Mildew grows in storage units because moisture gets trapped inside, and you can prevent it by controlling humidity, improving airflow, and keeping moisture-absorbing materials inside

Mildew thrives in dark, damp spaces with poor air circulation—which describes most storage units. The problem starts when warm, humid air enters the unit and has nowhere to go. As temperature drops at night or during cooler months, that moisture condenses on walls, floors, and your belongings. Within days, mildew spores (which are always present in the air) begin to grow. The good news is that mildew prevention is straightforward: you control three things—moisture entry, humidity inside the unit, and air movement—and mildew cannot establish itself.

You do not need expensive equipment or constant attention. Most prevention methods cost under fifty dollars and take less than an hour to set up. The key is starting before mildew appears, because once it takes hold on fabric, wood, or paper, it is difficult to remove completely.

Key Takeaways

  • Mildew needs moisture, darkness, and stillness to grow; removing any one of these three stops it from taking hold.
  • Moisture enters through open doors, gaps around seals, and humid outside air; seal gaps and limit door openings to reduce moisture entry.
  • Humidity inside the unit drops when you place desiccants (moisture-absorbing materials like silica gel or calcium chloride) throughout the space.
  • Air circulation inside the unit prevents moisture from pooling; a small battery-powered fan or open shelving allows air to move around stored items.
  • Check your unit monthly during humid months and after temperature swings, because condensation can appear suddenly even in a sealed unit.

Seal gaps and cracks around the door and walls

The door frame is where most moisture enters a storage unit. Even a small gap between the door and frame allows humid outside air to flow in, especially on warm days or after rain. Walk around the inside perimeter of your unit and look for light coming through cracks. If you see daylight, moisture is getting in.

Check the rubber seal (called a gasket) that runs around the door edge. If it is cracked, compressed, or missing in spots, ask the facility to replace it. This is the facility's responsibility, not yours—a damaged gasket means the unit is not functioning as designed. If the gasket is intact but the door does not close flush, the frame may be warped; report this to management as well.

For small gaps you can address yourself, weatherstripping tape (the adhesive-backed foam kind sold at hardware stores) fills gaps up to about a quarter-inch wide. Clean the surface first so the tape sticks, then press it firmly into place. This is temporary and will need replacing every year or two, but it works while it lasts. You can also use rope caulk, which is removable and does not leave residue when you leave the unit.

Place desiccants throughout the unit to absorb moisture

Desiccants are materials that pull moisture from the air. The most common types for storage are silica gel, calcium chloride, and activated charcoal. These work passively—you do not have to plug them in or maintain them—and they are inexpensive.

Silica gel comes in small packets or larger containers. One five-pound bucket of silica gel covers roughly 150 square feet. Spread containers around the unit: one near the door, one in each back corner, and one or two in the middle. As silica gel absorbs moisture, it changes color (usually from blue or orange to pink or clear, depending on the brand). When it is saturated, you can dry it out in an oven at low heat and reuse it, or replace it.

Calcium chloride (sold as DampRid or similar brands) works similarly but does not change color, so you have to monitor it by weight or by checking the collection tray underneath. It absorbs more moisture than silica gel but costs slightly more. Both are safe around most stored items, though you should keep them away from electronics and documents until you understand your specific items.

Replace or refresh desiccants every two to three months during humid seasons (spring through fall in most climates), and monthly if your unit is in a very humid location or near water.

Improve air circulation inside the unit

Still air allows moisture to settle and condense. Even a small amount of movement prevents this. If your facility allows it, place a small battery-powered or plug-in fan inside the unit and run it continuously or for several hours a day. Point it toward the back wall so air circulates around your stored items rather than blowing directly on one spot.

If a fan is not practical, arrange your items to allow air to move around them. Avoid stacking boxes directly against walls; leave a few inches of space. Use shelving units rather than floor stacking when possible, because air can flow underneath. Do not cover items with plastic sheeting or tarps, as this traps moisture against them; use breathable cloth covers instead if you need protection from dust.

Open the door briefly once a week on dry days to exchange some of the stale air inside, but do this only when the outside humidity is lower than the inside humidity. On hot, humid days, opening the door actually makes things worse by letting in more moisture. A good rule: open the door only on cool, dry mornings or after a cold night.

Monitor humidity levels and check for condensation

A hygrometer is a small, inexpensive device that measures humidity. Place one inside your unit and check it during monthly visits. Humidity above 60 percent creates conditions where mildew can grow; above 70 percent, mildew grows rapidly. If your reading is consistently high, you need more desiccant or better air circulation.

Even if your hygrometer reads normal, check the walls, floor, and ceiling for condensation or damp spots after temperature swings. Condensation often appears overnight when warm days cool down suddenly. If you see it, increase desiccant, improve ventilation, and open the door on the next dry day to exchange air.

Pay special attention during spring and fall, when temperature and humidity fluctuate most. These seasons are when most mildew problems start, because people assume the unit is fine and stop checking.

Choose what you store and how you pack it

Some items attract and hold moisture more than others. Avoid storing anything damp or wet. Wipe down furniture before storing it, and let it dry completely. Do not store plants, soil, or anything organic that will decompose and release moisture.

Pack items in breathable containers when possible. Cardboard boxes allow some air movement, while sealed plastic bins trap moisture inside. If you must use plastic bins, leave the lids slightly ajar (or drill small holes in the lids) so air can circulate. Wrap valuable items in acid-free paper or cloth rather than plastic wrap.

Store items off the floor on shelving or pallets. This keeps them away from any water that might pool if the floor develops a leak, and it allows air to flow underneath. Leave space between boxes and between boxes and walls—tight packing prevents air circulation and creates pockets where moisture collects.

Inspect your unit monthly and after weather changes

Set a calendar reminder to visit your unit once a month, more often during humid seasons. Spend ten minutes checking for condensation, musty smells, or visible mildew. If you find any, it means your current prevention method is not working, and you need to adjust.

After heavy rain, a heat wave, or a sudden temperature drop, check within a day or two. These weather events often trigger condensation inside sealed spaces. If you find moisture, open the door on the next dry day, increase desiccant, and run a fan if you have one.

If mildew has already started to grow on items, wipe it off with a cloth dampened in white vinegar or a dilute bleach solution (one part bleach to ten parts water), then dry the item thoroughly. For porous items like books or upholstered furniture, mildew is harder to stop once it starts, so prevention is much easier than treatment.

Frequently Asked Questions

Can I use a dehumidifier instead of desiccants?

Yes, but only if your unit has power and you can run it continuously. A plug-in dehumidifier works faster than desiccants and is more effective in very humid climates. However, it requires electricity, costs more upfront, and needs maintenance (emptying the collection tank or running a drain line). Desiccants are cheaper, require no power, and work well for most storage situations. Many people use both: desiccants as the main method and a dehumidifier as backup during peak humidity months.

What if my storage unit is in a basement or below ground?

Below-ground units are naturally more humid because they are closer to soil moisture. You will need more desiccant (roughly double the amount for above-ground units) and should check humidity monthly year-round, not just during humid seasons. A small dehumidifier or fan is especially helpful here. If the facility allows it, ask about units on upper floors, which tend to stay drier.

Will plastic sheeting on the floor prevent moisture from coming up?

Plastic sheeting can help if water is pooling on the floor, but it does not stop humidity in the air from condensing on your items. It may even trap moisture underneath itself. Use plastic only if there is an active leak or standing water; otherwise, let air flow across the floor. Pallets or shelving keep items off a damp floor without blocking air movement.

How do I know if the smell is mildew or just mustiness?

Mustiness is a stale smell from poor air circulation; mildew is a stronger, sour smell that often comes with visible growth or dark spots. If you smell mustiness, open the door, run a fan, and improve ventilation. If you smell mildew or see dark spots, you have active growth and need to act when ready: increase desiccant, improve airflow, and inspect items for damage.

Can I prevent mildew in a climate-controlled unit?

Climate-controlled units maintain steady temperature and humidity, which makes mildew much less likely. However, they are not mildew-proof—moisture can still enter through door gaps or be brought in on damp items. Follow the same prevention steps (seal gaps, use desiccants, check monthly) but you can do so less frequently. Many people find that climate-controlled storage eliminates the need for active prevention, but checking once every few months is still wise.