Humidity, Heating and Furniture

Wood is never finished absorbing and releasing water. It tracks the air around it for its whole life, and a heated flat in January is a desert compared with the same room in July.

What actually happens

Wood exchanges moisture with the air until the two are in balance. Dry air pulls water out and the timber shrinks; damp air puts it back and the timber swells.

The movement is almost entirely across the grain. Along the grain it is negligible. This is why a wide panel changes width noticeably between seasons while its length stays effectively constant, and why every sensible piece of wooden furniture is built to let that happen.

Heating is what makes winter severe. Warming cold outside air without adding moisture drops the relative humidity sharply, so a heated room in a cold climate can be drier than people assume.

What you will notice

• Creaking and the occasional sharp crack, usually at night as the heating cycles. Joints are adjusting. It is normal and it is not the sound of something breaking.
• Narrow gaps opening at panels or between boards in winter, closing again by summer.
• Drawers and doors running freely in winter and tightening in a humid summer.
• Surface checking in severe cases: fine cracks along the grain, usually at end grain where moisture leaves fastest.

The first three are the material doing its job. The fourth is a sign conditions have gone too far.

When it becomes damage

Movement causes damage only when it is restrained. If a panel is fixed so it cannot move, the stress has to go somewhere, and it goes into a split.

This is why construction matters more than conditions. A piece built with floating panels and fixings that allow movement rides out a dry winter. A piece glued rigidly across the grain cracks in its first heating season regardless of what you do.

The Side Shelf is solid oiled ash in three planes, shipped flat and assembled with one key, which means the fixings do the accommodating rather than the timber.

The things that cause real trouble

• Direct radiator heat. A piece standing against a hot radiator dries locally on one side, so it moves unevenly and cups. The simplest fix in this whole subject is moving it away.
• Underfloor heating with a piece sitting flat on the floor, which traps heat underneath and dries the base faster than the top.
• Wood burners, which produce the driest indoor air most homes ever see.
• Fast swings, an unheated holiday flat brought up to temperature in a day. Gradual change is manageable, rapid change is not.
• Direct sun through glass, which combines heat with UV and does both kinds of damage at once. Sun fading covers the other half.

What to actually do

1. Buy a hygrometer. They cost almost nothing and turn guessing into a number. Aim for roughly 40 to 60 percent.
2. Move pieces off radiators and out of direct beams of sun. This is free and does most of the work.
3. If the reading is persistently low, a humidifier in the room helps. Wood and people want the same range, so you are not doing this only for the furniture.
4. Avoid shocks. Bring the temperature of an unheated space up over days rather than hours.
5. Keep the finish maintained. An oil finish slows moisture exchange. It does not stop it, but it damps the speed of the swing, which is what actually causes checking. Re-oiling schedule has the timing.

A delivery in winter

A piece that has travelled in a cold lorry arrives at a different moisture content from the room it is entering. Give it time in the room before judging anything, and resist tightening fixings in the first fortnight, because the piece is still settling. What to check when furniture arrives covers the inspection that does matter on day one.

What does not move

Steel and concrete are dimensionally stable in a room. The only place humidity matters for them is corrosion on exposed steel, which is a coating question rather than a climate one. In a mixed piece, the design has to let the timber move against the steel, and on well-built furniture that allowance is invisible until you look for it. Steel versus wood frames covers the junction.

Reality check

• Wood moves across the grain, every season, permanently.
• Creaking and seasonal gaps are normal. Splits mean movement was restrained.
• Radiators, wood burners and underfloor heat are the local causes.
• Measure with a hygrometer and aim for 40 to 60 percent.
• Steel and concrete do not move. Mixed pieces must allow the timber to.