What humidity should a room be?

Between about 40 and 60 percent relative humidity suits both furniture and people. Below 30 percent in a heated flat in winter is where gaps open and joints get stressed.

Does wood move along its length?

Barely. Movement along the grain is small enough to ignore in furniture. Across the grain it can be several millimetres over a wide board between a damp summer and a dry winter.

Is a gap that opens in winter a defect?

Usually not. A gap that opens in the heating season and closes again in spring is the wood doing what wood does. A gap that keeps growing year after year is a different matter.

Does a finish stop movement?

No. Oil, lacquer and wax slow the rate at which wood exchanges moisture with the air. They do not stop it. Nothing you can apply to wood stops it moving.

Material care · 4 min read

What Indoor Humidity Does to Wood Furniture

Wood moves across the grain all year. What that means for joints, gaps and the heating season.

Wood is not inert. It takes moisture out of the air when the air is damp and gives it back when the air is dry, and it changes size as it does. Every joint in every wooden thing you own is designed around that, or has failed because it was not.

The direction that matters

Movement along the grain, end to end on a board, is negligible. Movement across the grain is not. A wide board can change several millimetres in width between a humid August and a January flat with the heating on.

That asymmetry is why furniture is built the way it is. A solid top is fixed to a frame with fittings that let it slide across its width. A panel sits in a groove rather than being glued in. Breadboard ends are pinned in slots. Every one of those details exists for the same reason.

The heating season is the hard part

Outdoor air in winter is cold and holds little water. Bring it inside, heat it to 21 degrees, and its relative humidity collapses. A flat with radiators running can sit at 20 to 30 percent for months, which is drier than many deserts.

What you see:

Gaps opening between boards, or between a panel and its frame.
A drawer that was tight in summer running freely.
Small checks at the end grain of a thick component.
Joints that creak when loaded, because glue lines are being stressed.

Then spring comes, humidity rises and most of it reverses. The cycle itself is normal. What does damage is speed and extremes.

Three things that cause real damage

A radiator directly under or beside a piece. The face nearest the heat dries faster than the far face, so the component cups or twists. Steady dryness is survivable. A gradient across one piece of wood is not.
Direct sun through glass. Same mechanism plus ultraviolet. See sunlight, fading and where not to put a piece.
Moving a piece between very different environments. A cold van into a hot flat, or an unheated storage room into a heated one, with no settling time.

What to do

Measure. A hygrometer costs very little and turns guessing into knowing. Aim for 40 to 60 percent.
Raise winter humidity if you are under 35 percent. Drying laundry indoors, plants and a humidifier all work. This is good for you as well as the furniture.
Keep a gap from radiators. Not a measured distance so much as a rule: nothing wooden hard against a heat source.
Let a new piece acclimatise for a few days in the room it will live in before you judge it or load it.
Do not block airflow to one face only. A board with one side sealed against a wall and the other open to a dry room will cup.

Oiled ash specifically

Ash is a dense, straight-grained hardwood that behaves predictably. An oil finish lets the surface breathe, which means the wood tracks the room rather than fighting it. That is the trade: slightly more seasonal movement than a thickly lacquered piece, and far easier repair, because an oiled surface can be fixed in one spot.

The Side Shelf is an asymmetric ash unit that can stand alone or hang from two screws. If it is hung on an outside wall, leave an air gap behind it rather than clamping it flat, so both faces see similar air.

Movement also changes how often a surface wants oil, because dry wood takes finish differently. The test is in when to re-oil ash furniture.

What is not humidity

Not every gap or mark is seasonal.

A joint that has opened and stayed open through a full year is a glue or fixing failure.
A white cloudy patch is moisture trapped in a finish, usually from a hot or wet object, and is dealt with locally.
A board that has cupped and never flattened across a summer has probably been wet on one side.

The honest test is time. One full year tells you whether you are watching a cycle or a fault.

Reality check

Across the grain, wood moves. Nothing stops it.
40 to 60 percent relative humidity is the target, and winter is when you lose it.
Gradients do damage, not dryness by itself. Keep heat and sun off one face.
A gap that opens in January and closes in May is the piece working correctly.
Give a new piece a few days in the room before you decide anything about it.

Common questions

What humidity should a room be?: Between about 40 and 60 percent relative humidity suits both furniture and people. Below 30 percent in a heated flat in winter is where gaps open and joints get stressed.
Does wood move along its length?: Barely. Movement along the grain is small enough to ignore in furniture. Across the grain it can be several millimetres over a wide board between a damp summer and a dry winter.
Is a gap that opens in winter a defect?: Usually not. A gap that opens in the heating season and closes again in spring is the wood doing what wood does. A gap that keeps growing year after year is a different matter.
Does a finish stop movement?: No. Oil, lacquer and wax slow the rate at which wood exchanges moisture with the air. They do not stop it. Nothing you can apply to wood stops it moving.