Safe places to hang a framed print: sun, damp, heat and insects

This page is a room-by-room check for where a framed paper print can hang safely in a home. It covers four risks from the Canadian Conservation Institute's basic care guide for works on paper (light, humidity, heat and insects) and gives the numbers behind each one: the humidity to aim for, how temperature changes the life of paper, and how a change of a few degrees changes the air around the frame. It ends with a table of rooms and a seasonal check. The main framing and hanging guide explains how the frame itself protects the print, and the page on how to light a framed portrait at home covers lamps and light levels.

Humidity: the number to watch

Relative humidity (RH) measures how much water the air holds compared with the most it could hold at that temperature. It is the number shown on a home humidity meter.

Health Canada's guide to moisture and mould advises keeping indoor relative humidity between 30% and 50%, and running a dehumidifier when it goes above 50% or when water condenses on cold windows. The Canadian Conservation Institute gives the conservation view. It calls air above 75% RH damp, and says damp causes mould, fast corrosion of metal and physical damage. At 60% RH, it says, visible mould is just possible on some surfaces, and a period below 55% stops the growth.

For a paper print, the Institute's basic care page describes what damp does. Paper absorbs moisture quickly. High humidity makes it swell and expand, so a framed print can look wavy or, in the worst cases, wrinkled. Long periods of high humidity speed up chemical decay and invite mould, and foxing (small reddish-brown spots) may become more pronounced. Keeping a room inside Health Canada's range protects the family and the print at the same time.

Temperature: slower ageing in cooler air

Heat speeds up the chemical reactions that yellow and weaken paper. The Institute's guide to temperature gives a rule of thumb: each reduction of 5 °C doubles the lifetime of an object. Its guidelines for paintings recommend a room temperature anywhere in the range of 16 to 25 °C.

Temperature and humidity move together. The Institute says that in a closed room at 20 °C and 50% RH, a change of 1 °C shifts the humidity by 3%, and a change of 5 °C shifts it by 15%. A temperature drop of more than 10 °C brings the air to 100% RH and water condenses on surfaces. This is why cold spots in a house, such as the inside of an outside wall in winter, collect moisture.

It also explains winter dryness. The Institute says heating cold air lowers its RH, and that in many areas of Canada indoor humidity can fall to 5% by December when a building is heated to 21 °C without a humidifier. The Institute's examples of damage from dry winter air are mostly wooden objects, such as barrels and wagon wheels, while paper-bound books came through well. For a framed print, the wooden frame is the part to watch. Its guidelines for paintings add that lowering the thermostat from 22 to 18 °C can raise the RH in an unhumidified room by up to 12%.

Sunbeams and frames

Direct sun is the worst place of all. The Institute's guide to temperature calls direct sunlight the single most damaging source of incorrect temperature. It says dark surfaces facing the sun can quickly reach 40 °C above the air around them, that surfaces enclosed by glass, as in picture frames, can get even hotter, and that a sunbeam striking an object regularly can make its decay run 100 times faster. It adds a point specific to framed paper: sheets in glazed and backed frames are especially vulnerable, because the frame that normally protects them also keeps in the moisture needed for fast decay.

So check a wall for sun before you put in a hook. Look at it in the morning, at midday and in the late afternoon, and in both summer and winter, because the sun's path changes with the season.

Heat sources, cold walls and cooking

The Institute's guide to storage and display of paintings, which its framing guide says applies to framed works on paper, names the places to avoid:

• Near radiators, air vents or fireplaces, and in draughty spots. Its guide to humidity adds that sources of heat, such as sunny windows and perimeter heaters, cause low RH close to them.
• Kitchens and cafeterias, and anywhere with cooking fumes or food.
• The inside of an outside wall, which gets cold in winter and can collect condensation. If you must use one, the Institute advises a backing board on the frame and cork or plastic spacers on the back, so air can move between the frame and the wall.

Health Canada's guide agrees on outside walls from a different angle. To stop condensation and mould, it advises keeping beds, bedding and furniture away from outside walls for good airflow, and keeping heating vents clear of furniture. The same reasoning applies to a frame hung flat against a cold outside wall.

Basements and attics

The Institute's basic care page advises against displaying or storing works of art on paper in basements or attics, where temperature and humidity change widely. Its guide to humidity explains each one. Basements, and ground floors of buildings without one, are commonly damp. Poorly ventilated attics get hot under the summer sun, which drives the humidity down. Insects also prefer cool, dark, moist places, which is another reason to keep prints out of basements.

If you want to use a finished basement room, put a humidity meter on the wall for a few weeks in summer before you hang a print there, and keep it only if the reading stays within Health Canada's 30% to 50%.

Dust and polluted air

The Institute's basic care page says paper is absorbent and takes in any liquid or gas around it. Air pollutants are a source of acid, which harms paper and pigments, and acid can also move into a print from poor-quality framing materials. A frame helps here. The Institute's guide to framing works on paper advises sealing the back of the frame with tape across the point where the backing board meets the frame, to keep out dust and polluted air, and says gummed linen tape is better than brown kraft tape because it is more stable. When you take a frame down for its yearly check, look at that seal. If the tape has lifted, ask a framer to replace it.

Insects

The Institute's guide to caring for paper objects says the most common insect enemies of paper are silverfish, cockroaches, booklice and bookworms, which are the larvae of many kinds of beetle. They are attracted to paper and also to glues, sizes and some media. The basic care page says an abraded, scuffed-looking surface is a sign of silverfish, and that squashed insect bodies are sometimes found on the paper or inside the frame. Booklice feed mainly on mould, and silverfish are also a sign of high humidity, so insects often point back to a damp problem.

The Institute's answer is good housekeeping: regular cleaning, because dust carries mould spores and food for pests, and regular inspection. Once or twice a year, take the frame down, look at the back and the edges of the glazing, and wipe the wall behind it.

Room by room

Pets add one more risk. For a home with cats, the Purrspectives article on cat wall art ideas covers where to hang a print out of reach of claws.

A seasonal check

1. Spring and summer: watch the humidity on muggy days and after showers. Above 50%, run a dehumidifier or open the bathroom fan, as Health Canada advises.
2. Autumn: check whether the lower sun now reaches the wall.
3. Winter: feel the wall behind the frame. If it is cold, move the frame to an inside wall or add spacers. Look for condensation on nearby windows, which Health Canada treats as a sign of high indoor humidity.
4. Once a year: take the frame down, check the back for insects or mould, and dust the glazing.

If you ever see mould on a print, the Institute's advice is to consult a paper conservator, because mould is harmful to the artwork and to people. Choosing the portrait comes before all of this; the gallery shows the finished styles, and the page on picture hanging hardware covers fixing the frame safely to the wall you choose.