Condensation and Mold in Closets: Why Humidity Rises and Guides by Location

Why humidity rises when air enters a cold place

The relative humidity (%) a hygrometer shows is the percentage of the most water vapor air can hold at that temperature. Air can hold less as temperature falls, so air with the same amount of vapor reaches a higher percentage (humidity) when it moves somewhere cold.

For example, room air at 20°C and 50% humidity contains about 8.6 g of water vapor per 1 m³. When this air enters a 12°C closet, where air can hold at most about 10.7 g, humidity rises to 83%. At 8°C, the air can hold less than 8.6 g, so humidity reaches 100% and the excess condenses on walls and bedding. That's why a closet can be thought of as window condensation without a window.

Humidity chart by location and temperature

These are calculated humidity levels when heated room air enters a cold spot. The left column gives the room's condition, and it gets colder to the right. Condensation occurs where it says 100%.

Location temperatures are guides that vary widely with outdoor temperature and the building's insulation. Leave a thermometer against the back wall of the closet for a few minutes to find your home's actual value. The table shows that just lowering room humidity from 50% to 40% drops the humidity of a 12°C closet from 83% to 67%, below the 70% mold threshold.

Humidity levels where mold and dust mites multiply

• Humidity above 60%: dust mites multiply more easily
• Humidity above 70%: mold grows more easily, and if it stays above 80%, it spreads quickly
• Humidity at 100% (condensation): moisture forms on walls and bedding, a breeding ground for mold until it dries

Mold grows without condensation droplets, simply when humidity near a surface stays high. Air hardly moves in closets and wardrobes, so once humidity rises it's slow to fall, and dust and skin oils on bedding and clothes feed the mold. "It got moldy though there were no droplets" is the result of this 70–99% condition lasting for days.

You can also calculate how much condenses. With the room at 20°C and 60%, when that air enters an 8°C spot, about 2.1 g per 1 m³ overflows and condenses. For one closet shelf level (about 1.5 m³), that's about 3 g. It looks small, but it repeats every time room air slowly replaces the air through gaps in the door, building up daily through the winter.

Why closets that store bedding get especially moldy

People sweat as they sleep, and about a glassful of moisture is said to pass into the bedding each night. Fold your futon straight away in the morning and put it in the closet, and you shut that moisture into a cold space. The closet's lower shelf and the back wall against the outer wall get especially cold, so moisture builds up behind the bedding and where it touches the wall, and mold appears.

1. 1

   Don't fold it as soon as you get up

   Turn back the covers and let the moisture escape for 30 minutes to an hour before putting it away.

2. 2

   Use slatted boards and keep it off the wall

   Lay slatted boards on the closet floor and lean them against the back wall so the bedding doesn't directly touch cold walls or floors. This creates an air path and prevents condensation at the contact points.

3. 3

   Upper shelf rather than lower, and don't stack too much

   The lower shelf near the floor gets cold and collects moisture, so put bedding you use on the upper shelf. Don't pack it tight; leave gaps for air to pass.

4. 4

   Air it on sunny days, or use a futon dryer

   Once or twice a week, drive out the moisture that's built up in the bedding.

Condensation in north-facing rooms and behind furniture

North-facing rooms get no sun and are often unheated, so they cool to around 10°C in winter. When warm, moist living room air flows in through gaps around the door, condensation starts on walls and windows and behind stored cardboard boxes. This is why wallpaper and belongings tend to get moldy in north-facing rooms used for storage.

• Keep the door to a north-facing room closed: don't let warm, moist air into a cold room
• Heat the north-facing room a little too, or let moisture out with an extractor fan or a slightly open window
• Keep dressers and bookcases at least 5 cm off outer walls: the air between wall and furniture stops moving, cools to around 13°C, and reaches nearly 80% humidity
• Put desiccant behind furniture and under the bed, and judge how damp it is by the water collected
• Cardboard absorbs moisture and feeds mold, so switch to plastic storage cases

Remedies, most effective first

Like windows, mold control for closets combines reducing the room's water vapor with raising the temperature of cold spots and moving the air. These are in order of greatest effect and lowest cost.

1. 1

   Keep the heated room's humidity at 40–50%

   When the room air's dew point falls, condensation is less likely wherever the air goes. Taking a 20°C room from 60% to 40% drops the humidity of a 12°C closet from 100% to 67%. Use Guide to Humidifier Output to avoid setting your humidifier too high.

2. 2

   Open closet doors regularly

   Opening the doors once a day, even for a few hours, lets warm room air in, raising the temperature inside and lowering the humidity. Keeping them closed and cold is the worst option.

3. 3

   Slatted boards, distance from walls, and no overpacking

   Create an air layer between cold walls or floors and your belongings. Aim to fill storage to about 80%.

4. 4

   Use desiccants and dehumidifiers

   Use standing desiccants in closets and a dehumidifier in north-facing rooms. Desiccant-type (zeolite) dehumidifiers work even at low winter temperatures.

5. 5

   If it still gets moldy, review the insulation

   Some homes have no insulation in the closet's back wall against the outer wall. Reduce the cold spots themselves, such as by fitting insulation boards or adding inner windows on north-side windows. Window remedies are compared in Preventing Window Condensation.

FAQ

Why do closets get condensation?

Because warm, moist air from a heated room enters the cold closet and cools. Air at 20°C and 50% humidity reaches 83% humidity in a 12°C closet, and at 8°C it reaches 100% and condenses.

At what humidity does mold start to grow?

Mold grows more easily above 70%, and spreads more easily if it stays above 80%. Dust mites multiply more easily above 60%. Even without droplets, sustained high humidity is enough for mold to grow.

What room humidity prevents mold in a closet?

At 20°C, aim for 40–50%. With the room at 20°C and 40%, even a closet cooled to 12°C has 67% humidity, below the 70% mold threshold. At 50%, it rises to 83%.

Is putting desiccant in the closet enough?

Desiccants alone can only absorb so much. It's more effective to combine them with other measures: don't let room humidity get too high, open the doors regularly, keep things off walls and floors with slatted boards, and let bedding air before storing it.

Why do walls in north-facing rooms get moldy?

Because a sunless, unheated north-facing room cools to around 10°C, and moist living room air flows in. Air at 20°C and 50% reaches 95% humidity at 10°C. Closing the door, keeping furniture off the wall and using a dehumidifier are effective.