Do the test before you call anyone
This is the one job on this site we would rather you did yourself. It is free, it takes two minutes of work and two days of waiting, and it answers the only question that matters. Damp surveyors use the same method on concrete floors to separate a slab that is still drying from one being wetted from underneath. It works just as well on a cistern wall, a pipe run or the loft boards beneath them.
- Dry the area completely. Not damp-to-the-touch. Wipe it, then leave it an hour with a fan or an open hatch until it is bone dry. Moisture left behind shows up under the foil and reads as a false positive.
- Cut a square about 150mm across. Kitchen foil works. A piece cut from a clear freezer bag works better, because you can read it without lifting it.
- Tape all four edges. Every edge, with no gaps, using gaffer or duct tape. Half-taped is worthless: air gets under it and both faces end up wet. The seal is the test.
- Leave it 48 hours. It needs to span at least one cold night and one stretch with the heating running, because those are the conditions that drive the two mechanisms.
- Read it without disturbing the tape first. Look at the outer face. Then peel one corner and look underneath.
| What the foil shows | What it means |
|---|
| Beads or a film on the outer face, dry underneath | Condensation. Moisture in the loft air is settling on a cold surface. Nothing is leaking. |
| Dry outer face, water trapped underneath | Water is arriving through or from the surface. That is a leak, or ingress through the roof. |
| Wet on both faces | Either both are happening, or the tape seal failed. Re-tape properly and repeat before concluding anything. |
| Dry on both faces after two days | Whatever wetted it has stopped, or it only happens in conditions you did not catch. Repeat in a hard frost. |
Two supporting checks cost nothing and sharpen the answer. Condensation appears evenly across a whole cold surface and is worst at the coldest part of it; a leak returns to one point and runs from there. And if your water meter keeps turning with everything off, you have a supply leak regardless of what the foil says, and the two are separate problems.
Why a cold tank in a loft sweats
Warm air holds more water vapour than cold air, and when it touches a surface below its dew point the vapour condenses out. Air at 15°C carrying 70% relative humidity reaches its dew point at roughly 10°C, and mains water arriving into a London loft in January is comfortably below that. The tank wall works as a dehumidifier plate.
The moisture is coming from your house. Every shower, kettle, wash load and breathing occupant puts vapour into the air, and it migrates upwards. A loft hatch that does not seal, a bathroom extractor ducted into the loft rather than through the roof, downlighters cut through a bedroom ceiling — each one is a route for warm, wet air to reach the coldest surfaces in the building.
The regulations acknowledge the problem from the other direction. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 requires that a cold water pipe to a tap is installed so that, so far as reasonably practicable, the water is not warmed above 25°C. Loft tanks and cold runs are kept cold on purpose, for water quality. Keeping them cold is exactly what makes them condense.
Why it gets reported as a leak
Condensation on a tank does not stay on the tank. It forms on the underside and the lower third of the walls, gathers, and drips. It lands on a joist, runs along it, and leaves the tank behind. By the time it shows on the ceiling below it can be two metres away from the thing that produced it, and it looks exactly like a slow leak on a pipe run.
The same happens on pipework. A cold run crossing a warm loft, or a boxed-in riser, condenses along its length and the water tracks to the lowest point, usually a clip or a joint. A wet joint gets read as a weeping joint. It very often is not. If your damp is on a ceiling with mould around it rather than a clean stain, the separate question of mould on a ceiling versus a leak above it is worth reading first.
Bad insulation makes condensation worse, not better
Lagging a cold pipe or jacketing a tank only helps if the insulation is continuous and sealed against air. If humid air can still reach the cold surface, you have not stopped condensation — you have moved it somewhere you cannot see and given it something absorbent to soak. The failures we find most often in London lofts:
- Split lagging with open seams. Foam tube slit down the side and pushed on, seams left unjoined and untaped. Air moves along the annular gap between foam and pipe, the pipe condenses inside the lagging, and the foam holds the water against the metal for months.
- Nothing at the fittings. Straight runs get lagged; elbows, tees, valves and the tank connections do not, because they are fiddly. Those are the coldest, most exposed points on the run.
- Loft insulation laid under the tank. The tank needs the warmth rising from the house, mainly so it does not freeze. Insulation belongs around and over the tank, not in the floor beneath it. Done the wrong way round, the tank is cut off from that heat and sits in cold loft air, which makes freezing and sweating both more likely.
- A jacket over the sides only. The underside condenses hardest and is the face nobody covers.
- No lid. Schedule 2 requires a storage cistern to have a cover positioned to exclude light and insects, and thermal insulation to minimise freezing or undue warming. An open cistern in a loft is also evaporating continuously into that loft, raising the humidity that then condenses on every cold surface up there, including the tank itself.
The general rule is the same one that governs lagging against frost: an insulation job that is 90% complete is not 90% effective. Our note on lagging that works and lagging that does nothing covers the fitting detail.
Condensation is not harmless, and it still needs fixing
Telling you it is not a leak is not telling you to ignore it. Persistent condensation in a loft does real damage on a slower clock:
- Loft insulation that gets wet loses most of its thermal value and does not recover, so the loft gets colder and the condensation gets worse.
- Timber held above about 20% moisture content for long enough supports rot and, in the right conditions, woodworm.
- Plasterboard ceilings stain, then sag, and the staining returns through paint until the source stops.
- Metal pipework held wet under saturated lagging corrodes from the outside in. A pinhole that appears under old wet foam has usually been made there, not by the water inside the pipe.
So the finding "it is condensation" converts a leak-detection problem into a ventilation and insulation problem. That is a cheaper problem, but it is still a problem, and it is one that gets worse every winter it is left.
What actually fixes it
- Cut the moisture at source. Bathroom and kitchen extractors ducted through the roof or wall, never terminated in the loft. Doors shut and fans left running after showers.
- Seal the route upwards. A draught-sealed, insulated loft hatch is the single biggest win in most houses. Then any service penetration, downlighter or pipe hole through the ceiling.
- Let the loft breathe. Eaves ventilation blocked by insulation stuffed into the corners is very common. Vent trays restore the airflow that carries the vapour out.
- Insulate the cold surfaces properly. Continuous lagging with sealed joints, fittings included, and a tank jacket that covers the sides, the underside and a lid. Floor insulation carried around the tank, not under it.
- Re-test. Dry it, re-tape the foil, read it after the next cold spell. A dry outer face means you fixed the right thing.
When it turns out to be a leak after all
Around a cold tank there are genuine leaks that present the same way. A passing float valve holds the level at the overflow so the warning pipe dribbles, soaking the fascia or the loft edge rather than the tank. A perished washer at the tank connector weeps at one point. A galvanised cistern rusts through at the base seam. A compression joint on a mains run that has crept will weep.
The pattern separates them: a leak wets one place, at any temperature, humid loft or not. If the foil comes back wet underneath and dry on top, or the tank level drops and refills with nothing drawn off, that is a real loss and worth someone attending.
If you want us to look
We charge £150 per hour, agreed before we attend, with no call-out fee. If we do not find a leak, you do not pay for the time we spent looking — and "there is no leak, this is condensation, here is what to do about it" is a legitimate outcome, not a failed visit. Where there is a leak we repair it, and our own handymen and builders make good the ceiling, plaster or boards afterwards. The rate is set out in full on pricing.
