Ventilation Assessment: Airflow, Compliance and Health

Why measured ventilation matters

Ventilation is the lever that decides how every other pollutant behaves. The same source — a printer, a kitchen, ten occupants — produces an unremarkable indoor environment in a well-ventilated room and an unacceptable one in a tightly sealed building.

Designed airflow rarely equals delivered airflow. Fan belts slip, dampers stick partially closed, terminal boxes are commissioned to nominal values that drift over years, and zoning that suited the original tenant is rarely re-tuned for the current occupant. The only way to know what is actually being delivered is to measure.

A ventilation assessment quantifies that gap. It tells you which zones meet design, which under-perform, by how much, and what the cheapest path to compliance looks like.

What the assessment covers

Scope is shaped to the building type, but four elements appear in nearly every project.

Outdoor air supply. Total fresh-air rate per AHU, per zone and per person, measured and compared with design intent and the relevant standard.

Distribution and balance. Terminal-by-terminal supply and extract, plotted against design schedules, with hot or cold spots and recirculation paths flagged.

Effectiveness. Whether the supplied air actually reaches the breathing zone — measured by CO₂ decay or tracer techniques, not assumed from diffuser geometry.

Filtration and air quality. Filter classification (ISO ePM rating), pressure drop, bypass and the resulting indoor PM2.5 and NO₂ relative to outdoor air. HVAC and air quality →

Measurement methods used

The right method depends on the question being asked.

CO₂ tracer-gas decay. A controlled CO₂ release is followed as it dilutes, yielding a true air-change rate per hour for the occupied zone. Works in any building type, including naturally ventilated spaces.

Direct anemometry. Hot-wire or vane anemometers at supply diffusers and extract grilles, or balometer hoods over terminal boxes, give per-terminal flow rates. Essential for balancing problems.

Pressure mapping. Differential pressure between zones identifies short-circuiting, infiltration paths and over-pressurised cores that starve perimeter offices of fresh air.

Continuous CO₂ logging. Occupied-period CO₂ profiles across a working week reveal demand-response failures that point-in-time measurements always miss.

Natural and mixed-mode buildings

Naturally ventilated buildings present a different problem. There is no AHU schedule to benchmark against, and air-change rate varies with wind, temperature and which windows are open.

We assess the openable area against floor area (typically 5% is a CIBSE rule of thumb), measure achieved air-change rate under typical conditions, and quantify how the rate collapses on still summer days. Where night purging is used, we verify the thermal mass actually benefits.

Mixed-mode buildings add a control-strategy question. The assessment confirms whether changeover thresholds match real occupancy and outdoor conditions, or whether mechanical and natural modes fight each other in shoulder seasons.

What you receive

The deliverable is a structured report that any facilities team, contractor or assessor can act on.

Per-zone compliance table. Measured rate, design rate, benchmark rate and a clear pass / partial / fail flag.

Floor-plan overlays. Supply and extract values mapped onto drawings so under-ventilated rooms are visible at a glance.

Remediation hierarchy. Findings ranked by health impact and intervention cost, from terminal rebalancing through filter upgrades to AHU replacement.

Verification protocol. A short, repeatable method for confirming improvements after works — so you do not need a full re-survey to check one floor.

Standards the assessment is benchmarked against

The Workplace (Health, Safety and Welfare) Regulations 1992 and ACOP L24 require effective and sufficient ventilation in every workplace. CIBSE Guide A and TM40 set design targets that UK practice follows. BS EN 16798-1 places non-residential buildings in Categories I–IV based on per-person and per-area airflow.

For commercial work, ASHRAE 62.1 is widely referenced because the per-person plus per-area additive model is more granular than the older flat per-person rule. WELL v2 Air requires verified outdoor air rates 30% above ASHRAE 62.1; BREEAM Hea 02 has its own credits for ventilation effectiveness.

The report shows which standard each finding is measured against, so the reader can see at a glance whether they are looking at a legal duty, a design recommendation or a certification threshold.

From assessment to a healthier building

A ventilation assessment is the start of an improvement loop, not a one-off deliverable.

Commission the recommended works — often a combination of rebalancing, filter upgrades and control tuning rather than capital replacement.

Re-verify the changed zones. A targeted re-measurement confirms the design intent has been delivered.

Move to continuous oversight with CO₂ and PM2.5 monitoring, so drift is caught in weeks not years.

Frequently asked questions

What is the difference between an air quality test and a ventilation assessment?

An IAQ test measures what is in the air. A ventilation assessment measures whether the system is delivering enough outdoor air to dilute it. The two answer different questions and are most informative when run together.

How do you measure airflow without disrupting occupants?

Tracer-gas decay using CO₂ pulses or SF6 runs silently in the background and resolves real air-change rates without occupant involvement. Direct anemometry at terminals is done with handheld balometers during quiet periods.

What air change rate should a UK office achieve?

BS EN 16798-1 Category II suggests around 1.4 L/s/m² for an open-plan office, or roughly 8–10 L/s per person. CIBSE Guide A gives equivalent targets. Below 5 L/s per person, CO₂ routinely exceeds 1000 ppm and complaints rise.

Can you assess natural and mixed-mode buildings?

Yes. Naturally ventilated buildings need CO₂ decay measurement, opening-area assessment and wind-driven flow modelling. Mixed-mode buildings add control-strategy review so the changeover between modes can be tuned to actual occupancy.

Will you specify remedial works yourself?

We specify the engineering intent — required outdoor air rates, filter grades, terminal balancing targets, control logic — and leave the mechanical installation to your contractor. We can verify the post-works performance independently.

When should I commission a ventilation assessment for my building?

A ventilation assessment is the right tool when CO₂ readings sit persistently above 1000 ppm, when occupants cluster complaints by zone, after a fit-out or layout change, when adopting hybrid working patterns that shift peak occupancy, or before any BREEAM, WELL or HSE-driven audit. Pair the ventilation assessment with continuous indoor air monitoring to confirm the engineering changes actually delivered.

Next step

Quantify your building's real ventilation performance

Request an assessment

Ventilation Assessment: Quantifying Airflow Across Your Building