A 4m by 4m wool carpet holds somewhere around one and a half litres of water after a competent hot water extraction. That is the figure worth sitting with. Not a damp feeling underfoot, not a bit of moisture in the pile – a litre and a half of water, plus whatever is sitting in the underlay, that has to leave the room through the air and nowhere else.
In June, in a first-floor flat with the sash up, that water is gone in four hours and nobody thinks about it again. In February, in a converted flat on Chamberlayne Road with sealed replacement units, no background ventilation and a boiler running eleven hours a day, the same litre and a half is still in the room on Thursday.
Why does a carpet dry so slowly once the temperature drops?
Drying is evaporation, and evaporation depends on how much more water the air in the room is capable of holding. Warm air holds a great deal. Cold air holds very little. Air at 20°C can carry roughly four times the moisture of air at 0°C, and that ratio is the whole story of winter drying work in this city.
Here is where most people go wrong. They turn the heating up, close the windows because it is cold, and assume warmth equals drying. Warming the room does raise its capacity to hold moisture, so the carpet gives up water quickly for the first hour or two. Then the air in the room reaches its limit. With no route out, that moisture stays in the flat, and the moment the heating drops overnight it condenses on the coldest surface available – usually the window reveal, sometimes the external corner of a bedroom wall, occasionally the back of a wardrobe pushed against a party wall.
I have been called back to two flats where a customer complained of a musty smell three weeks after a clean and the smell was coming from the skirting behind a sofa, not the carpet. The carpet had dried. The water had moved house.
Flats built or converted before the current ventilation guidance are the difficult ones. Approved Document F was updated in 2021 and took effect from 15 June 2022, requiring background ventilators in most replacement windows in England. Anything glazed before that date, and any flat where only the sealed unit was swapped rather than the whole window, has no permanent air path at all. In a London January that flat is a sealed box with a wet floor in it.
What “dry to the touch” is hiding
Press the back of your hand into the pile and it reads dry long before the job is done. The pile is the fastest-drying part of the assembly and always has been. Underneath it sits the primary and secondary backing, then the underlay, and PU foam underlay in particular will hold moisture for a day and a half after the surface reads perfectly dry.
I meter through the pile into the underlay with a pinless meter, in three places per room, and I write the reading down. If the underlay is wet and the pile is dry, the room is not finished.
How long should you realistically expect it to take?
Numbers, with the caveats they deserve.
Polypropylene cut pile, thin foam underlay, heated flat with some ventilation: five to eight hours. Wool twist on 10mm PU underlay in the same flat: twelve to sixteen. Wool on old hessian-backed felt underlay, ground floor, no trickle vents, January: twenty-four to thirty-six hours, and I say thirty-six to the customer so that twenty-four feels like good news.
Basement and lower ground floor flats sit in their own category. Rupack Street in Rotherhithe, SE16, a lower ground flat with one openable window and a light well: the bedroom took two full days with an air mover running, and that was with a low-moisture method rather than extraction.
The variable that moves the number most is not the machine or the operator. It is air changes per hour. A room with the door open onto a hallway with a stairwell will dry at perhaps double the rate of the same room with the door shut, before you plug anything in.
What people get wrong about airflow
Air movers are directional. Pointing one at the middle of the carpet dries a circle and does very little else. The point of the machine is to lift the boundary layer of saturated air sitting immediately above the pile and replace it, which means the airflow needs to travel along the floor, wall to wall, at a shallow angle. One unit in the corner aimed diagonally across the room beats two units aimed downward at the wet patches.
Move it every couple of hours. Nobody does, and it costs them half a day.
Do dehumidifiers help, and does the type matter?
Enormously, and yes, and this is the part the hire shops on the Old Kent Road will not tell you.
Refrigerant dehumidifiers work by chilling a coil below the dew point so moisture condenses out of the air passing over it. In a warm room they are excellent. In a cold room the coil ices up, the machine spends most of its cycle defrosting itself rather than collecting water, and the extraction rate falls off a cliff below about 15°C. The 20-litre unit rented to a customer for a January bedroom in an unheated flat might pull three or four litres a day.
Desiccant machines use a rotor of moisture-absorbing material and a heater, so ambient temperature barely troubles them. They keep working at 5°C. They also put heat back into the room, which raises air temperature and accelerates evaporation at the same time. For carpet drying in a London January, in a flat with no background ventilation, a desiccant unit is the correct tool and a refrigerant unit is a compromise.
The catch is that a dehumidifier only works in a closed room. Running one with the window open is a way of dehumidifying Greater London at your own expense, and it will not go well.
Where to put it
Same room as the wet carpet, door shut, unit raised off the wet floor on a chair or a box so it is sampling the air above the pile rather than the coldest layer. Empty the tank or run a hose to the sink. A unit that switches off on a full tank at 2am has given you six wasted hours and a room at 80% relative humidity.
Should you open the windows, or keep them shut?
Depends entirely on what the air outside is doing, and half the advice given on this subject in London is wrong.
Ventilation dries a room only when the outside air is capable of absorbing more moisture than the inside air already holds. On a cold, bright, breezy day in February, outside air at 5°C and 60% relative humidity will warm to 20°C indoors and drop to around 22% RH, which makes it a superb drying medium. Open everything.
On one of those still, grey, saturated days that sit over the river for a week at a time – outside air at 9°C and 95% RH, fog on the Isle of Dogs, everything beaded with damp – you are importing water. I have stood in a flat on Manchester Road, E14, watching a customer’s hygrometer climb after they opened the balcony door to help.
Buy a £12 hygrometer. Compare inside and out. Ventilate when the numbers favour it and shut up when they do not.
The internal door trick
Where a flat has a bathroom or kitchen extractor fan, that fan is a permanent, weather-independent air path. Shut the front door, open the internal doors, crack one window an inch at the far end of the flat and leave the extractor running on continuous trickle. You get a slow, steady cross-flow through the wet room and out through a duct that does not care about the fog. It is not fast. It runs all night without anyone having to think about it.
When is it better to postpone the job?
I turn winter work down, and I would rather say so before somebody books.
A ground-floor flat with existing black mould at the window reveals, no extractor, no trickle vents, and a tenant who works nights and cannot leave the heating on is a flat where a full extraction clean in a London January creates a condensation problem worse than the soiling it fixes. That flat gets encapsulation, or it gets a booking in April.
The pressure point is check-out. Letting agents put professional carpet cleaning into inventory clauses with a hard deadline, and tenants ring me on a Sunday for a Monday clean and a Tuesday key handover. Thirty-six hours in a sealed flat in February is not enough, and a carpet still damp at the inventory inspection reads as a failed clean to a clerk with a checklist. I did a Culmington Road flat in W5 last winter where the clerk marked the carpet down as unclean because it was cold and dark underfoot at the door threshold. It had been cleaned properly nineteen hours earlier.
What the inventory clerk is looking at
Clerks work from a checklist and a torch, and moisture reads as dirt to a torch beam at an angle. They are not testing the fibre. They are looking at colour depth, at whether the pile lies flat, and at whether the threshold strip feels cold underfoot on the way in. A carpet that is 90% dry photographs darker at the doorway than a carpet that is bone dry, and that photograph goes into a report that decides who keeps a deposit.
Book the clean for four days before the handover, not the day before. Take a photo of the meter reading with the date on it. Alma Road, SW18, three weeks ago: eighteen hours, wool, ground floor, desiccant unit running from six in the evening. Pile at 6% by lunchtime.
