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Hygrometer, damp and mould in a UK home

Updated: October 2026

The drying rack is up, or a bedroom corner looks tired, and a TFA Dostmann hygrometer is how you decide whether to heat or open a window. The dial cannot take water out of plaster, and a number in the comfort band is not a certificate that the wall is dry. This page is about the building, using the dials in the [TFA Dostmann hygrometer comparison](/hub/climate-measuring/tfa-dostmann-hygrometer). It does not rank them. The cloth is on the [how to use](/hub/climate-measuring/tfa-dostmann-hygrometer/sub/tfa-dostmann-hygrometer-how-to-use) page. Products Compared used GOV.UK, the Centre for Sustainable Energy, and TFA’s English note. We did not survey a house. A qualifying purchase can earn us a commission at no extra cost to you.

The drying rack is up, or a bedroom corner looks tired, and a TFA Dostmann hygrometer is how you decide whether to heat or open a window.

Our top picks

TFA TFA 45.2027 indoor hygrometer

Best overall

TFA TFA 45.2027 indoor hygrometer
8.0/10about £17.39
View offer
TFA TFA 45.2045.02 indoor-outdoor hygrometer

Best value

TFA TFA 45.2045.02 indoor-outdoor hygrometer
7.9/10about £9.71
View offer
TFA TFA 45.2024 metal-ring hygrometer

Alternative

TFA TFA 45.2024 metal-ring hygrometer
7.5/10about £10.94
View offer
On this page
  1. 1.What the dial is for
  2. 2.Whose 40–60 % is this
  3. 3.A cold room and a washing load
  4. 4.When to put the dial down

What the dial is for

A TFA Dostmann hygrometer in this set shows relative humidity and temperature on one analogue face. You glance, then you heat or you ventilate. Relative humidity is how full the air is. It is not a measure of water stored in the wall. High relative humidity is one sign in GOV.UK’s guidance on damp and mould in rented homes, and the page we used does not publish a single percentage. A reading is a prompt to change the room or to look harder at the building. The dial does not prevent mould, extract moisture, or heat a wall. A green band is print, not a certificate for a landlord. Where a bullet prints an optimum, it is often 18–22 °C and 40–60 % humidity. The 45.2043.51 uses Dry, Ideal, and wet. If you are still choosing which of the five to hang, the buying guide is the order.

Whose 40–60 % is this

The figures disagree because they come from different desks. The listing graphic, where a bullet has one, is 18–22 °C and 40–60 % relative humidity. The Centre for Sustainable Energy aims at about 40–60 % RH and about 18–21 °C as a household target. CSE’s range is advice, not a statute. GOV.UK treats high relative humidity as one sign and does not hand you a single percentage on the page cited here. Wiltshire Council treats about 40 % RH as a household level and long spells at 65–70 % as mould risk. Westmorland and Furness speaks of an average around 40–65 %. A council leaflet is local advice, not the legal limit for England, Scotland, Wales, or Northern Ireland. BS 5250:2021 is the moisture code. A dial on a shelf is not a way to score it.
TFA TFA 45.2027 indoor hygrometer

Testsieger

TFA TFA 45.2027 indoor hygrometer
8.0/10about £17.39
View offer

A cold room and a washing load

The same moisture can read as higher relative humidity when the air is colder. A bedroom on a January morning can show a higher number than the same air after the heating has run, without any new water. If the rack of washing is in a small flat, watch whether the number is still high long after the clothes have gone, or whether it falls once a window or an extractor runs. The pointer times that fall. It does not dry the clothes. TFA’s English note treats sustained indoor humidity above 65 % RH as a condition that encourages mould. That is the maker’s condition, not a prosecution threshold, and not Wiltshire’s long spells at 65–70 %. A spike from a kettle or a shower is ordinary. A week above that line, in a room you also heat, is a reason to change the habit and to look at the fabric if the number never comes down. Approved Document F, Volume 1, is guidance for dwellings in England. Scotland, Wales, and Northern Ireland use their own regimes.

When to put the dial down

Stop once the problem is bigger than a room you can air: mould that is more than a small patch or returns after cleaning, a landlord who has not acted, a suspected leak, rising damp, or a failed damp-proof course. Stop if the dial is meant as evidence. A surveyor uses their own method, and 50 % RH does not prove a dry wall. Match the listing on the best-for-the-job page. None of these five keeps mould away.

Common questions

What should a UK home aim at for indoor humidity?
The Centre for Sustainable Energy aims at about 40–60 % relative humidity and about 18–21 °C. Several of these listings print 18–22 °C and 40–60 % as a comfort graphic. Wiltshire’s note treats about 40 % as a household level, and Westmorland and Furness speaks of about 40–65 %. Use CSE’s band as a household aim, and keep each source’s numbers with that source.
Is 40–60 % a legal limit?
CSE’s 40–60 % is advice. The band on a dial is print. GOV.UK’s damp guidance, on the page used here, does not set a single percentage. A long spell above the ranges councils mention is a reason to act. It is not, by itself, proof that a landlord is in breach.
Does a reading under 60 % mean the wall is dry?
It means the air around the dial is under that number. The same moisture can read higher in a cold room, and a comfortable percentage does not prove the plaster is dry. BS 5250:2021 is the moisture code. A room dial is not a pass mark against it, and it is not a reason to skip opening a wall if you suspect a leak.
Why can the same room read higher on a cold day?
Relative humidity rises when air cools if the moisture in it stays the same. A cold bedroom can show a higher number in the morning than after the heating has run, with no new water added. Heat the room and then ventilate. Watching the dial while the room stays cold keeps showing you the cold-air figure.
When do I stop and get help?
If you need the number as evidence, a surveyor brings their own method. Take dated photos of what you saw. A reading is not, by itself, a finding that a landlord is in breach.

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Sources

Manufacturer and public UK sources used for this comparison. Listing prices, stars, and review counts are from amazon.co.uk on 15 October 2026.

  1. TFA — English hygrometer instructions (retrieved 16 October 2026)
  2. TFA — thermo-hygrometers (retrieved 16 October 2026)
  3. TFA — frequently asked questions (retrieved 16 October 2026)
  4. TFA — analogue thermo-hygrometer 45.2027 (retrieved 16 October 2026)
  5. TFA — analogue thermo-hygrometer 45.2043 (retrieved 16 October 2026)
  6. Centre for Sustainable Energy — temperature and humidity at home (retrieved 16 October 2026)
  7. GOV.UK — damp and mould in the home (retrieved 16 October 2026)
  8. Consumer Rights Act 2015, section 9 (retrieved 16 October 2026)