UK Part L drives ever-tighter airtightness targets (commonly <5 m³/(h·m²) and increasingly <3). At that level the building is no longer self-ventilating; performance depends entirely on the mechanical ventilation system being designed, installed, balanced and commissioned correctly. NHBC's 2021 IAQ research and the BEIS Smart Meter and Building Performance Evaluation programmes both show that a substantial proportion of new MVHR installations under-deliver on extract rates and that CO₂ in bedrooms routinely exceeds 2,000 ppm overnight in the first heating season.
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Why new builds need a dedicated IAQ test
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What a commissioning IAQ test should cover
- Flow-rate verification at every supply and extract valve, against the dwelling design schedule (Approved Document F Table 1.3).
- Continuous CO₂ logging in the master bedroom and a second bedroom for 7–14 nights.
- Relative humidity in bathrooms, bedrooms and the living room across one heating cycle.
- Formaldehyde and TVOC in the kitchen and living room (new MDF, paint, flooring).
- PM2.5 and NO₂ if the home has a gas hob or sits on a busy road.
- Report against AD F, BS EN 16798-1, WHO IAQ Guidelines with a prioritised remedial action list.
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MVHR — the most common new-build failure
Mechanical Ventilation with Heat Recovery is the dominant whole-house strategy in new UK builds. The four failure modes a test reliably exposes: (1) supply and extract flows not balanced — the dwelling becomes positively or negatively pressured, dragging air through cracks and around windows; (2) flexible ductwork instead of rigid, halving the design flow rate; (3) summer bypass not functioning, leading to overheating; (4) filter not changed since handover, choking flows. All four are routine on the handover snag list and all four show up clearly in a 7-day CO₂ + RH trace.
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Using the result with the developer
NHBC Buildmark covers defects for the first 2 years and structural for 10. Submit the IAQ report against the specific Approved Document F clause it breaches and reference the developer's commissioning certificate. A written test result almost always resolves a dispute that endless emails never do.
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Frequently asked questions
Why are new-build homes a higher IAQ risk?
Modern UK new builds are highly airtight to meet Part L energy targets — but airtightness without correctly designed, installed and commissioned ventilation traps moisture, CO₂ and VOCs indoors. NHBC and BRE research consistently find that a high proportion of new-build MVHR systems are commissioned incorrectly or never balanced, producing CO₂ and humidity levels that breach Approved Document F within weeks of occupation.
What does Approved Document F require for new homes?
Whole-dwelling ventilation rate sized to the number of bedrooms (e.g. 25 l/s for a 3-bed), purge ventilation via openable windows, extract from kitchens, bathrooms and utility rooms, and a CO₂ daily average target below 1,000 ppm with peaks below 1,500 ppm. New builds must be commissioned and the commissioning certificate provided to the homeowner.
When should a new-build IAQ test be commissioned?
Twice. First, a commissioning verification within 1–3 months of occupation — measures actual MVHR flow rates and CO₂/humidity/VOC behaviour with the family living in the home. Second, a follow-up at 9–12 months once seasonal heating and weatherproofing have stabilised the building physics.
What does a new-build IAQ test catch?
Under-ventilated bedrooms (CO₂ over 2,000 ppm overnight), excess winter humidity leading to interstitial condensation, MVHR not balanced (one half of the dwelling positively pressured against the other), elevated formaldehyde from MDF kitchen units, and bypass paths around fire dampers and service penetrations.
Is my developer obliged to fix issues found?
Within the NHBC 2-year defects period, ventilation that does not meet Approved Document F at commissioning is a defect the developer must remedy. A documented IAQ test forms the technical basis of the claim — a desk-based complaint with no data rarely succeeds.