Most of the mould I’m asked to look at is condensation mould, and condensation is a timing problem. The room isn’t wet at two in the afternoon when I’m standing in it — it’s wet at two in the morning, when the heating has been off for four hours, two people have been breathing in a closed bedroom, and the air against the cold external wall has quietly slid past its dew point. A spot reading taken during a daytime visit misses that entirely. A logging hygrometer, left in the room for a fortnight, catches it every single night and draws you a graph.
That is why the humble room humidity meter has become one of the most useful diagnostic tools in housing — and why the useful ones are not necessarily the expensive ones. A £12 Bluetooth sensor that records a reading every few minutes and exports the lot to a spreadsheet tells you more about a mouldy bedroom than any instrument you can hold in your hand for an hour. When I’m assessing ventilation for retrofit work under PAS 2035, or trying to separate condensation from penetrating damp before anyone spends money on the wrong fix, humidity data over time is the evidence that settles the argument. A damp meter tells you where the moisture ended up; a hygrometer tells you where it came from and when.
One note on specifications before the recommendations. Relative humidity is genuinely hard to measure. A claimed accuracy of ±2–3% RH in the middle of the range is honest, decent performance — that’s roughly what the sensor elements in all of these units can do. Anything advertising ±1% RH at this money is marketing, not metrology. Happily, for damp diagnosis you mostly care about the shape of the graph, not the third significant figure.
The one I’d buy
Govee H5075 Bluetooth thermo-hygrometer
A clear on-device display, a claimed ±3% RH sensor, and — the part that matters — proper logging: readings stored on the unit for around 20 days, two years of history in the app, and CSV export for evidence. At the price of a takeaway you can afford three of them, which is exactly how humidity monitoring should be done.
Typically £10–£15
Check price on AmazonHow I judge a hygrometer
Logging, above everything. The whole case for these instruments is the record. A monitor that stores readings on the device and syncs a long history to a phone turns “the bedroom feels damp” into a graph showing RH climbing from 55% at ten o’clock to 78% by 2am, every night, with the window shut. That pattern is a diagnosis. A monitor that shows only the current number — however accurately — is a barometer for arguments, not a tool.
Export for evidence. If the data can leave the app as a CSV file, it can go in a report, an email to a landlord, or a ventilation assessment. Deposit disputes, disrepair claims and retrofit ventilation decisions have all turned on a humidity log. Check the export exists before buying; it’s the first thing cut from cheap apps.
Honest accuracy, sensible drift. Claimed ±2–3% RH mid-range is the honest standard; treat anything tighter with suspicion. Just as important is whether the app lets you apply a calibration offset, because all RH sensors drift over time — a unit that can be checked against a salt test (more below) and corrected in software stays useful for years.
Dew point, or the data to derive it. Mould on a cold external wall is governed by the wall surface temperature relative to the dew point of the room air, not by RH alone. A monitor that displays or logs dew point directly saves arithmetic; failing that, logged temperature plus RH lets you calculate it. Either way you need both numbers recorded together.
Battery life and manners. These things live on shelves and wall brackets for months. Common replaceable batteries, a display readable without picking the unit up, and a sensor that doesn’t need a subscription or a cloud account to give you your own data. Owner feedback is the best guide here — battery-life complaints show up fast in reviews.
The comparison
| Model | Logging & export | Claimed RH accuracy | Typical price | Best for | |
|---|---|---|---|---|---|
| Govee H5075 | On-device + app, CSV export | ±3% | ~£12 | Most people, most rooms | View |
| SwitchBot Meter Plus | 68 days on-device, CSV via app | ±2% mid-range | ~£18 | Big readable display + logging | View |
| Govee H5179 (Wi-Fi) | App history + export, remote alerts | ±3% | ~£25 | Landlords, second homes, voids | View |
| Inkbird IBS-TH2 Plus | On-device + app, CSV, calibration offset | ±3% | ~£20 | Probe work — voids, cold spots | View |
| Testo 608-H1 | None — live display incl. dew point | ±3% | ~£55 | Professional reference on the wall | View |
| ThermoPro TP357 | App history via Bluetooth | ±2–3% | ~£12 | Cheap logging, simple app | View |
| ThermoPro TP49 | None — spot readings only | ±2–3% | ~£10 | Best on a budget, per-room dials | View |
The picks in detail
Govee H5075 Bluetooth thermo-hygrometer
Best overall — logging, export and a proper display, for about £12
The H5075 is the default answer for a reason. Big clear digits with min/max on the face, a claimed ±3% RH sensor, and the full logging package: around 20 days of readings held on the device itself, up to two years in the app once synced, and CSV export so the data can leave the phone and go into a report. Owner feedback across tens of thousands of reviews is consistent that units track each other closely and batteries last many months. Its limits are Bluetooth ones — you sync when you’re in range, not from the office — and the app wants an account, which is the modern tax on everything. For diagnosing a damp bedroom, buy two or three, place them as described below, and come back in a fortnight.
For
- On-device storage plus long app history and CSV export
- Readable display with min/max — works standalone
- Cheap enough to deploy several per house
Against
- Bluetooth only — must visit to sync
- App requires an account for export
Typically £10–£15
Check price on AmazonSwitchBot Meter Plus
Best display-and-logging combination — the one to leave with a client
SwitchBot’s Meter Plus uses a Swiss Sensirion sensor element — the same family found in far dearer instruments — with a claimed ±2% RH in the middle of the range, and stores 68 days of readings on the device itself. The three-inch display is the best of this group: readable across a room, which matters when the point is a tenant or homeowner actually glancing at it before deciding whether to open a window. Data exports as CSV through the app. The catch is the ecosystem: remote access and alerts away from home need a SwitchBot Hub, sold separately, and the Matter smart-home integration likewise. As a standalone logger synced by Bluetooth it needs no hub at all, and that’s how I’d use it.
For
- Quality sensor, honest spec, 68-day on-device memory
- Large display people actually look at
- CSV export via the app
Against
- Remote monitoring needs the separate hub
- Slightly dearer than the Govee for similar data
Typically £15–£22
Check price on AmazonGovee H5179 Wi-Fi thermo-hygrometer
Best for remote monitoring — empty properties, rentals and problem rooms you can’t keep visiting
The H5179 trades the display for a Wi-Fi radio: it reports straight to the router, so the graph updates in the app wherever you are, with push alerts when humidity crosses a threshold you set. That makes it the right tool for a void property between tenants, a holiday let over winter, or an elderly relative’s house — situations where the 2am spike is happening and nobody is there to see it. Same app, same long history and export as the H5075. The trade-offs: no screen for the person in the room, it needs 2.4GHz Wi-Fi to be up, and like all cloud-connected kit the alerts are only as reliable as the service behind them. Owner feedback rates the sensor; occasional grumbles about firmware and connectivity come with the territory.
For
- Genuine remote monitoring with threshold alerts
- Same logging and export pipeline as the H5075
- Right answer for landlords and vacant stock
Against
- No display — useless to the occupant standing next to it
- Dependent on Wi-Fi and Govee’s cloud
Typically £20–£30
Check price on AmazonInkbird IBS-TH2 Plus
Best for awkward spots — the one with the external probe and a calibration menu
Inkbird’s little logger has two features the mainstream units skip. First, the Plus version ships with an external probe on a lead, so the sensor can go where the unit can’t — through a hatch into a sub-floor void, into the back of a built-in wardrobe on an external wall, into a loft while the logger stays reachable. Second, the app supports user calibration offsets, so after a salt test you can correct the reading rather than merely knowing it’s off. Add a magnetic back, on-device storage with CSV export, and a price under £25, and it’s the tinkerer’s pick of this list. The display is small and the app is more functional than pretty, which for this money is the right compromise.
For
- External probe reaches voids and cold corners
- Calibration offset in the app — rare at any price
- Magnet mount, CSV export, replaceable batteries
Against
- Small display, workmanlike app
- Bluetooth range through solid walls is modest
Typically £15–£25
Check price on AmazonTesto 608-H1
Best professional wall unit — the reference instrument that shows dew point
Testo make the environmental instruments half the building-services trade carries, and the 608-H1 is their no-nonsense wall-mount thermohygrometer: humidity, temperature and — the reason it earns its place here — calculated dew point, continuously displayed. For anyone thinking about cold-wall mould, having the dew point on the wall next to the actual wall is quietly instructive. Claimed accuracy is ±3% RH from a properly characterised sensor, batteries last around a year, and calibration certificates are available for the versions that need to live in a QA system. What it doesn’t do is log — no memory, no app, no export — which is why a £55 professional instrument sits at number five in a guide about data over time. Pair it with a cheap logger and you have both the trustworthy live number and the history.
For
- Continuous dew point display — the cold-wall mould number
- Professional build and sensor pedigree; calibration available
- Around a year per set of batteries
Against
- No logging, app or export at five times the Govee’s price
- Plain to the point of austerity
Typically £45–£70
Check price on AmazonThermoPro TP357
The Govee alternative — cheap Bluetooth logging from the other big name
ThermoPro’s TP357 does broadly what the H5075 does — Bluetooth, app graphs, claimed ±2–3% RH, a clear little display with a comfort face — often for a pound or two less, and it’s frequently sold in multipacks that bring the per-room cost right down. Owner feedback is solid on sensor agreement between units and battery life; the recurring criticism is the app, which keeps a shorter, less exportable history than Govee’s. If the plan is serious evidence-gathering, the Govee’s CSV export wins. If the plan is a sensor in every bedroom so the household can see what the shower and the tumble dryer actually do, a TP357 multipack is excellent value.
For
- Cheap, especially in multipacks
- Good display; units track each other well per owner feedback
Against
- App history and export weaker than Govee’s
- Bluetooth range is line-of-sight optimistic
Typically £10–£14
Check price on AmazonThermoPro TP49
Best on a budget — the honest £10 dial for every room
No app, no logging, no radio — just a small, competent thermo-hygrometer with the same claimed ±2–3% RH class of sensor as its Bluetooth siblings and a comfort icon anyone can read. It can’t catch the 2am spike, so it isn’t a diagnostic tool in the sense this guide cares about. What it’s for is behaviour: one in the bathroom, one in the bedroom, and the household learns in a week what drying washing on radiators does to a house. As a first response to “we keep getting mould” while the loggers gather their fortnight of data, a handful of these is money well spent.
For
- About £10, runs for ages on a AAA battery
- Simple enough for anyone to act on
Against
- No logging — a spot reading with all its blind spots
- Min/max only; no export, no evidence
Typically £9–£12
Check price on AmazonWhat actually matters — and what doesn’t
Placement is half the measurement. One sensor in the middle of the house tells you about the middle of the house. For a mould investigation you want at least three: one in the affected room, at the cold spot — low on the external wall, near the corner where the mould grows, not on a warm internal wall at eye level; one in the moisture-producing room, bathroom or kitchen, to see how high the spikes go and how long they take to clear; and one in a hallway or living room as the reference. The difference between the affected room and the reference is usually the whole story. Keep sensors out of direct sunlight, away from radiators, and off the top of the router.
Think in dew point on cold walls. Relative humidity is relative to air temperature, which is why “60% RH” can be fine in one room and a mould farm in another. Air at 20°C and 60% RH has a dew point around 12°C — and the inside face of an uninsulated solid brick wall, behind a wardrobe, in January, can sit at or below that all night. The air doesn’t have to feel damp; the wall just has to be cold. This is why the fix for corner mould is sometimes insulation and heating rather than a dehumidifier, and it’s why a logger recording temperature and RH together — or a display showing dew point, like the Testo — earns its keep.
Check your sensor with a salt test. Put a couple of tablespoons of table salt in a bottle cap, wet it to a slush, and seal it in a sandwich box or zip-lock bag with the hygrometer for eight to twelve hours. Physics holds the air above saturated salt water at very close to 75% RH at room temperature. If the unit reads 71%, it reads 4% low; either apply the offset in the app (the Inkbird lets you) or write it on a label on the back. Do it annually — every RH sensor drifts.
What you can skip. Weather-station bundles with rain gauges you’ll never site properly; colour screens and smart-home theatrics that halve battery life; “comfort zone” algorithms doing what a threshold alert does; and above all, claimed ±1% RH accuracy on a £15 unit — the sensor element inside doesn’t support it, whatever the listing says. Spend the money on more sensors instead. Three adequate loggers beat one “precise” monitor in every diagnostic scenario I can think of.
Questions I get asked
What is a healthy indoor humidity level in a UK home? Aim for roughly 40–60% RH in living spaces. Sustained levels above 65–70%, particularly overnight in bedrooms, put you in mould-risk territory on any cold surface; below about 40% is needlessly dry and hard on skin and joinery. But the single number matters less than the pattern — a house that averages 55% but spends every night at 80% in one bedroom has a problem the average hides, which is precisely why logging matters.
Will a hygrometer tell me if I have damp? It tells you which kind you might have, which is more useful. High humidity with night-time spikes points at condensation — a ventilation, heating and moisture-behaviour problem. Normal air humidity alongside a wet patch on a wall points at penetrating damp, plumbing or a defect, which is damp meter and investigation territory. Running both instruments together — air data over time, wall readings by location — is how condensation gets separated from building defects without guesswork.
Where exactly should I put the sensor in a bedroom? At the problem, not at the door. Mould grows where surfaces are coldest: the corners of external walls, behind furniture pushed against them, around window reveals. Place the sensor low on the external wall, near (not touching) the affected corner, out of sunlight and away from heat sources. If you have a second unit, put it centrally in the room — the gap between the two readings is a measure of how cold that wall really runs.
Are cheap hygrometers accurate enough to trust? For diagnosis, yes — with two caveats. The claimed ±2–3% RH of the units above is fine, because mould risk lives in 10–15% bands, not decimal places, and the diagnostic signal is the shape of the graph over days rather than any single reading. The caveats: verify yours with the salt test rather than assuming, and don’t compare absolute numbers between two different brands of sensor — compare each sensor with itself over time, and sensors of the same model with each other.
If the graph confirms what you feared — humidity camped above 70% every night and mould creeping across the cold corner — the answer is usually ventilation, heat and insulation in some combination, and working out which is the day job. The practice does ventilation assessments and PAS 2035 retrofit assessments where moisture risk is dealt with before any insulation goes near a wall — get in touch. For measuring what’s in the fabric rather than the air, there’s the damp meter guide; for checking whether the extract fan actually extracts, the anemometer guide; and there are more field-kit guides if you’re building out the survey bag.
Small print. This guide is general information, not a survey of your property; condensation and mould diagnosis depend on the building and household in front of you, and humidity data alone is never the whole answer. 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.