Radiant heating and humidity: the link most people miss

Humidity is the part of indoor comfort people notice only when it goes wrong. Too high and you get condensation, mould and that heavy, close feeling in a room. Too low and your eyes, throat and skin dry out.

ASHRAE puts the comfortable band at 40 to 60% relative humidity. Getting a house to sit inside it is usually treated as a ventilation or dehumidification problem. It is also, to a surprising degree, a heating problem.

Condensation is a surface problem, not an air problem

Water does not condense out of warm air on its own. It condenses when warm, moist air meets a surface that is cold enough to bring it below its dew point. The cold surface is the cause; the humidity in the air is only the fuel.

This is why condensation shows up where it does: window reveals, the corner behind a wardrobe on an external wall, the cold patch above a lintel. Those are the coldest surfaces in the room, and they are where the moisture lands first.

A convection system heats the air and leaves the surfaces to catch up. Cold surfaces stay cold for a long time — sometimes permanently, in the corners the warm air never really reaches.

Radiant heating inverts that. It warms the surfaces directly, which is precisely what stops them reaching dew point. The mechanism that makes a radiant room feel comfortable is the same mechanism that keeps the walls dry.

Why it matters beyond comfort

Persistently high humidity means mould and mildew, which is a respiratory problem as much as a decorating one. Persistently low humidity is uncomfortable in its own right, and dry air is one of the most common complaints about forced-air heating — a system that moves large volumes of air through a house will strip moisture out of it.

Radiant heating does neither. It does not blow air around, so it does not dry a room out, and it does not leave the cold surfaces that produce condensation.

There is a related benefit: because nothing is circulating, dust and allergens are not being lifted off surfaces and carried around the room. For anyone with allergies or asthma, that is not a marginal difference.

Where it matters most

Bathrooms and kitchens are the obvious cases — high moisture load, and often the coldest external wall in the house. A heated wall or ceiling in a bathroom does two jobs at once: it makes the room comfortable to be in undressed, and it keeps the surfaces above dew point so the room actually dries out between uses.

Basements, north-facing rooms and heritage buildings with solid walls are the other cases worth flagging. Solid-wall properties in particular are where condensation is a chronic problem, and where a warm wall surface changes the behaviour of the room completely.

Design decides the result

None of this happens automatically. It depends on the elements being placed where the cold surfaces actually are — on and around the external walls, not on the internal partition that happened to be convenient.

That is what a layout is for. Send us a plan of your building and we will do the heat loss room by room and put the elements where the building needs them, rather than where they are easiest to install.