How to protect cards from light and humidity
How to measure humidity and trust the reading, when silica gel helps, what graders and supply makers say their holders block, and how to handle curl and damp.
By the Rip.fan Editorial Team.
Published 8 min read
Protect cards from light by keeping them dark most of the time: a closed box or binder beats any holder’s UV label. Protect them from humidity by keeping the room steady and somewhere between the drier targets archives set and the mold line they warn about, then checking it with a hygrometer you have tested yourself. Control the room first, with a dehumidifier if it runs damp, and the container second, with a well-closed box and, only if needed, silica gel.
Our guide to storing trading cards covers sleeves, boxes and where in a house to put them.
What humidity should cards be kept at?
The sources do not agree on one number, partly because they are giving different kinds of figure. Cards are printed card stock, so Rip.fan treats the guidance for paper as the closest match.
| Source | Figure | What kind of figure it is |
|---|---|---|
| Library of Congress | About 35% relative humidity, at room temperature or below | A target for keeping paper long term |
| Canadian Conservation Institute (CCI) | 50% relative humidity, 15 to 25°C | The setpoint its table gives for general museums, libraries and archives |
| National Archives | Below 65% relative humidity, under 75°F | A ceiling, to prevent mold and cut insect activity |
| National Archives | Not below 15% relative humidity | A floor: drier air than that can make paper brittle |
| Canadian Conservation Institute | 75% relative humidity | The practical boundary for damp; readings held above it give mold what it needs |
Read together, the figures set a band. A steady reading in the 35% to 50% range sits at or near the targets, readings creeping toward 65% call for action, and anything held above 75% counts as damp. Paper and other organic materials take up and give off moisture as the air changes, and those small, repeated size changes can start or widen cracks, which is why swings matter. With cards, warped foils follow changing weather and temperatures.
How do you measure humidity you can trust?
With some skepticism. CCI says that measuring relative humidity precisely takes a meter that costs roughly $100 or more, plus the accessories to calibrate it, and calls the low-cost hygrometers sold in hardware stores, and desktop weather stations, unreliable. Its silica gel bulletin adds that a 5% shift in a hygrometer’s reading is common. On a meter that reads 60%, the room could be 65%.
Three checks cost almost nothing:
- Your nose and hands. CCI says that if a space or object feels damp or smells musty, it usually is.
- The salt-on-tape test. CCI describes sticking a few grains of table salt to clear tape and leaving it in a suspect spot. Salt turns to droplets at 75% relative humidity, and even if the air dries out again before you look, the grains will have lost their crystal shape.
- The salt box check. Conservation Physics, an independent conservation reference site, explains how to check a meter: put dry salt in a shallow tray, in a layer about 3 mm deep, inside an airtight box, add just enough water to make it look damp, and rest the meter above it on a grid. Allow one hour for a bulky instrument and 20 minutes for a small sensor. Its table puts the air over damp table salt (sodium chloride) at 75.3% to 75.7% between 5°C and 25°C, so a meter that settles far from 75% is off by about that much.
CCI’s monitoring guidance also separates two kinds of device. A spot-reading meter shows the air right now. A recording device, or data logger, logs many readings and catches changes that last only minutes as well as trends over months. Either way, CCI says to replace batteries or recharge regularly and to calibrate from time to time. A logger in the room where your cards live, checked against damp salt now and then, tells you far more than a dial glanced at once.
How do you bring humidity down, or keep it steady?
Start with the room. CCI calls portable household dehumidifiers and humidifiers cost-effective for small museums, with one warning: take care that the device itself does not become a source of water damage. It also says fixing the source of water, such as drainage or gutters, beats running a dehumidifier against it. The National Archives adds the placement rules: keep collections out of damp basements, hot attics and garages, away from pipes, windows and roof leaks, and on a shelf so they do not get wet.
Then the container. CCI’s silica gel bulletin explains when a box with gel helps, and its humidity guidance shows when it doesn’t need to:
- Airtightness comes first. CCI says articles on silica gel often do not stress enough how much the container’s airtightness decides the result. In its example, a book in an acrylic box that is visibly airtight at the base buffers itself and needs no extra gel.
- The amount depends on the box. The bulletin’s formula weighs the box’s volume and leakiness, the gap between the room’s humidity and the target, and how many days the target must hold. It suggests about 30% extra gel as a margin, and calls a box that leaks little the best way to need less gel.
- Dry gel gets used up. In CCI’s scenario of a box that keeps inorganic objects drier than the room, the dry gel has to be replaced once the humidity inside reaches the highest level allowed. CCI calls this manageable for a temporary display or transport, but time-consuming as a permanent setup, since without monitoring the humidity can drift to the wrong level.
- Too dry does harm too. CCI says very dry air makes many organic materials lose flexibility, and the National Archives sets 15% as the floor for paper.
So a few gel packets in a leaky shoe box do little for long, while a well-closed tote in a steady room may need none. If you use gel, put a small hygrometer in the box with it. And a storage unit sold as climate controlled may control only temperature: CubeSmart’s climate-controlled units, for one, are not humidity controlled. Our guide to moving a card collection compares what storage firms promise.
What do holders block, and what don’t they?
Each maker describes its holder’s protection differently, and none of them claims to make light or damp harmless.
| Holder | What its maker says about light | What it says about moisture |
|---|---|---|
| TAG slab | Acrylic rated about 99% UV resistant, blocking most UV below 400 nanometers | Sealed by ultrasonic welding to guard against dust, handling and general weather, but not hermetically sealed |
| SGC holder | Not UV-resistant | Not waterproof |
| CGC Cards holder | Archival materials and clear optics; no claim that the holder blocks UV light | No moisture claim |
| Ultra PRO products marked UV Protection | Up to 99.9% of UV-C and UV-B blocked, some UV-A, tapering off into visible light | Not addressed |
The table explains why a UV claim is not the end of the matter. CCI says UV usually causes less than half of the fading of light-sensitive colors, and often only a tenth, so a filter does not change the need to cut light exposure overall. It also calls fading utterly irreversible. Ultra PRO itself advises keeping UV-protected products out of direct sunlight, both for the UV that still gets through and for the heat. The Library of Congress asks for minimal exposure to all kinds of light and none to direct or intense light. For how fast colors fade at different light levels, see our guide to storing graded cards, and for frames and cases, displaying trading cards.
What do the warning signs mean, and what helps?
| What you see | Likely cause | What to know |
|---|---|---|
| A foil card bowing in its sleeve | Weather and temperature changes; foils also warp on their own | It usually flattens after a couple of days pressed in a heavy hardcover book under more books; TCGplayer counts a center lift of more than 5mm as Damaged |
| Faded colors or yellowing | Light, adding up hour by hour | Fading cannot be reversed; TCGplayer treats fading or yellowing from extreme or prolonged sun as damage |
| A musty smell or fuzzy spots | Damp, often with still air and darkness | Active mold looks fuzzy or slimy and should not be removed, since that may only smear it around; TCGplayer treats mold, like other foreign substances, as damage |
| Damp at one end of a box | A box against a cold wall or floor | The colder end of a package gathers damp; leave an air gap of 10 cm or more from outside walls and cold floors |
| A warped slab, toploader or case | Heat, such as sun or a car | Many common plastics, with heat distortion temperatures in the 65 to 90°C range, distort quickly and permanently at those temperatures |
| Droplets on your salt test tape | Humidity at or above 75% | Dry the room before anything else |
For mold on a valuable card, the National Archives’ flood salvage guidance, written in August 1993, says wet, moldy items that cannot be dried at once can be frozen, which does not kill mold but puts it in a dormant state, and that items of high value should be treated only with a conservator’s advice. Our guide to card condition problems shows how each kind of damage affects a card’s grade, and the glossary explains card condition and the NM grade.
What does a good setup look like?
- Keep cards in a lived-in room, on a shelf, off outside walls and away from windows and vents.
- Put a recording hygrometer beside them, and check it against damp salt from time to time.
- Keep the room steady, aiming low in the band and never letting it sit above 65%.
- Keep cards closed away from light, and treat UV labels as an extra, not a shield.
- Look for curl, smells and droplets on a salt strip every few weeks, and act early.
Frequently asked questions
How long does mold take to appear in damp storage?
Anywhere from months to days. On mold-prone surfaces such as starched or dirty paper, visible mold takes about 100 days at a steady 70% relative humidity and only about 2 days at 90%, so the last few points of damp matter most.
Does a damp room that dries out now and then still grow mold?
Less readily. A space that sits near 70% but drops into the safe range every few days resets the mold growth clock to zero, which makes it safer than one held steady at 70%.
Does ordinary window glass block UV light?
Only partly. Window glass filters shortwave UV, but not enough for museum use, which is why laminated glass with a UV filter layer, or a self-adhesive UV film, is fitted where windows light a collection.
Does a dry climate keep indoor humidity low?
Not by itself. Even in a dry climate such as the foothills of Alberta, relative humidity on most days sits between 40% and 70%, and very low readings indoors mostly come from winter heating.