Why does mould in a storage unit or garage show up on boxes you packed away clean just months earlier?
It’s one of the most common surprises renters face when they open a rented storage unit or an unused garage after a Melbourne winter — and the cause has almost nothing to do with how carefully the items were packed. It comes down to what the space itself is doing to the air trapped inside it.
Storage Spaces Are Built to Trap Moisture, Not Release It
Garages and storage units share a design flaw for mould purposes: they’re sealed boxes with almost no airflow, no heating, and often a concrete slab that sits directly on the ground. Concrete is porous. It draws up ground moisture and releases it slowly into the air above it, and with the door shut for weeks at a time, that moisture has nowhere to go.
Add cardboard boxes, fabric furniture covers, and stacked textiles — all materials that hold moisture and provide an organic food source — and a sealed storage space becomes close to ideal mould conditions without a single leak ever occurring.
What’s Actually at Risk Inside
Cardboard, fabric, leather, and timber furniture are the first things affected, often before you notice any smell. As explained in Mould on Clothes, Furniture, and Carpet: Can It Be Saved?, porous materials absorb spores into their fibres rather than just growing mould on the surface, which is why a quick wipe-down often isn’t enough once mould has taken hold.
Modern, well-insulated garages can paradoxically make this worse, not better. The same airtightness that keeps a home efficient — covered in Sealed House Syndrome: Is Energy Efficiency Breeding Mould? — applies just as much to an attached garage with a solid door and no vents, trapping whatever moisture the concrete slab releases.
Signs You Should Check Before It Spreads Further
A musty smell the moment you open the door is usually the first sign, well before any visible growth. Condensation on the inside of windows or on metal shelving, a powdery white or black bloom on cardboard, and a damp, cool feel to the concrete floor are all worth investigating rather than dismissing as “just an old garage smell.”
If you’re storing items long-term or have noticed any of these signs, testing the air and surfaces gives you a clear answer rather than a guess, particularly before deciding whether stored items are safe to bring back into the home.
Reducing the Risk Going Forward
Elevating boxes off the concrete floor on shelving or pallets, using plastic storage tubs instead of cardboard for anything staying more than a season, and cracking a window or vent on dry days all reduce the moisture load without needing any structural work. None of these fixes the underlying slab moisture on their own, but together they remove most of the materials mould needs to take hold. Boats stored over winter face a similar sealed-space problem, covered in winter boat storage mould.
Conclusion
A garage or storage unit doesn’t need a leak to grow mould — a sealed door and a concrete floor are often enough on their own. If you’ve noticed a musty smell or visible growth on stored items, an inspection can confirm the source and whether anything else in the space needs attention before you store anything else in there.
Frequently Asked Questions (FAQs)
Why does my garage get mouldy even though there’s no leak?
Concrete slabs draw up ground moisture and release it into an enclosed space with nowhere to escape. Combined with poor airflow, that’s often enough to grow mould without any actual water leak involved.
Can mould on cardboard boxes spread to other stored items?
Yes. Mould spores travel through the air inside an enclosed space and can settle on nearby porous items like fabric, timber, or paper, even if those items weren’t directly touching the affected box.
Is it safe to bring mouldy stored items back into my home?
Items with visible mould growth, especially porous ones like cardboard, fabric, or untreated timber, should be assessed before being brought back inside. Some can be cleaned; others are safer to dispose of.
How can I stop mould in a storage unit or garage without heating it?
Elevate items off the concrete floor, use sealed plastic tubs rather than cardboard, and allow occasional airflow on dry days. These reduce moisture exposure even without adding heating or ventilation systems.