The best
CO2 monitors
for finding out if a room actually breathes

Most of the condensation calls I get to start the same way: someone describing a bedroom that’s wet on the inside of the glass by morning, or a room that never quite loses its stuffiness no matter how often the door’s left open in the day. The moisture meter and the thermal camera tell me what’s already happened to the plaster and the paint. What they don’t tell me is why — whether the room’s simply too cold for the moisture load, or whether nobody’s actually getting fresh air in and stale air out of it overnight. That second question has an answer that doesn’t involve any guesswork at all, and it isn’t a damp reading: it’s the carbon dioxide level.

Exhaled breath runs somewhere around 40,000 parts per million of CO2, against roughly 420ppm in outdoor air. Shut a bedroom door on two sleeping adults for eight hours and the CO2 in that room climbs at a rate that depends on exactly one thing — how much fresh air is getting in, through a trickle vent, an extractor, a gap under the door, or nothing at all. It isn’t a proxy for damp, dust or allergens; it measures ventilation rate, directly, and it does it more honestly than almost anything else that comes out of the van. A chart that sits at 400–800ppm all night means the room breathes. One that climbs past 1,500ppm and stays there means it doesn’t, and that’s very often the missing half of a condensation diagnosis rather than the mould itself being the real problem.

The complication with this category is that a lot of what Amazon lists as a “CO2 monitor” for £15–£20 isn’t measuring CO2 at all — it’s a cheap VOC sensor extrapolating an estimated “eCO2” figure, which is worse than no reading if a real ventilation decision hangs on it. The seven below all measure real carbon dioxide: most with a manufacturer-stated NDIR (non-dispersive infrared) sensor, the only technology that actually detects CO2 directly, and the one exception here still ships a dedicated CO2 sensor with its own published accuracy rather than a VOC-derived guess. They span a £30 plug-in traffic light to a £190 portable with its own app.

The one I’d buy

Aranet4 Home

The Aranet4 runs the same NDIR sensor and firmware lineage as the version ventilation engineers and researchers use professionally, in a battery-powered case small enough to carry from room to room rather than commit to one windowsill. The e-ink display stays legible from across a room in daylight and doesn’t glow in a dark bedroom, and a plain green/amber/red bar under the number means it can be read at a glance without unlocking a phone at 2am.

Typically £170–£200

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How I judge a CO2 monitor

A genuine NDIR sensor, not an estimate. This is the one spec that matters more than any other. Cheap sensors infer an “eCO2” number from a VOC reading, which drifts and bears no fixed relationship to a room's real ventilation rate. NDIR shines infrared light through the air and measures how much of it CO2 itself absorbs — it's measuring the gas, not guessing at it from a different one.

A number you can read without your phone. The whole point of a bedroom test is what happens overnight, unattended. A monitor with its own display — ideally one readable in the dark without a glaring backlight — answers the question on the spot; an app-only monitor means checking a screen before the number means anything.

Battery or mains, depending on the job. A battery unit moves between rooms over a few nights, suiting a specific bedroom or an intermittent complaint. A mains or USB unit suits living permanently in one spot — a kitchen, a living room, wherever the household spends its evenings.

Logging, not just a live number. A single reading tells you the room right now, which might just be the moment someone opened a window. A history — on the device, an app, or an SD card — shows whether the level climbs all night or briefly spikes and recovers, and that shape matters more than any single figure.

A sane alarm, not a constant one. A threshold that flags a room past roughly 1,000–1,500ppm is useful. A unit that beeps every time the number ticks up gets muted within a week and stops being read at all.

The comparison

The picks in detail

01

Aranet4 Home

Best overall — accurate, portable and reads at a glance without an app

Aranet built its name on the NDIR sensor inside this unit, the same sensor family used in its professional Aranet4 PRO line, rated across a 0–9999ppm range with a stated accuracy around ±50ppm. What sets the Home version apart from most of the rest of this list is the e-ink screen: no backlight, fully legible in daylight from across a room, and unobtrusive enough to sit on a bedside table overnight without glowing in the dark. Two AA batteries are claimed to run for roughly two years at the default reporting interval, and the free Aranet app pulls history over Bluetooth with no account, subscription or wifi required.

Because it’s genuinely portable and doesn’t depend on a home network, it’s the one I’d reach for to test a specific complaint — a stuffy bedroom, a windowless bathroom, a home office that feels stale after a few hours on a call. The trade-off is that it only ever reports on the one room it’s sitting in, and there’s no built-in audible alarm, just the colour bar under the number.

For

• NDIR sensor with a genuinely useful 0–9999ppm range
• E-ink display readable in daylight, unobtrusive at night
• Roughly two years from two AA batteries, no subscription
• Small and light enough to move between rooms or jobs

Against

• Bluetooth only — no standalone wifi logging without a phone nearby
• One room at a time unless more than one unit is bought
• No audible alarm, just a visual colour bar

Typically £170–£200

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02

Airthings View Plus

Best for a full air-quality picture — CO2 alongside radon, VOC and particulates

The View Plus is a different proposition to the rest of this list: an NDIR CO2 sensor (400–5000ppm) sits alongside a radon diffusion chamber, a laser-based PM1/PM2.5 particle counter, a VOC sensor, and temperature, humidity and pressure, all reporting to a rotating on-device display and a wifi-connected app with long-term history. For a household that already wants a radon check — genuinely relevant across large parts of the South East — getting the ventilation number in the same box rather than buying two separate devices is a real convenience.

It's built to live permanently in one spot, powered by six AA batteries or a USB cable, and the trade-off for covering seven metrics is that none gets the dedicated full-time display an Aranet4 gives CO2 alone — the screen cycles between readings. For anyone whose only question is “is this room ventilated,” it’s more monitor than the job needs; for anyone weighing radon, damp risk and ventilation together, it earns its place.

For

• Seven sensors in one unit, including a genuine radon diffusion chamber
• Laser PM2.5 particle counter, not an estimate
• Long-term wifi dashboard with history for every metric

Against

• Display rotates between metrics rather than showing CO2 continuously
• Six AA batteries or a permanent USB tether — not pocket-portable
• More device, and more cost, than a CO2-only question needs

Typically £150–£185

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03

TFA Dostmann AIRCO2NTROL 5000

Best for logging evidence — an actual CO2 trace, not just a spot reading

Where the rest of this list logs to a phone app, the AIRCO2NTROL 5000 logs to a micro SD card as a CSV file — over a million records at a default five-second interval, readable straight in a spreadsheet. That makes it the one I’d point a landlord or a genuinely sceptical homeowner towards: pull the card after a week, plot it, and there's an actual chart of what a bedroom or living room did overnight, rather than one number that could have been taken at any moment. The NDIR sensor covers 0–5000ppm, with a large LCD showing live ppm, temperature and humidity together, plus a traffic-light indicator against user-set thresholds.

It runs from USB rather than batteries, so it's a fixed-location unit rather than something to carry room to room, and reading the log means pulling the SD card and opening it on a laptop — there's no app doing that automatically. For what it is, though, a genuinely professional-feeling data logger at a price well below what dedicated building-monitoring kit usually costs.

For

• Onboard SD card logger — real evidence, not just a live number
• No app or account needed to use the core function
• Adjustable logging interval and configurable alarm thresholds

Against

• Mains/USB powered, not portable between rooms
• Reading the log means an SD card and a laptop, not a phone
• Dated LCD interface next to the touchscreen rivals here

Typically £110–£135

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04

Qingping Air Monitor Gen 2

Best for a shared room — a full-colour display that doubles as a clock

Qingping fits an NDIR sensor (the widely-used Sensirion SCD40) behind a 4-inch colour IPS touchscreen that shows CO2 alongside PM2.5, PM10, estimated VOC, noise, temperature and humidity, with a clock face and weather readout that make it double as something worth actually leaving out on a kitchen counter rather than tucking away. Auto-adjusting brightness means it doesn’t glare in a dim room, and the same data logs to an app for longer-term history.

The touchscreen and extra sensors put it toward the top of this list on price, and a glass screen on a counter is more likely to pick up a scratch than a fixed LCD. For a living room or kitchen where several people glance at it through the day — rather than a single bedroom test overnight — the always-on colour display earns its keep.

For

• Genuine NDIR sensor (Sensirion SCD40) behind a colour touchscreen
• PM2.5, PM10, eTVOC and noise alongside CO2 for the same footprint
• Doubles as a clock/weather display, worth leaving out permanently

Against

• Touchscreen is more fragile than a fixed LCD or e-ink panel
• Mains/USB powered only, not battery-portable
• Among the pricier options here once the extra sensors are paid for

Typically £145–£175

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05

Inkbird INK-CO2W

Best budget wifi monitor — a real NDIR sensor well under £100

The INK-CO2W is the cheapest unit here with a genuine NDIR sensor rather than an estimated eCO2 figure, covering 400–5000ppm at a stated ±(50ppm + 5% of reading). A colour LCD shows the live number against a green-to-red gauge, a three-stage alarm flags normal, warning and danger bands, and it connects over 2.4GHz wifi to the Inkbird app for graphed history and remote checking — all from a built-in 2400mAh rechargeable battery rather than a permanent mains lead.

The app and casing both feel a step down from the Aranet4 or Airthings in polish, and wifi setup is 2.4GHz-only, which trips up some routers running a combined 2.4/5GHz network name. For roughly a third of the top pick's price, though, a real NDIR sensor with app history and a rechargeable battery is a fair trade.

For

• Real NDIR sensor at a genuinely budget price
• Wifi app history and remote alerts, no subscription
• Built-in rechargeable battery, not tied to a socket

Against

• App and build quality noticeably behind the pricier picks
• 2.4GHz wifi only — can trip up some router setups
• Three-stage alarm is coarser than an adjustable threshold

Typically £70–£90

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06

Netatmo Smart Indoor Air Quality Monitor

Best for a smart-home household — HomeKit-compatible, checked from a phone

Netatmo doesn’t publish its CO2 sensor as an NDIR spec the way its rivals here do, but the reading carries its own stated accuracy — roughly ±100ppm up to 1,000ppm and ±10% above that — rather than being calculated from an unrelated VOC signal. There's no numeric display on the unit itself, just a single status LED; every actual reading, along with temperature, humidity and noise, lives in the Netatmo app, which also supports Apple HomeKit and configurable room profiles for a baby's room, an allergy sufferer, or an elderly relative.

That app-only approach is the main trade-off: there’s no glance-at-the-bedside-table number, so an overnight bedroom test means checking the app the next morning rather than reading a screen at 2am. For a household already leaning on Apple Home and wanting air quality folded into that routine rather than a separate gadget, it's the natural fit here.

For

• Apple HomeKit compatible, fits an existing smart-home setup
• Minimal, unobtrusive design rather than looking like test equipment
• Configurable room profiles (baby room, allergy sufferer, and so on)

Against

• No on-device numeric display — every reading needs the app
• Sensor isn't published as an NDIR spec the way its rivals are
• Wifi-dependent; no standalone use without a home network

Typically £80–£110

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07

TFA Dostmann AIRCO2NTROL Mini

Best budget pick — the cheapest genuine sensor on this list, nothing else

Strip away the app, the wifi, the touchscreen and the extra sensors, and the AIRCO2NTROL Mini is what's left: an NDIR sensor covering 0–3000ppm, a small LCD showing the live number, and two LED dots that flip between green and red against fixed thresholds around 800ppm and 1,200ppm. There's no account to set up, no app to install, and no subscription to worry about — plug it into any USB port or charger and it starts reading within about a minute.

The 3,000ppm ceiling is plenty for a bedroom but tight for a packed meeting room, and there's no logging — whatever the number is when you look is all it tells you. For a straightforward first check of whether a room is under-ventilated, without spending £100-plus to find out, it does the one job it's built for and nothing more.

For

• Genuine NDIR sensor at the lowest price on this list
• No app, account or subscription — plug in and read
• Small and unobtrusive, easy to tuck on a shelf or windowsill

Against

• 3,000ppm ceiling, lower than the rest of this list
• No history or logging — a spot reading only
• USB powered, no battery, so it stops reading if unplugged

Typically £25–£35

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What actually matters — and what doesn’t

“eCO2” is not CO2. Plenty of cheap air-quality gadgets, including some sold explicitly as CO2 monitors, use a VOC sensor and an algorithm to estimate a CO2-equivalent figure. It drifts, it isn't calibrated against real carbon dioxide, and it can read a room as fine when it genuinely isn't. Anything that doesn't state NDIR, or a dedicated CO2 sensor with its own published accuracy, should be treated as decoration rather than a measurement.

Chasing lab-grade accuracy is pointless here. Every NDIR sensor on this list lands around ±50ppm, or a few percent of the reading. That's precise enough to tell 600ppm from 1,400ppm without doubt, which is the only distinction that actually changes a ventilation decision. A cheaper unit with a slightly wider tolerance isn't meaningfully less useful for that.

A colour touchscreen is a convenience, not a better measurement. The Qingping's screen is pleasant to look at, but underneath it's the same class of NDIR sensor as the plainer units here. Pay for a nicer display because it suits where it'll sit, not because it implies more accurate data.

This is not a smoke, heat or CO alarm. Carbon dioxide and carbon monoxide are different gases entirely, and nothing here meets EN 50291 or satisfies a landlord's legal alarm duties. A CO2 monitor is a ventilation diagnostic; a carbon monoxide alarm is a life-safety device, and a property needs both for entirely different reasons.

Logging beats a single number for anything you'll need to explain to someone else. A one-off reading of 1,800ppm is easy to dismiss as a bad moment. A graph showing a bedroom above 1,500ppm for six hours every night is much harder to argue with, whether the person on the other end is a tenant, a landlord, or a builder who fitted a trickle vent that never gets opened.

Questions I get asked

What CO2 level counts as bad in a bedroom? Below 800ppm is good ventilation. 800–1,000ppm is acceptable for most of the night. Above 1,000–1,500ppm for sustained periods is where school ventilation guidance (Building Bulletin 101) and workplace standards start flagging a room as under-ventilated, and it lines up with the groggy, stuffy feeling people report on waking. It isn't a health hazard at typical indoor levels, but it's a reliable sign the air isn't being replaced fast enough — usually also the condition mould and condensation prefer.

Will a CO2 monitor fix my condensation problem? No — it only diagnoses the ventilation half of it. If the overnight reading is consistently high, the fix is more air movement: a trickle vent that actually opens, an extractor on a humidistat, or a ventilation assessment to work out whether the property needs something more than a single room's fan, right up to what MVHR costs if the fabric's tight enough to justify it.

Do I need to test every room, or just the bedroom? Bedrooms overnight are the priority, because that's where a room is sealed up with people for the longest single stretch, and the CO2 climb is most consistent and easiest to read. A living room in the evening with several people and the telly on is the next most useful check; box rooms and hallways rarely show anything a bedroom hasn't already.

How do I tell a real CO2 monitor from a cheap one before buying? Look for the word NDIR, or “non-dispersive infrared,” explicitly in the listing or spec sheet. If a product only says “eCO2” or “equivalent CO2,” or doesn't mention the sensor type at all despite a suspiciously low price, it's very likely the VOC-estimate kind rather than a genuine measurement.

A CO2 monitor only ever answers the ventilation half of a damp or condensation question — if the overnight readings come back high, a ventilation assessment is the logical next step, and what MVHR costs is worth reading alongside it if a single fan clearly won't be enough. For the wider air-quality picture, including radon and particulates, the best air quality monitors guide covers that broader category, and a hygrometer is the natural companion for tracking the humidity side of the same problem. Get in touch for a second opinion, and there are more field-kit guides if you're building out the rest of the list.

Small print. This guide is general information, not an assessment of your own property or its ventilation. CO2 monitors are diagnostic tools, not smoke, heat or carbon monoxide alarms, and don't replace either. Prices are indicative, move constantly and are checked at the time of writing; the linked page always shows the current price. As an Amazon Associate I earn from qualifying purchases.