Humidity control

Of everything you can measure in a collection space, relative humidity is the one that does the most cumulative damage and the one most people get wrong in the most expensive direction. Wood, ivory, parchment, paper and textiles all take up and give off moisture with the air around them, and they change size as they do it. When a composite object cannot move uniformly, something has to give: a joint, a ground layer, a veneer, a binding.

Why stability beats the number

The classic instruction was to hold 50 percent relative humidity and not deviate. It produced a generation of expensive plant, and the evidence base for the precise figure was always thinner than its confidence suggested.

Current practice has moved. Broad agreement now sits closer to the following: most mixed collections tolerate a band rather than a point, sudden change is worse than steady deviation, and seasonal drift that an object can follow slowly is far less damaging than a weekly sawtooth of the same amplitude. Objects that have sat in an unheated country house at 60 percent for two centuries are usually in equilibrium with it, and moving them to a textbook 50 is an intervention, not a rescue.

The nuance matters because it changes what you buy. Chasing a fixed setpoint needs powerful, continuously running equipment. Holding a band and damping the rate of change needs much less. Target humidity ranges sets out the current positions and where they conflict.

Too damp: the options

Mould germinates on most organic substrates somewhere above roughly 65 to 70 percent RH sustained, and metals corrode faster as humidity rises. Once monitoring shows a genuinely damp space, there are three levers, and they are worth trying in this order.

Fix the source. A blocked gutter, a failed damp-proof course, a store room below ground with no ventilation. Mechanical dehumidification of a room with an active water ingress is an expensive way to run a permanent pump.

Change the air handling. Ventilating with outside air helps only when the outside air is drier in absolute terms, which in a maritime climate is often not the case in summer even when it feels drier. Check dew point, not relative humidity, before opening anything.

Dehumidify. Compressor units are cheap and effective in warm rooms and poor in cold ones. Desiccant units work at low temperature but consume more energy and add heat. The dehumidifiers page covers sizing to the measured moisture load rather than the room volume, which is where most purchases go wrong.

Too dry: the harder problem

Winter heating is the usual culprit. Warm a cold building without adding moisture and the relative humidity falls, often into the twenties. Wood shrinks, glue joints fail, panel paintings cup, parchment cockles.

Adding moisture back is technically easy and practically fraught. Evaporative and steam humidifiers both work; ultrasonic units can distribute dissolved minerals as a fine white dust over everything in the room, which is a conservation problem of its own. Any humidifier that raises room humidity while a cold external wall stays cold will produce condensation on that wall, and in time mould behind it. Humidifiers covers the trade-offs.

Often the better answer is not to humidify at all but to heat less, or to heat with a different strategy. Conservation heating, which controls the heating to a humidity setpoint rather than a temperature one, keeps historic interiors within a sane band at a fraction of the energy cost and without adding water to a fabric that cannot take it.

The case for microclimates

A sealed display case with an appropriate quantity of conditioned silica gel is a remarkably good humidity buffer. It has no moving parts, consumes no energy, fails gracefully rather than catastrophically, and treats the volume that actually contains the object rather than the several hundred cubic metres that do not.

The trade-off is maintenance and honesty about leak rate. A case that exchanges its air once a day will overwhelm any reasonable quantity of gel; a case at one air change per day or better can hold a band for months. Silica gel and sorbents covers how much you need and why the answer depends on the case, and showcase microclimates covers the construction side.

Before you buy anything

Every option above costs money to run, and several of them can cause damage if sized or sited wrongly. The order that avoids waste has not changed: log the space for a year with a calibrated instrument, work out whether you have a moisture problem or a temperature problem, then decide.