Hygrometer probe

This tool as saved my career many times over, and countless properties from rot and misdiagnoses rising damp.

Contents hide


  1. Look up Hygrometer probe on eBay. The link may become out of date. The cheaper ones come from China are made for fridges and cost about £3.50 – compare to the image.
  2. There is nothing especially clever about hygrometers, they have a piece of plastic that curls-up with humidity and a thermocouple to measure temperature.
  3. I buy in bulk as I probably break one or two every week trying to probe through blocked sub-floor vents, or between the gaps in floor boards.
  4. I remove the plastic guard (gentle twist and pull), unknot it and attach to a thin extendable car arial.
  5. Ideally measure as far in as possible, at least 1M from the sub-floor, or the under stair cupboard and leave for 20 – 30 minute to acclimatise (so it should be the first job on site).
  6. Over 85%RH needs further investigation, normally a sign of penetrating damp or leak – but could be wet rot, or worse still Dry rot.
  7. Below 75%RH is normally incompatible with rising damp.
  8. There will be variation over time.
  9. As a home owner, be sure to have a hygrometer probe in your sub-floor and check it regularly, say weekly or monthly,
    1. if there is any risk of water leaking into the sub-floor, such as penetrating damp, waste water or mains water or
    2. you have an extension,
    3. decking, or
    4. otherwise blocked sub-floor vent.
  10. I plan to use these embedded long-term, deep behind insulation to ensure water doesn’t become trapped between a vapour barrier and wall, resulting in rotten joists.

Leave a Comment

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.