Humidifiers, and the alternative

Winter dryness is real and it does real damage. Wood shrinks, glue joints open, panel paintings cup, parchment cockles, and furniture that survived two centuries can fail in a decade of centrally heated display. The instinct is to add moisture back. That instinct is right about the problem and frequently wrong about the solution.

Why the building got dry

Nothing removed water from the air. Cold winter air contains very little moisture in absolute terms. Bring it inside, warm it to 20 °C, and its capacity to hold water rises sharply while the actual quantity of water stays the same. The ratio, which is what relative humidity measures, falls accordingly.

This matters because it identifies the lever. The dryness was caused by heating, so the cheapest fix is usually to heat differently rather than to add water.

Conservation heating, the option people skip

Conservation heating controls the heat to humidity rather than to comfort. The heating comes on when relative humidity rises above the target and otherwise stays off. The building runs cooler than a comfort-heated one, sometimes considerably, and sits inside a sensible humidity band without any water being added.

For unoccupied or lightly occupied historic interiors this is the well-established answer. It costs less to run than comfort heating, it needs no water supply, and it cannot produce condensation because it never raises the moisture content. Its limitation is obvious: it produces a cool building, which is fine for a store or a house museum in winter and unacceptable for a busy gallery.

If you do humidify

Evaporative units pass air over a wetted surface. They self-limit, because evaporation slows as the air approaches saturation, and they do not produce mineral dust. They need regular cleaning, because a permanently wet wick in a warm room is a microbiological culture.

Steam units boil water and inject vapour. Clean output, precise control, high energy consumption. They are the usual choice in specified installations.

Ultrasonic units atomise water with a vibrating element. They are cheap and efficient and they are the wrong choice in a collection: everything dissolved in the water is atomised too and settles as a fine white deposit on every surface in the room, including the objects. Demineralised water reduces this but the maintenance discipline required is rarely sustained.

The condensation trap

This is the risk that catches people out, and it is worse than a dry gallery.

Raise the moisture content of the air in a building with cold external walls, and the air in contact with those walls will reach its dew point. Water condenses on and within the wall. The visible result may be nothing at all for a year; the invisible result is moisture in the fabric, mould behind panelling, and salt movement in masonry.

The check is arithmetic, not judgement: compare the dew point you will create against the surface temperature of the coldest external wall. If the dew point is above that surface temperature, you will get condensation. A surface temperature probe and a handheld with dew point will answer it in an afternoon.

The order to work through

1. Confirm from logged data that the space is genuinely too dry, and for how long. A fortnight at 35 percent is not the same problem as four months at 25.
2. Ask whether the heating can be reduced or controlled differently. This is free or nearly free and removes the cause.
3. Improve the envelope. Draught sealing reduces the volume of cold dry air being drawn in and warmed.
4. Consider buffering the objects rather than the room. A well-sealed case with conditioned silica gel holds a stable band around the object regardless of what the gallery does.
5. Only then humidify, with a type that does not produce dust, and only after checking the dew point against the coldest surface.