Damp is the single biggest killer of motorhome resale value. It starts as a 19% reading on a damp meter and ends as a £3,000 wall-panel rebuild. A £15–£40 damp meter and ten minutes a year is the cheapest insurance you'll ever buy. Here's which ones actually give accurate readings — and how to use them.
A pin-type damp meter measures electrical resistance between two prongs pushed into the wood or wood-backed panel. Resistance drops with moisture. Most reliable meters express results as %WME (wood moisture equivalent). Under 15%: dry. 16–20%: 'monitor closely'. 21–25%: active ingress, repair needed. Above 25%: structural damage likely already underway.
Pinless meters scan a wider area without leaving holes, but campers and motorhomes have so many laminated, foil-backed and insulated panels that the pinless readings are unreliable. Pin meters give true substrate readings. The pinholes are tiny and inside cupboards no one sees.
A proper habitation check includes a damp survey at 20+ predefined points: window frames, roof seams, skylight, wardrobe corners, under sinks, around the front bulkhead, around any seams or moulding joins. Doing this yourself once a year between paid checks catches problems early.
Musty smell after a closed-up week. Soft-feeling carpets near the wall edges. Springy floor near the door entry step. Discoloured wallpaper around windows. Black mould speckles in corners. Any of these and the meter is confirming what you already know — get a professional habitation engineer involved.
Around every roof-light. Around every window frame (inside and out edges). Wardrobe back wall (top corners). Under-sink cabinet rear. Around the fridge vent. The over-cab bed front edge (luton-style). Around the rear panel join. Floor edges near the door entry. Around the shower tray. Beneath any external skirt locker. Around the front bulkhead seal. The bed-base panels (lift them up and probe underneath).
Anything above 20% in multiple connected points means the seal has failed and water has tracked behind the panel. Surface drying is hiding the issue. A workshop will strip the panel, identify the leak source (usually a perished sealant), re-bed the offending fitting, replace any rotten timber and refit. Catch it at 18% and it's often just a re-sealant. Catch it at 28% and the floor or wall structure may need rebuilding.
Under 15% is dry. 16–20% is 'monitor and act'. Above 20% needs professional attention.
Surface drying and re-sealing a window or roof-light is DIYable. Anything involving panel removal or floor structure is a workshop job — done wrong, you trap moisture and accelerate the damage.
Twice a year minimum — once before storage, once after winter. Plus before every long trip.
It supplements it. Pros are thorough but only see your van for an hour a year. Your own monthly nose-and-eyes check catches drips before they read as damp.
Only as a supplement to a pin meter — too unreliable on the laminated panels typical of campers to use alone.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email [email protected].