Tall rooms are harder to heat than their floor area suggests, and in the period houses of Hampstead that has a direct bearing on where mould grows. Warm air rises and collects near the ceiling, cooler air settles at floor level, and in a room three or four metres high that separation is pronounced. The thermostat in the middle of the room may read comfortably while the surfaces near the ceiling perimeter and the surfaces near the floor are both several degrees colder than the air people are actually standing in.
Layers of air in a high room
Heat delivered by a radiator rises in a plume, spreads across the ceiling and then falls slowly down the external walls as it cools. In a room of ordinary height that circuit mixes the whole volume. In a very tall room, the upper layer stays warm and the lower layer barely joins in, so the coldest surfaces are the bottom of the external walls and the corners the circulation never reaches. Add a large window and the falling cold air along it reinforces the effect, holding the lower part of the room down.
Why the top of the wall goes as well
The other place growth appears in these rooms is the opposite extreme: the angle between the external wall and the ceiling, and the decorative perimeter beside it. That surface is warm from the air but cold from the structure, because a wall head, a parapet or a roof junction behind it is losing heat to outside, and the humid air collecting at high level is the wettest air in the room. Speckling along a ceiling perimeter with a clear ceiling in the middle is a reliable indicator of that combination.
Intermittent heating in a large volume
A big room heated for a few hours in the evening never reaches equilibrium. The air warms quickly, which feels like success, while the masonry behind the plaster takes far longer and may not warm through at all before the heating goes off again. Surfaces therefore spend most of the day cold in a house whose air is often humid, and that is a very favourable situation for growth. Steadier heating at a lower level does more for these rooms than short intense bursts, even though it is counter-intuitive.
Rooms that are shut up for months
Large houses have rooms that are barely used, and those are frequently where the problem is found. A closed drawing room or a spare bedroom is unheated but not sealed, so humid air from the rest of the house drifts into it and meets surfaces at their coldest. Furniture is often left in place against the walls and covered over, which removes what little air movement there was. A room being left alone is not the same as a room being protected.
Working within a protected interior
Much of the stock here is listed or in a conservation area, with historic plaster, joinery and decorative detail that cannot simply be stripped out. Treatment is therefore local and reversible wherever possible: growth removed with the surface damp, minimal mechanical action, and no abrasive work near mouldings. Where plaster has failed and has to come off, the area is kept as small as possible and replaced in a compatible material. Consent may be needed for anything beyond like-for-like repair, and the general order of assessment and works is described under black mould removal.
What a visit to a large, tall room involves
Access at height is the first consideration, since readings and treatment at a ceiling perimeter need a working platform rather than a stepladder, and that has to be planned rather than improvised. Temperature and humidity are recorded at floor level, at standing height and as high as can be reached, because the gradient is the evidence. Costs reflect the height and area worked rather than the number of rooms, so a single tall room can be a larger job than three ordinary ones. Careful cleaning of affected contents and soft furnishings afterwards is the kind of task the cleaning services team in Belsize Park takes on, refitting joinery around a repaired area is a job for Hampstead handyman specialists, what the service covers covers activity found in a closed-up room, and mould removal services in Belsize Park deals with the same stratification problem in the converted houses below.