The best
air quality monitors,
for ventilation and condensation work

Air quality monitors didn’t used to come up much in domestic survey work. They do now, mostly because of retrofit. Once a property has cavity or internal wall insulation, new windows and a bit of draught-proofing, the background air changes that used to carry moisture and CO2 out of a house quietly stop happening — and nobody notices until the mould comes back, or a bedroom starts feeling stuffy by nine in the evening. A CO2 monitor is the cheapest, fastest proxy I know for “is this room actually getting fresh air”, which is exactly the question PAS 2035 retrofit assessments and any sensible ventilation testing are trying to answer.

A CO2 monitor with a genuine sensor tells you, reliably, how much carbon dioxide is in the air — which correlates closely with occupancy and air change rate, because people are the main CO2 source indoors and ventilation is what removes it. A particulate or PM2.5 reading tells you something about dust, smoke and cooking fumes instead. Neither measures damp, mould spores or radon directly, though some multi-sensor units bundle radon detection in alongside everything else. Buying the wrong type of monitor for the question you’re actually asking is the first common mistake here; the second is buying one that doesn’t really measure CO2 at all — more on that below, because it is the whole game.

How I judge an air quality monitor

A real sensor, not an estimate. This is the whole ballgame. Genuine CO2 measurement uses an NDIR (non-dispersive infrared) sensor, which shines infrared light through a sample of air and measures how much of it CO2 absorbs. Cheap “CO2 monitors” often use a VOC sensor instead and run the reading through an algorithm that guesses a CO2-equivalent number from it. That number can look plausible on the box and be nowhere near correct in the room. If a spec sheet doesn’t say NDIR, assume it’s a guess.

Evidence you can actually use. A colour changing on a screen is fine for a homeowner deciding whether to open a window. For anything that ends up in a report — a retrofit ventilation assessment, a dispute over a mould complaint — you want logged, timestamped, exportable data, not a memory of what the light was doing on Tuesday.

Field manners. Battery life measured in years rather than days, a screen that’s legible without squinting, and ideally no account or subscription standing between you and a number. A monitor you have to charge every three days is a monitor that’s flat in the van when you need it.

Honesty about scope. A single-sensor CO2 monitor and a seven-sensor whole-home unit are answering different questions, and the price difference is mostly the extra sensors, not marketing. Buy for the question in front of you, not the longest spec sheet.

What it’s actually for. A spot-check tool you carry room to room on a walk-through, a permanent bedroom monitor a client leaves running, and a logging instrument left in situ for a week before-and-after an intervention are three different jobs. Some units do all three; most do one well.

The comparison

The picks in detail

What actually matters — and what doesn’t

NDIR versus everything else. Worth repeating, because it’s the whole category: a genuine CO2 reading comes from an NDIR sensor. Budget “air quality monitors” often carry a cheap VOC sensor instead and run its output through an algorithm that estimates a CO2-equivalent figure — sometimes labelled “eCO2” in the small print, if it’s labelled at all. That number can drift a long way from reality in a room with cooking, cleaning products or aerosols in the air. If the listing doesn’t say NDIR, assume it’s a guess and price it accordingly.

Radon needs patience. Any unit here that includes radon sensing takes weeks, not minutes, to produce a reading worth acting on — the sensor is counting decay events over time. Long-term averaging is normal and expected, not a fault.

Particulate readings are softer than they look. PM2.5 sensors at consumer prices use light-scattering to estimate particle mass, and independent lab comparisons regularly find they run high against reference-grade equipment — useful for spotting a trend, less useful as an exact figure to quote in a report.

What the marketing pushes that you can skip. “AI insights”, elaborate app dashboards and IFTTT integrations are decoration if the sensor underneath is estimating rather than measuring. Spend the money on the sensor and the battery, not the software wrapped around it.

One high reading isn’t a verdict. A CO2 spike after a room fills with people for an evening says nothing on its own. What matters is the baseline once the room empties and how quickly it recovers — a baseline that never drops much below 1000ppm even unoccupied points at a genuine background ventilation shortfall. Confirming that properly is what a proper extract-rate test with an anemometer is for — see the guide to anemometers for ventilation testing.

Questions I get asked

What’s a normal CO2 level in a bedroom? As a rough guide: under about 800ppm is good, 800–1000ppm suggests background ventilation is working but not generously, and sustained readings above 1000ppm — especially above 1500ppm for hours at a stretch — point to a room not getting enough fresh air for the number of people in it. A short spike overnight is normal; a baseline that never really drops is the thing worth investigating.

Will an air quality monitor tell me if I have mould or damp? Not directly — it measures what’s in the air, not moisture in the walls, so it can’t diagnose rising damp, a leak or a cold bridge on its own. What it can do is flag the ventilation shortfall that so often sits behind a condensation-driven mould problem: persistently high humidity and CO2 together is exactly the signature of stale, damp air that isn’t being replaced.

Do I need a CO2 monitor for a PAS 2035 retrofit assessment? The formal compliance measurement for background ventilation under PAS 2035 is an extract and background ventilator air-flow test, typically done with a calibrated anemometer — that’s the number that goes in the report. A CO2 monitor sits alongside that as a cheap way to sanity-check whether the ventilation strategy is actually working once people are living in the house day to day.

Is a cheap air quality monitor worth buying? It depends entirely on the sensor, not the price bracket. A genuinely cheap NDIR unit, like the INKBIRDPLUS or Qingping here, gives you a real number for well under £100. A monitor with a VOC sensor pretending to be a CO2 monitor gives you a number that looks the same on the screen and means considerably less — and that trap exists at every price point.

If the reading you’re chasing is why a property feels stuffy, damp or both, that’s the day job. The practice carries out ventilation testing and PAS 2035 retrofit assessments where background ventilation is measured properly rather than guessed at — get in touch. And if you’re kitting out a survey bag more generally, there are more field-kit guides.

Small print. This guide is general information, not a survey or ventilation assessment of your property; a single air quality reading is never a diagnosis on its own. Prices are indicative, move constantly and were checked at the time of writing; the linked page always shows the current price. As an Amazon Associate I earn from qualifying purchases.