To prevent mold in storage, keep moisture out. Mold needs water to grow, so store only clean, fully dry items, pack them in sealed containers, lift everything off the floor, leave room for air, and keep humidity between 30% and 50% where you can. A climate-controlled unit makes that last part far easier, especially in humid South Texas.
This guide walks through the whole process in the order you will do it: choosing the unit, prepping your belongings, packing, controlling humidity, and checking in later. It also covers what to do if you open the door and smell something musty.
TL;DR
- Moisture is the cause. The EPA says controlling moisture is how you control indoor mold growth.
- Clean and fully dry everything before it goes in. Damp, dusty or food-stained items are what mold feeds on.
- Use sealed plastic bins, lift boxes off the concrete, and leave a gap along the walls for air.
- Aim for relative humidity of 30% to 50%. Climate-controlled storage, an absorber or a dehumidifier can help you stay there.
- If you find mold, fix the moisture source first, clean hard surfaces with detergent and water, and throw out porous items you cannot dry.
Table of Contents
- Why mold grows in storage units
- Choose the right unit before you pack
- Prep your belongings: clean and dry first
- Pack to keep air and moisture out
- Control humidity in the unit
- Special cases: RVs, boats and vehicles
- Check in on your unit
- If you find mold
- Perspective
- Rent a unit that fits your belongings
- FAQ
- Sources
Why mold grows in storage units
Mold is not something a unit "has" or "doesn't have." Spores are everywhere, and the EPA notes there is no practical way to eliminate all mold and spores indoors. What you can control is whether those spores find enough moisture to grow.
A closed storage unit is a small, still space. Anything damp you bring in, such as a couch cushion that never fully dried or a box that sat in a humid garage, releases that moisture into the air with nowhere to go. Temperature swings add to it: warm daytime air cools overnight and condensation forms on metal, concrete and the sides of containers.
And mold is not picky. According to the EPA, it can grow on virtually any substance when moisture is present, including wood, paper, carpet and food. That is why a box of old paperwork, a wooden dresser and a leather jacket are all at risk in the same unit.
The EPA also gives a useful timeline: to prevent mold, find and fix the water source and dry the area quickly, within 24 to 48 hours if possible. Apply that to storage. Anything that gets wet on moving day needs to dry before it goes in the unit, not after.
Choose the right unit before you pack
The cheapest mold prevention happens before you carry a single box.
Climate-controlled or standard?
If you are storing things that react badly to moisture and heat swings, climate-controlled storage is the simplest answer. Wood furniture, mattresses, upholstered pieces, books, photographs, important papers, fabrics, leather and electronics are the usual candidates. Our guide on what needs climate-controlled storage breaks down the categories for South Texas conditions, and our piece on climate-controlled storage explains how to compare options.
Sturdier items, such as metal shelving, plastic totes of tools or sealed outdoor gear, do well in a standard drive-up unit when they are packed properly.
Size matters for airflow
A unit packed wall to wall has no airflow. Pick a size that lets you leave a walkway and a few inches along the walls. If you are between two sizes, go up. Our furniture storage comparison of 10x10 and 10x20 units shows what fits in each, and for a lighter load, small apartment storage covers 5x5 and 5x10 choices.
Look at the unit before you move in
Before loading, look at the floor, walls and ceiling of the unit. A dry, clean concrete floor and no sign of water stains or drips is what you want. If you see damp patches or a musty smell, tell the office and ask for a different unit rather than loading in. Doing this takes two minutes and avoids the most common source of trouble: a leak you did not notice.
Prep your belongings: clean and dry first
This is the step people skip and the one that matters most. Mold needs moisture and something to eat. Dirt, food residue, skin oils and spills all give it that something.
Furniture and household items
- Wipe down everything. Use mild soap and water on hard surfaces, then let items air dry completely, ideally for a full day.
- Empty and dry appliances. Wash out the refrigerator, freezer and washing machine, dry the inside, and prop the doors open slightly or use a vented cover so air can move.
- Vacuum upholstery and let it air out. Never store a cushion that feels even slightly cool or damp to the touch.
- Clean tools and outdoor gear. Hose off, dry, and wipe metal with a light coat of oil if you like.
If you want a full checklist for the unit itself, see our step-by-step guide to cleaning a storage unit.
Clothes, linens and fabrics
Wash or dry-clean before storage, and make sure they are bone dry. University of Missouri Extension reports that clean synthetic fabrics such as acetate, acrylic, polyester and nylon resist mildew better than natural fibers, while cotton, linen, rayon, silk, wool, leather, wood and paper are common mildew-prone materials. It also notes that soil can support mold even on resistant synthetics, so clean comes first whatever the fabric.
That shapes how you pack. Put natural-fiber items in sealed bins or sealed bags, and don't tuck anything leather or wool into a box with leftover moisture.
Mattresses
Mattresses soak up humidity and are hard to dry once damp. Let them air out first, use a protective cover that is fully closed, and store them flat or on their side off the floor. Our mattress storage tips cover the details.
Heavy and valuable pieces
Large display pieces, statues and other heavy items need clean, dry surfaces and stable support as much as anything else. If you collect large figures or props, this guide to storing heavy collectibles shows how to protect them in storage. Art, photographs and documents may justify a climate-controlled unit; see our overview of art storage units.
Pack to keep air and moisture out
Containers: plastic beats cardboard
Cardboard is paper, and paper is one of the materials the EPA lists as mold-friendly when it gets damp. It also absorbs moisture from the air. For anything you want to keep long-term, use lidded plastic bins with tight seals. Use cardboard only for short stays, and if you do, keep it off the floor and away from the walls.
Sealed bins help, but they are not magic. If you close a bin around slightly damp items, you have created a small humid chamber. Dry first, then seal. One commonly cited trick is to pack bins on a dry, low-humidity day rather than a rainy one.
Do airtight containers stop mold?
They reduce the risk by keeping outside humidity away from your items. They do not stop mold that is already in the box or growing on a damp item inside. That is why prep matters more than the container.
Elevate everything
Concrete floors can hold and transmit moisture. Put pallets, plastic risers or sturdy shelving under boxes and furniture so nothing sits directly on the floor. Raise furniture legs on small blocks or plastic feet if you can.
Leave space for air
Set a few inches of gap between your stacks and the walls, and keep a path down the middle. Stacking tightly traps still air, and still air is where humidity settles. Our five-step guide on packing a storage unit for drive-up access includes a layout that keeps an aisle open and the things you need most near the door.
A quick packing checklist
| Item | Best container | Extra step |
|---|---|---|
| Clothes, linens | Sealed plastic bin | Wash and dry fully first |
| Books, papers | Plastic bin, off the floor | Keep out of cardboard long-term |
| Furniture (wood) | Breathable cover, not plastic wrap | Wipe clean, dry, leave airflow |
| Mattress | Closed mattress bag | Store flat, raised off floor |
| Electronics | Original box or sealed bin | Climate-controlled recommended |
| Tools, outdoor gear | Plastic tote or shelf | Dry and wipe metal |
| Appliances | Open slightly or vented cover | Empty and dry the inside |
Use a breathable cover, such as a cotton sheet, over furniture rather than tight plastic wrap, because wrap can hold moisture against the surface.
Control humidity in the unit
Here are the numbers to aim for. The EPA recommends keeping indoor relative humidity below 60%, ideally between 30% and 50%. The CDC says to keep humidity as low as possible, no higher than 50% all day. Those targets are written for homes, but they are the practical benchmark for anything you care about storing.
A small digital hygrometer costs little and tells you what is going on. Put one on a shelf at eye level, note the reading each visit, and watch for trends rather than one-off spikes.
Climate-controlled storage
The most hands-off approach is a unit built to keep temperature and humidity in a steadier range. We offer climate-controlled units at our South San Antonio facility for belongings that are sensitive to moisture and heat.
Moisture absorbers and desiccants
Absorbers are inexpensive and useful for small, closed spaces: a bin, a wardrobe box, a cabinet. One manufacturer, Dri-It, says its pouches collect up to 300% of their weight in moisture and suggests one to two pouches per 50 cubic feet, replacing a pouch once it feels like gel. A 10x10 unit with an 8-foot ceiling holds 800 cubic feet, so absorbers alone are better thought of as a supplement for containers and corners than as a way to dry a whole unit.
Silica gel packs work similarly inside sealed packages. Benchmark Industrial explains that silica gel lowers humidity by adsorbing water vapor and works passively, and that a desiccant cannot make up for a package that keeps letting humid air in. In practice: seal the bin properly, then add the packet.
Some practical tips:
- Put hanging or tub-style absorbers up high in wardrobes and cabinets, and tubs on shelves rather than the floor.
- Check them on every visit and replace them when they are full.
- Use silica packs in bins of documents, photos, electronics and leather goods.
Dehumidifiers
A dehumidifier moves much more water than an absorber, but it needs electricity and a way to drain. Before you plan on one, ask our office whether powered equipment is permitted in your unit. Many storage units have no outlet, and that is a question for the facility rather than something to assume.
If you can use one, ENERGY STAR explains that portable dehumidifier capacity is measured in pints per 24 hours and depends on room size and moisture conditions, so size it carefully. A hose drain avoids emptying the bucket by hand, and below 65°F you should consider a model rated for lower temperatures. Follow the electrical-safety and drainage precautions on the product label.
Type matters too. Equipment rental company United Rentals notes that desiccant dehumidifiers suit cold or unheated spaces and can be more efficient at or below 50°F, while refrigerant units are more efficient above 50°F. In South San Antonio most days are well above that, so a standard refrigerant unit is the usual fit for warm months.
Never leave an unattended electrical device running in a unit unless the facility has confirmed it is allowed.
Airflow
The cheapest control is the one you already did: gaps along walls, a center aisle, and nothing resting on the floor. If you visit, open the door for a few minutes on a dry day to let the air exchange. Avoid doing that on a rainy, muggy day, which just brings humid air in.
Special cases: RVs, boats and vehicles
Vehicles are a different problem. Interiors trap moisture, and a sealed RV or boat can grow mold on upholstery, carpet and canvas. Before parking:
- Empty all food, drinks and trash.
- Clean and fully dry the interior, bilge and cushions.
- Open cabinets and closets slightly and prop cushions on edge so air moves.
- Add a moisture absorber in cabins, and check it on each visit.
We offer RV storage and boat storage in South San Antonio, with gated entry and video surveillance on site.
Check in on your unit
Prevention is not a one-time job. A short visit every month or two catches problems while they are small.
- Smell the air when you open the door. A musty odor is an early warning.
- Read your hygrometer and compare it with 30% to 50%.
- Look at the floor and the bottom of boxes for dampness or discoloration.
- Swap out full absorbers.
- After heavy rain, check for water near the door seal and under the doors.
Our 24-hour kiosk and drive-up access make short check-ins easy, since you are not tied to office hours.
If you find mold
Stay calm and work in order.
- Fix the moisture first. The EPA's guidance is to find and fix the water source and dry the area quickly. Cleaning without fixing the cause just brings the mold back. Tell our office so we can look at the unit.
- Clean hard surfaces. Mold on hard surfaces can be removed with mild detergent and water, per the same EPA guidance.
- Decide what to discard. Porous items such as ceiling-type materials, carpet, upholstered pieces or cardboard that cannot be cleaned and dried should be thrown out.
- Skip the testing. The CDC does not recommend mold testing. It says to remove existing mold safely and prevent future growth, and that sampling cannot show whether someone might become sick, since health effects differ among people.
- Protect yourself. Wear gloves, eye protection and a well-fitting mask while cleaning. If you have asthma, allergies, a weakened immune system or a lung condition, ask your doctor before handling moldy items, or have someone else do it.
If the problem is widespread, or mold is hidden behind surfaces, a professional is the right call: EPA training material notes that hidden mold can occur behind walls and in crawl spaces.
If damaged items are valuable, check your policy. Our guide to storage unit insurance explains what to confirm in writing.
Perspective
We see the same pattern again and again: mold in storage is almost never a mystery. It comes from something damp going in, or something staying damp once it is there. That is why we tell customers to spend their effort before move-in day.
Match the unit to the belongings. If you are storing wood furniture, mattresses, books, photos or fabrics for more than a few weeks in South Texas, we think climate-controlled storage is the safest place for them. It takes the guesswork out of humidity and gives your packing work the best chance of paying off.
Then pack the way a careful mover would: clean, dry, sealed, lifted off the floor, with an aisle for air. Add a hygrometer and check in on a schedule. Our Touchless Rentals™ system, drive-up access and 24-hour kiosk mean visiting is quick, so there is no reason to leave a unit unchecked for months.
Month-to-month leases also mean you can move things out as soon as your project ends, whether that is a renovation, a move or a summer break. The shorter the stay and the better the prep, the lower the risk.
Ian
Rent a unit that fits your belongings
If you are ready to store, we have sizes from lockers and 5x5 units up to 10x30, plus climate-controlled units, RV spaces and boat storage in South San Antonio. Gated access, video surveillance and flexible month-to-month leases come standard to how we operate, and you can reserve your unit online with Touchless Rentals™ without a visit to the office.
Browse our self-storage units to choose a size, or use the 24/7 contactless kiosk to rent on your schedule. Planning a bigger load? Look at our 10x15 units or 10x20 units, where there is room to leave an aisle for airflow.
FAQ
What should I put in storage to avoid mold?
Store items that are clean and completely dry, in sealed plastic bins or on shelves off the floor. Synthetic fabrics resist mildew better than cotton, wool, leather, wood or paper, but only when they are clean. Keep anything sensitive, such as photos, documents and wood furniture, in climate-controlled storage.
Does mold grow in airtight containers?
It can, if the items inside were damp, dirty or carried moisture in when you sealed the lid. An airtight bin keeps outside humidity away from clean, dry belongings, but it cannot dry something that is already wet. Add a silica gel packet for papers, photos and electronics.
What humidity level should I keep in a storage unit?
The EPA says to keep relative humidity below 60%, ideally between 30% and 50%, and the CDC advises no higher than 50%. Those are home guidelines, but they work well as a target for storage. A small hygrometer shows you where your unit stands.
Is a moisture absorber enough to keep a unit dry?
Absorbers help in bins, wardrobes and corners, and you should check and replace them regularly. For a whole unit, they work best alongside good packing and airflow, and for sensitive items, a climate-controlled unit gives steadier conditions. A dehumidifier is another option if your facility allows powered equipment.
What should I do if I find mold in my unit?
Fix the moisture source and dry the area quickly. Clean hard surfaces with mild detergent and water, and discard porous items that cannot be cleaned and dried. The CDC does not recommend testing for mold. If you have a respiratory condition, ask your doctor before cleaning.
Sources
- Ten Things You Should Know about Mold, US EPA
- Tribal IAQ Resource Directory: Mold and Moisture, US EPA
- Mold, CDC
- Mold Web Course, EPA
- How to Prevent and Remove Mildew, University of Missouri Extension
- Dehumidifiers, ENERGY STAR
- Suggested Uses, Dri-It Moisture Absorber
- What Is a Desiccant Pack?, Benchmark Industrial
- Desiccant Dehumidifiers and When to Use Them, United Rentals