Where a picture can hang: light, humidity and the walls to avoid
A wall is a set of conditions, and some walls cannot be made safe by any frame. This chapter rates every candidate wall for light, humidity and heat, measures the light, and turns a reading into a yearly exposure you can compare with a published budget.
- Start here: the essentials
- The walls to avoid, and why
- What the published light figures say
- Measuring the light
- The exposure budget
- Rotation: months on, years off
- Treating the window instead of the picture
- Lighting the picture
- Temperature and humidity in a house
- See it
- Practice
- Your pictures
- Check your work
- Knowledge check and scenario
Start here Essentials
At minimum this chapter asks you to take one light reading and rule out the walls that cannot be made safe. Everything marked Go deeper is optional on a first pass.
- Rule out the walls that no frame can fix. Write down which and why (The walls to avoid).
- Measure the light on each remaining wall at its brightest hour. In lux or footcandles (Measuring the light).
- Work out the yearly exposure. Compare it with the published annual budget for the medium (the exposure box).
- Write the rating for each wall in the hanging plan. Include the reading and the hours (Hanging plan).
Minimum time: about 30 minutes with a meter. Done when: every candidate wall has a row in the hanging plan with a light reading, a humidity note and a verdict in your own words.
1. The walls to avoid, and why Understand
The list below is remarkably consistent across publishers, and what makes it useful is the mechanism beside each entry rather than the entry itself.
| Place | Mechanism | Who says so |
|---|---|---|
| Exterior walls | Warm humid indoor air meets a cold surface, so relative humidity climbs locally and condensation becomes possible. A conservation institute notes that a building's perimeter walls can experience fluctuations in temperature and humidity and can be a source of moisture | A preservation center, a national library, a conservation institute, a professional wiki |
| Bathrooms and kitchens | Repeated humidity spikes, and condensation on cold glazing. A professional wiki names bathrooms, poolhouses, gazebos and waterfront homes as damp environments that framing cannot permanently protect against | The same four |
| Above a fireplace, radiator or vent | Heat and dry air, plus soot and airborne particles. A conservation institute warns against walls above or beside fireplaces, radiators and heating vents | A national library, a preservation center, a conservation institute |
| Any wall direct sun reaches | Two separate agents at once: light dose, and heat. A conservation institute calls direct sunlight the single most damaging source of incorrect temperature, reports that dark surfaces facing the sun can quickly reach 40 degrees Celsius above the air temperature and 75 degrees on a warm summer day, and adds that where such surfaces are enclosed by glass, as in display cases and picture frames, even higher temperatures are possible | Every publisher in the research |
| Attics | Extremes of heat and dryness | A preservation center, a conservation institute |
| Basements and garages | Damp, and vulnerability to flooding from ground water or overhead pipes | The same |
| Near an air intake, or near cooking | Airborne dust and greasy contaminants. A conservation institute names air intakes and wood fires or food preparation areas for unglazed paintings | A conservation institute |
The honest framing of this list comes from the professional wiki, which says plainly what framing can and cannot do: matting and framing may temporarily mitigate the effects of uncontrolled environments but cannot permanently protect objects hung on exterior walls subject to condensation when indoor and outdoor temperatures differ greatly, light-sensitive objects hung in unfiltered light, or objects in damp environments. Physical principle
Mold is the figure that makes damp concrete. A federal leaflet gives the household version: spore germination is less likely if relative humidity is controlled between 45 and 55 percent, humidity should always be kept below 65 percent, and above 65 percent a dehumidifier is necessary, with a target temperature of 18 to 20 degrees Celsius. A conservation institute gives it as time to growth, which is more useful for judging a room: on organic materials with a surface rich in soluble protein, starch or sugar, visible mold takes about 100 days at 70 percent relative humidity, about 10 days at 80 percent, and about 2 days at 90 to 100 percent, at around 25 degrees. Clean plant-based materials such as clean paper, textiles and wood typically need 80 to 85 percent before growth is likely at all, and any intermittent period below 55 percent stops growth.
That is why a bathroom is a bad idea and why a steamy bathroom is a worse one than a damp cellar at the same average reading: it is the spikes that count.
2. What the published light figures say
There is a ladder of recommended maximum light levels, it appears in near-identical wording across several publishers, and it is worth seeing as one inherited convention copied forward rather than as several independent findings.
| Material | Maximum | Published by |
|---|---|---|
| Sensitive: works on paper, photographs, textiles | 50 lux (5 footcandles) | A conservation center, a state agency, a museum's duration guidelines, a state museum body |
| Sensitive, as a band | 50 to 100 lux | A conservation center, as its standard recommendation |
| Most collection material, with a duration | 50 to 150 lux for three to four months, with no fading expected | A preservation leaflet |
| Less sensitive: oil paintings, wood, leather | 150 lux (15 footcandles) | A conservation center, a state agency, a preservation leaflet |
| Moderately sensitive, competing figure | 200 lux | An Australian state museum body, reprinted by a US state agency |
| Least sensitive: metal, ceramics, stone, glass | 300 lux (30 footcandles) | A conservation center, a state agency, a museum's duration guidelines |
| Stone, indoors | 1000 lux | The Australian body |
| In footcandles instead | 3 footcandles maximum for very light sensitive; 5 to 10 for moderately sensitive | A national library |
Two things to notice. First, the 150 against 200 lux difference for moderately sensitive material is a real conflict between the North American and Australian conventions, and the manual shows both rather than averaging them. The US state agency that prints 200 is reprinting the Australian text, so it is not independent corroboration.
Second, only one source explains where 50 lux came from. A conservation institute describes it as a visibility benchmark, the light at which a 25-year-old viewer can see adequately, adjustable upward by up to three times each for dark surfaces, low contrast, very fine or time-limited tasks, and older viewers, which multiplies out to about 4,000 lux at the extreme. The same institute says plainly that a single lux ceiling is the wrong tool, which is why this chapter moves on to dose.
For scale, from the same source: a candle at a metre is 1 lux; the traditional categories are 50, 150 and 300; an office is 500; a window or a study lamp about 5,000; average daylight 30,000; noon sun about 100,000. A preservation leaflet gives the household anchor: 50 lux is similar to a home living room in the evening.
3. Measuring the light
You cannot manage this without a number, and the number is cheap to get.
What to buy. A national museum development body gives the buying specification: you will likely want an instrument that is most accurate within the 50 to 200 lux range, which is the band that matters for display. It also gives two habits: each time you use a meter, cover the cell completely with your hand and check the reading is zero, and send light and ultraviolet meters for a calibration check every couple of years.
A phone app. A conservation center says in print that less accurate light measurements can be taken with a smartphone app. That is a reason to use one for comparing walls, which is mostly what a household needs, rather than for deciding whether a number clears a published ceiling.
Where to hold it. The same center says readings should be taken slightly in front of the actual object. The museum body adds: place the meter so nothing is blocking or shadowing it, and so it points in the same direction as the object faces.
When to read. At the brightest hour, not an average one, and again at the hour you actually look at the picture. Those are two different numbers and both are useful: the first tells you the dose, the second tells you whether you can see it.
Ultraviolet is not something you can measure usefully at home. The published limit is 75 microwatts per lumen from five bodies, and 50 from the federal agency that alone shows its derivation: in the late 1970s the standard was based on the ultraviolet output of an incandescent bulb, between 40 and 70 microwatts per lumen, so the acceptable level was set at 50. The museum body notes that some ultraviolet meters struggle to measure accurately at the low light levels typical of displays. Treat ultraviolet as a glazing choice rather than a measurement, and use the glazing chapter for what the choice buys.
One cheap at-home dosimeter does exist: a Blue Wool card, a set of blue-dyed wool samples of known fastness that fade at known rates, which the museum body describes as the best known dosimeter system. A conservation center adds the caution that goes with it: the most sensitive blue wool swatch is still more lightfast than many common colorants, so it should not be used as the only indicator. Test it
4. The exposure budget
This is the part that turns a reading into a decision, and the arithmetic is simple enough to do in your head.
Dose is level multiplied by time. A conservation center states it and works it twice: 100,000 lux for one hour is the same dose as 50 lux for 2,000 hours, so exhibiting an object for one hour in direct sunlight is equivalent to exhibiting it for 200 hours at 50 lux. The conservation institute's own light damage calculator publishes the same formula: illuminance times hours a day times days a year times years. Physical principle
The annual budget. A preservation leaflet gives it with a worked example: an object lit for 10 hours a day at 50 lux for 100 days has a dose of 50,000 lux hours, and ideally light-sensitive materials would have an annual exposure of only 50,000 lux hours, regardless of whether they will be displayed annually. A professional group's exhibition guidelines give the same figure in imperial units, 5,000 footcandle hours a year for very light-sensitive material with a minimum of three years of rest between display cycles, and 10,000 footcandle hours a year for moderately light-sensitive material with two years of rest.
What that means on an ordinary wall. Put your own reading into the exposure box. A wall at 450 lux lit six hours a day all year spends about 985,000 lux hours, which is close to twenty times the published annual budget for light-sensitive material. That is not a verdict on the picture; it is a statement about the wall, and it is the number that makes "it is a bit bright in there" into something you can act on.
The same box prints the fade doses with and without ultraviolet filtering, from the conservation institute's published table, so you can see what the glazing upgrade buys at each end of the sensitivity range. What the ultraviolet claim covers has that table in full.
Every lux figure in this chapter is a recommendation published for institutional collections on exhibition. This manual uses them because they are the best figures anyone publishes, and a house is not a gallery. If your only wall is a bright one, nothing here says the picture is ruined. It says what the yearly exposure is, what the published budget says that costs, and what rotation or a copy on the wall would buy instead. Those are the three things you can actually do.
5. Rotation: months on, years off
If a wall is brighter than the budget allows, the published answer is not a better frame. It is less time.
A museum publishes a duration table by material category, and it is the most explicit rotation guidance found in this research. For works of art: category A, the most fugitive pigments and dyes and highly degraded paper and silk, three months of display in every five years at 50 lux; category B, watercolors, organic pigments and dyes, pastels and textiles, six months in five years at 50 lux; category C, light-stable pigments or carbon ink, pencil and tempera, twelve months in five years at 50 to 100 lux; category D, ivory, bone, wood and undyed leather, twenty-four months at 50 to 150 lux; category E, oil paintings, stone, metals, glass and ceramics, unlimited display at 300 lux. A parallel table does the same for photographs by process.
Two of its own rules matter as much as the numbers. The light category for an object is determined by its most sensitive material component. And anything hand-colored, with a tinted base, on colored paper or mount, or with other components, moves to the most restrictive category.
A national library gives the same shape in footcandles with rest periods attached: very light sensitive at a maximum of 3 footcandles and only while being viewed, for three to five months, then a minimum of three years of dark storage; moderately light sensitive at 5 to 10 footcandles for six to nine months, then two years dark. A conservation center gives three months every five years for sensitive collections at 50 lux maximum, and three months every two years for less sensitive at 150. A state museum body puts it as a display fraction: an item may be on display three months per year, and if it is displayed for a year it should not be displayed for the next three.
The rotation box takes a category, your measured level and a start date, and returns the date to take the piece down, scaling the published months by reciprocity when your wall is brighter than the table assumes. That scaling is this manual's arithmetic on their figures, and the box says so.
And the sentence underneath all of it, from a preservation leaflet: no paper, wood, leather, textile or other organic object should be on permanent display. A national library's framing guidance says the same, and names the alternative: store the original well and frame a well-made reproduction.
6. Treating the window instead of the picture
Often the cheapest intervention is not on the picture.
Ultraviolet-filtering window film is well covered and actually measured. A federal leaflet measured thirty commercial films and publishes the table, and explains why a product label misleads: most product literature states 98 percent of ultraviolet filtered, but most films filter 98 percent of ultraviolet in the 325 to 380 nanometre range, which for a museum is not enough, because very few films filter the complete ultraviolet spectrum. A conservation institute reports neutral-density ultraviolet window films at 96 percent plus or minus 2 across 300 to 400 nanometres, from independent testing, and notes that films were found up to 5 percent less efficient than claimed, with one filter 15 percent less efficient and one tube reducing ultraviolet by only 32 percent.
Service life splits four ways across the publishers, which is worth knowing before anyone quotes you one: 8 to 15 years from the federal leaflet, about 10 years from a conservation center, 10 to 15 years indoors from a conservation institute, and 15 to 20 years from a preservation leaflet. Nobody is wrong; nobody agrees. Whatever you install, write the date on the window frame.
Curtains and blinds are the obvious household answer and no conservation figure for them was found in this research. That is a gap, not a verdict: reducing the hours the sun is on the wall reduces the dose by exactly the proportion you reduce them, which is arithmetic you can do from the exposure box. A conservation institute's own worked example mentions a window screen reducing light by about one half.
7. Lighting the picture
Lighting is a room subject, and Learn Interior Lighting's chapter on layers and glare covers choosing and aiming a fixture properly, along with working with an electrician. What belongs here is the handful of figures specific to lighting art, and one claim this manual will not repeat.
The figures reach this manual through a public agency design standard that quotes a lighting society's museum recommended practice, because that recommended practice is paywalled; the manual states that route rather than pretending to cite the standard. From it: the industry standard aiming angle is 30 to 35 degrees to the artwork, to prevent glare and veiling reflections on glass; track is located to give a nominal 45 degree angle to the artwork, with lower angles giving less uniform light and higher angles more glare; light levels on artwork walls at five to ten times the average ambient for feature emphasis; 60 to 80 footcandles average and 140 maximum; uniformity across an individual artwork of two to one, average to minimum; dimmable LEDs at 3000 to 3500 kelvin, a color rendering index of 85 or better, and a rated life over 50,000 hours. Convention
Note the tension: 60 to 80 footcandles is roughly 650 to 860 lux, which is far above the conservation ceilings in section 2. That is a public building lighting a lobby, not a house preserving a watercolor, and the gap between the two numbers is the whole argument of this chapter. If you light a sensitive piece to a gallery's display standard you have spent its budget faster, and the exposure box will tell you how much faster.
One claim this manual does not repeat. A preservation leaflet states that LEDs do not emit ultraviolet or infrared radiation and that the light does not generate heat. A conservation institute's own table contradicts the first half: white LEDs are given at 0 to 75 microwatts per lumen of ultraviolet, which means some LEDs are as high as a bare incandescent bulb. The honest version is the one a pair of conservation institutes use in their solid-state lighting guidelines: LEDs have little or no infrared radiation, and their ultraviolet output varies by product. Ask for the figure, or use a filter.
Two more, from a conservation institute's framing note and its textile note, both saying the same thing: do not attach light fixtures to frames, because such lights produce excessive light levels, tend to create a hot spot on the picture, and contribute to the fading of dyes and pigments.
Go deeper 8. Temperature and humidity in a house
Light is the agent households can measure and act on most easily. Temperature and humidity are two more, and three figures make them concrete.
Each 5 degrees Celsius of reduction roughly doubles an object's lifetime. A conservation institute gives it as a practical rule of thumb and says that given a chosen criterion there is no doubt the rule holds. Its own table shows the consequence: material of medium sensitivity lasting around 150 years at 25 degrees and around 1,500 at 10.
Temperature and humidity are coupled. In a closed room at 20 degrees and 50 percent relative humidity, the same institute gives a 1 degree fluctuation as about 3 percent relative humidity, 5 degrees as about 15 percent, and says the most dangerous fluctuation, a temperature drop of over 10 degrees, will cause 100 percent relative humidity and condensation. That is the number behind condensation on cold glazing, and it is why an exterior wall in winter is a humidity problem rather than a temperature one.
A history of fluctuation matters more than a single reading. The institute's fluctuation table grades swings of plus or minus 40, 20, 10 and 5 percent relative humidity from severe damage to none, depending on the material's sensitivity, and its author notes elsewhere that collections can acclimatize to an average annual humidity other than the conventional 50 percent. For a household that means: a house that sits at 45 percent all year is better than one that swings between 30 and 65, even if the second one averages better.
Target figures, for reference. A preservation leaflet gives a general set point of 64 to 72 degrees Fahrenheit, not exceeding 75, with paper, books and textiles in combined-use areas at 68 plus or minus 4 degrees and 30 to 60 percent relative humidity, and paintings in storage at 68 plus or minus 4 and 45 to 55 percent. A national library asks for 30 to 50 percent for photographs at 70 degrees or below, with daily fluctuation under 5 percent, and calls relative humidity the single most important factor in preserving most photographic prints. A professional wiki recommends about 70 degrees and 45 to 55 percent for paper objects.
None of that is achievable in most houses, and saying so is part of being honest about it. What is achievable: a cheap hygrometer in the room, a dehumidifier when it reads above 65 percent, and not hanging anything irreplaceable on the walls in the first table.
9. See it See it
The Okonkwos measure their sofa wall. Nia and Marcus want fourteen photographs on the wall above the sofa, in a room with a west window. Known: the meter reads 450 lux on the wall at four in the afternoon in July and about 60 lux at the same spot in the morning; the sun is on the left third of the wall for roughly six hours. Assumed: that six hours a day for 365 days is a fair annual figure, which overstates winter and understates midsummer. Open: which of their prints are at the fugitive end, which nobody can settle without testing. The exposure box returns about 985,000 lux hours a year, close to twenty times the published 50,000 lux hour budget for light-sensitive material. Under these assumptions, the most fugitive colorants would reach a just-noticeable fade in a few months at that exposure and the most stable in over a century, which is a range rather than an answer. Their decision: the eleven reprintable prints go up, the three irreplaceable ones get scanned, and the scans go on the sofa wall while the originals go flat in a box. Nothing about the frames changed.
- Protection from light damage, a preservation center, updated 2024: the clearest statement of the annual lux-hour budget, with the arithmetic worked out.
- Light duration guidelines, a museum's conservation department: the rotation table by material category, for works of art and for photographs, with its own category rules.
- Light exposure for artifacts on exhibition, a conservation center: the lux-hour arithmetic worked twice, the three-category table, and the remark about smartphone apps.
- Monitoring light and UV radiation, a national museum development body: what to look for in a meter, how often to recalibrate, and the dosimeter principle.
10. Practice Practice
Minimum: one wall measured at its brightest hour and rated in the hanging plan. Time: about 40 minutes, spread across a day. Materials: a lux meter or a phone app, a notebook, and a hygrometer if you have one.
- Walk the house and rule out. Write down every wall you were considering, and strike through the ones in the first table with the reason beside each.
- Measure each remaining wall twice. Once at the brightest hour and once at the hour you will usually look at the picture. Hold the meter slightly in front of where the piece will be, facing the way the piece will face.
- Count the hours. How many hours a day is that wall lit, by daylight and by lamps together? An estimate to the nearest hour is enough.
- Run the exposure box for your most sensitive piece. Write the yearly figure and the share of the budget into the hanging plan.
- Leave a hygrometer in the room for a month (Go deeper; optional). Note the high and the low. A swing matters more than an average, and a reading above 65 percent is the one that needs acting on.
11. Your pictures Your pictures
12. Check your work Check your work
Criteria
- Every candidate wall has a row, including the ones you ruled out, with the reason.
- Each remaining wall has a light reading, the hour it was taken, and the hours a day it is lit.
- The yearly exposure is written down for at least the most sensitive piece.
- Where the exposure is over budget, there is a decision written beside it: rotate, move, copy, or accept with eyes open.
- Nothing irreplaceable is going on a wall in the first table.
Worked example
Hal Brewer (fictional) has one wall in mind for his grandmother's watercolor: the one with the south window, on an outside wall. Both of those are in the first table. His reading at two in the afternoon is 1,200 lux. He does not need the exposure box to know the answer, but he runs it anyway and gets about 1.75 million lux hours a year at four hours a day, which is thirty-five times the budget for a watercolor. The watercolor goes on an interior wall across the room that reads 90 lux at its brightest, which is about 131,000 lux hours a year: still over the 50,000 budget, so he notes a rotation date. The south wall gets a reprintable poster, and he writes both decisions down with the readings beside them.
Common mistakes
- Measuring at the wrong hour. The average is not the problem; the brightest hour is.
- Treating ultraviolet filtering as the fix for a bright wall. Visible light fades things too, and on fugitive material removing ultraviolet buys very little.
- Judging damp by feel. A hygrometer costs less than a mat and gives you the number that the 65 percent threshold refers to.
- Lighting a sensitive piece to a gallery's display standard. The lighting figures in section 7 are for a public building, and they are an order of magnitude above the conservation ceilings.
- Accepting a wall because the alternative is a bare room. A copy on the bright wall and the original in a box is a real option, and it is the one the conservation sources keep arriving at.
What is still unresolved
- What the wall is made of and what it can hold: Hanging it.
- How a group fits on the wall you chose: Arranging pictures.
- When each sensitive piece comes down: Rotation.
- Choosing and aiming a fixture, and glare: Learn Interior Lighting.