How do I ventilate a shed properly?

Where to put vents so air actually moves, how much vent area a shed needs, and keeping mice and wasps out while letting air in.

Difficulty: beginner
Time: 1 hr
Read: 3 min

Short answer

Two vents, diagonally opposite, one low and one high. Low on one wall for air in, high on the opposite wall or in the gable for air out. Around 1% of the floor area in total free vent area is a reasonable target, with stainless mesh behind every opening.

One vent does almost nothing. Air only moves through a shed if there is somewhere for it to leave as well as to enter, and if the two openings are at different heights so warm air rising drives the flow.

What you'll need

Hit-and-miss or louvred vents, or a soffit vent strip
Fine stainless or galvanised mesh
A hole saw or jigsaw
Exterior sealant and screws

Step by step

Work out roughly how much vent you need.About 1% of the floor area in free vent area is a workable rule for an unheated store — a 2m x 3m shed wants around 600 square centimetres, split between the two vents. Free area is the actual open area, which is much less than the vent's outside size.
Put the inlet low on one wall.Near the floor on the side that gets the prevailing wind. Cool air enters here.
Put the outlet high on the opposite wall or gable.As high as the construction allows. Warm moist air leaves here, and the height difference drives the flow even with no wind.
Mesh every opening.Stainless or galvanised insect mesh behind the vent stops wasps, mice and birds. Plastic mesh is chewed through by mice within a season.
Cut the openings cleanly and seal the edges.A hole saw or a jigsaw with a pilot hole. Treat the exposed cut edges of the cladding with preservative before fitting the vent.
Fit the vents with a weather drip.Louvres face downwards outside so rain cannot drive in. Seal the top and sides of the vent frame but leave the bottom edge unsealed so any water that does get behind can escape.
Ventilate the floor void too.If the shed sits on bearers, keep the gap beneath open and clear of leaves and stored items. A blocked underfloor void puts moisture straight into the floorboards.
Keep the vents clear.Do not stack anything against a vent inside. It is the single easiest way to undo the whole job.

Tips

Hit-and-miss vents can be shut. Useful during a gale, but they are much more often left shut by accident and forgotten.
A ridge or gable vent works even on a still day, because it exploits the temperature difference rather than the wind.
If you keep a mower or a fuel can in the shed, ventilation is a safety matter as well as a damp one — petrol vapour is heavier than air and needs a low-level vent.

Common mistakes

Fitting a single vent — Air cannot enter and leave through the same hole. One vent moves almost nothing; two at different heights on opposite walls move a great deal.
Meshing with plastic insect netting — Mice chew straight through it. Stainless or galvanised mesh keeps rodents out for the life of the shed.

If it doesn't work

Vents fitted but the shed is still damp

Cause: Both vents on the same wall or at the same height — Fix: Move one vent to the opposite wall and as high as possible, so there is a genuine cross-flow.

Rain driving in through a vent

Cause: Louvres facing the wrong way or no drip detail — Fix: Louvres must slope downwards to the outside. Add a small hood or drip over exposed vents.

Wasps nesting behind a vent

Cause: Unmeshed opening — Fix: Mesh it after the nest is dealt with. Do not seal a vent to keep wasps out — mesh it instead.

Questions people ask

How many vents does a shed need?

At least two, diagonally opposite and at different heights. A larger shed benefits from a pair at each end. Total free vent area of around 1% of the floor area is a sensible target.

Will ventilating make the shed colder?

Marginally, and it does not matter in an unheated store. In a heated garden room the answer is insulation plus controlled ventilation, not a sealed box.