smoke is pushed out. We use a combination of smoke pens (precise, low-volume) and theatrical fog (whole-room visualisation) depending on the situation.What happens after the blower door test?+
We compile the data into a written report containing the air-change rate at 50 Pa (n50/ACH50), the volumetric leakage (m³/h at 50 Pa), the calculated permeability (m³/(h·m²) at 50 Pa for Part L), a photographic + thermal leak inventory, and a prioritised air-tightening plan. The report is yours to share with architects, contractors or surveyors.Do I receive a report after the test?+
Yes — a full written report with the measured airtightness figures, the leakage inventory, photographs of every meaningful leak path, and a costed prioritised remediation plan. The report follows the ATTMA TSL1 format and is acceptable evidence for Part L compliance, Passive House certification, EnerPHit and insurance claims.Is blower door testing useful before insulation works?+
Yes — it is essential. Adding insulation to a leaky envelope is one of the most common (and most expensive) retrofit mistakes. The insulation cannot perform if cold outside air is bypassing it through gaps and penetrations. A pre-retrofit blower door test identifies every leakage path so they can be sealed *under* the insulation, not on top of it.Is blower door testing useful after retrofit works?+
Yes — verified outcomes are the whole point of the 'Verify' stage in our diagnostic process. A post-works blower door re-test under the same protocol measures exactly how much the airtightness has improved (e.g. from 9.2 ACH50 down to 3.4 ACH50) and produces documented evidence for the client, the architect or the Passive House certifier.Can airtightness problems cause condensation?+
Yes — and this is one of the most underappreciated mechanisms. Warm moist indoor air carried through air-leakage paths into the cold loft or wall cavity carries vapour with it; when that air cools below dewpoint inside the cavity, the vapour condenses on cold structural elements (joists, rafters, the back of plasterboard). The resulting interstitial condensation is invisible from inside the room but slowly destroys timber, plaster and insulation. Airtightness is therefore a moisture-control measure as well as a heat-loss measure.What is a good airtightness result for a UK home?+
For context: a leaky Victorian terrace typically tests at 8-15 ACH50. Approved Document L requires new builds to achieve 8 m³/(h·m²) at 50 Pa or better. AECB Silver target is 1.5 ACH50. Passive House requires ≤0.6 ACH50. EnerPHit (Passive House retrofit standard) is 1.0 ACH50. A realistic retrofit target for an existing London property is 3-5 ACH50 — achievable, transformative and significantly cheaper to heat.Do you do blower door testing across all London postcodes?+
Yes — we cover every London postcode (E, EC, N, NW, SE, SW, W, WC) plus Essex, Kent, Surrey and Hertfordshire. Our equipment is van-mounted and tests can be arranged with 7-14 days' notice for most projects.
Next step
One company. One process. One point of responsibility.
We don’t simply identify problems. We investigate, diagnose, design solutions, carry out the work and verify the results. Book a Home Health Diagnostic Survey and we’ll tell you exactly which remedial works (if any) are actually needed.