Why is my shed damp inside?

Telling a leak from condensation from ground moisture, and the ventilation and base changes that dry a shed out for good.

Difficulty: beginner
Time: 1 hr
Read: 3 min

Short answer

If the damp appears on the underside of the roof and on metal tools overnight, it is condensation and the answer is ventilation. If it is in one place and follows rain, it is a leak. If the floor and the bottom of the walls are damp, moisture is coming up from the base.

Sheds are uninsulated boxes that cool below outdoor air temperature on a clear night, which makes them condensation machines. Most shed damp is not a leak, and treating it as one — more felt, more sealant — makes it worse by closing off the airflow that would fix it.

Telling shed condensation from ground moisture from a leak

Step by step

Check when the damp appears.Damp after rain and in one place is a leak. Damp on clear cold mornings, evenly spread, and heaviest on the roof underside and on metal, is condensation. Damp that is worst at the floor and lowest boards is ground moisture.
Look for the leak first if it follows rain.Check the felt at the ridge and eaves, around any window, and where the roof meets the walls. Water tracks along timber, so the wet patch is often downhill of the actual hole.
Check what the shed is standing on.A shed on bare soil or on slabs with no damp-proof membrane wicks moisture up through the bearers into the floor. The base is the most common cause of persistent low-level damp.
Add high and low ventilation.Two vents diagonally opposite — one low on one wall, one high on the other — create cross-flow. Hit-and-miss vents or simple soffit vents with mesh work. This is the single most effective change for condensation.
Get contents off the floor and off the walls.Air needs to move behind and under stored things. A shed packed tight to the walls has still air at every surface, which is where condensation forms.
Stop bringing water in.Wet coats, wet grass boxes, damp compost bags and freshly washed tools all evaporate into a closed box. Let things dry outside before they go in.
Insulate the roof if condensation persists.It is the roof that drops below dew point first. Even a layer of foil-faced insulation board between the rafters, with a ventilated gap above it, stops most roof drip.
Recheck through a cold snap.Judge the fix in December, not August. The test of a dry shed is a clear frosty morning with no drips.

Tips

A cheap hygrometer in the shed tells you far more than guessing. Persistent readings above 70% mean ventilation, not sealing.
Never use plastic sheeting inside a shed to keep damp out. It traps moisture against the timber and rots it from behind.
A small vent low down on the windward side and one high on the leeward side works better than two vents on the same wall.

Common mistakes

Sealing every gap to keep damp out — Shed damp is usually moist air condensing on cold surfaces. Sealing the shed traps the moisture and makes it worse. Ventilation is the fix.
Standing the shed directly on soil or grass — Moisture wicks up into the bearers and floor continuously, and the timber never dries. A shed needs a base that lifts it clear with an air gap or a membrane.

If it doesn't work

Water dripping from the roof underside on cold mornings

Cause: Condensation on a cold uninsulated roof — Fix: Add ridge or gable ventilation, and insulate the roof with a ventilated gap above the insulation.

Floor and bottom 200mm of the walls damp

Cause: Ground moisture through the base — Fix: Lift the shed if you can and add a damp-proof membrane or bearers that raise it clear of the ground with an air gap.

Tools rusting despite the shed being dry to look at

Cause: High humidity rather than liquid water — Fix: Ventilate, get items off the floor, and use a moisture absorber or a low-wattage tube heater in winter.

Mould on stored fabric and paper

Cause: Still, humid air — Fix: Cross-ventilation and spacing items off the walls. Anything that must stay dry should not live in an unheated shed.

Questions people ask

Should I put a dehumidifier in a shed?

Only in a sealed, insulated garden room with power. In a draughty shed it runs continuously drying the outside air and achieves very little. Ventilation and a base that keeps ground moisture out come first.

Does insulating a shed cause condensation?

Insulating without ventilating does, because you move the cold surface rather than remove it. Insulation plus a ventilated gap above it, plus cross-ventilation inside, is what works.