Canvas, Damp and Humidity, from the VanArtHub custom framing guides

Canvas, Stretching & Care · Care & repair

Canvas, Damp and Humidity

What a wet climate does to a painting on fabric, which rooms are worst, and how a sealed back changes it.

Humidity is the main thing that moves a painting on canvas, because both the fabric and the wooden bars take up moisture from the air and release it again, at different speeds. VanArtHub frames and stretches in Vancouver, where the wet season runs most of the winter and indoor heating then dries the air out, so the swing is larger here than in most places a painting is likely to live. The result shows up as a canvas that is slack in one season and taut in another.

What actually happens to the painting

Canvas is woven fabric. When the air is damp the fibres take up moisture, swell slightly across their width, and the weave shortens, which pulls the canvas tighter. When the air dries the fibres shrink and the fabric lengthens and goes slack. The wooden bars do the same thing more slowly and in their own direction.

One cycle does nothing. Repeated over years, the paint layer, which is far less flexible than the fabric under it, is asked to expand and contract more than it can. That is the origin of the fine web of cracking you see on older paintings, and it is why the same painting ages differently in a damp flat and in a dry one.

The rooms where a canvas should not hang

  • A bathroom. The swing between a hot shower and the rest of the day is the sharpest in the house.
  • Directly above a radiator or a heating vent. A rising column of dry warm air across the back of the canvas, every day of winter.
  • An unheated room that is heated only sometimes. A spare room brought up to temperature for visitors and left cold otherwise is worse than one that is consistently cold.
  • Against an uninsulated exterior wall, especially a north-facing one. The wall stays colder than the room and moisture condenses in the gap behind the painting.
  • A basement suite without dehumidification. Consistently high humidity is less damaging than a swing, but it brings mould, and mould on the back of a canvas is very difficult to deal with.

What helps, in order of how much difference it makes

  1. Move it. An interior wall in a room with steady temperature solves most of the problem for free.
  2. Hold it off the wall. Small spacers behind the bottom corners let air move behind the painting instead of trapping it. This is the cheapest real improvement you can make.
  3. Seal the back. A backing board across the back of the stretcher slows the rate at which the canvas sees a change in humidity. It does not stop it, it damps it, and damping the swing is most of the benefit.
  4. Deal with the room. A dehumidifier in a damp flat protects everything in it, paintings included.

What not to do

Do not key a canvas tighter every time it goes slack. If it is responding to the season it will come back on its own, and keying it at its slackest leaves it over-tensioned when the humidity returns. Two or three rounds of that and the piece is permanently too tight.

Related

See why a canvas goes slack for diagnosis, framing art for kitchens and bathrooms for the sealed-package approach, and canvas stretching in Vancouver.

Unsealed moist air gets in, then condenses on the glass Sealed, with a spacer stopped outside, and the art never touches glass
In a room that swings between wet and dry, moist air moves in and out of an unsealed frame and condenses on the cold inner face of the glazing. Sealing the package and holding the glazing off the surface with a spacer keeps the swing outside, where it cannot reach the work.

Common questions

Does humidity really affect a painting on canvas?

Yes, and it is the main thing that moves one. Canvas is woven fabric, so damp air makes the fibres swell and the weave tighten, and dry air makes it slacken. The wooden bars do the same more slowly. A wet winter followed by dry indoor heating drives that cycle hard.

Why does repeated humidity change cause cracking?

Because the paint layer is far less flexible than the fabric underneath it. One cycle does nothing. Repeated over years, the paint is asked to expand and contract more than it can, and the result is the fine web of cracking seen on older paintings.

Which rooms should a canvas not hang in?

Bathrooms, directly above a radiator or heating vent, a spare room that is heated only occasionally, against an uninsulated exterior wall, and an undehumidified basement. The common factor in the first four is a large swing rather than a high level.

Is steady high humidity worse than a swing?

No, a swing is worse for the paint layer. Steady high humidity brings a different problem, which is mould, and mould on the back of a canvas is very difficult to deal with. Neither is good, and they need different fixes.

What is the cheapest thing that actually helps?

Small spacers behind the bottom corners, so the painting sits a few millimetres off the wall and air can move behind it instead of being trapped. It costs almost nothing and it is the most effective single change after moving the piece.

Does sealing the back of a canvas help?

Yes, by damping the swing rather than stopping it. A backing board across the back of the stretcher slows how quickly the canvas sees a change in the air, and slowing the change is most of the benefit, because it is the speed and repetition that does the damage.

Should I key the canvas every time it goes slack?

No. If it is responding to the season it will come back on its own, and keying it at its slackest leaves it over-tensioned when the humidity returns. Two or three rounds of that and the painting is permanently too tight.

Is an interior wall really better than an exterior one?

Yes, and noticeably so. An uninsulated exterior wall stays colder than the room, so moisture condenses in the gap behind the painting. An interior wall in a room with a steady temperature solves most of the problem and costs nothing.

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