How to stop condensation in metal sheds

Imagine a steaming hot cup of coffee on your kitchen window sill on a frosty morning. As the steam rises from the hot liquid it condenses on the cold window leaving a plume of condensed steam.

Now drink the coffee.

By the time that you have drunk the coffee that plume of condensation on the window pane has gone, evaporated back into the air.

The same principle that applies to steam from your coffee also applies to metal sheds.

Condensation in metal sheds is a commonly reported problem

The inside of the shed roof is running with water and this damp atmosphere is bad for the items that are stored there. Metal tools go rusty, fabric becomes mildewed, there must be a way to solve this problem.

Why does condensation happen particularly with metal sheds?

The first thing is to understand how the condensation forms in the first place. Water vapour is always in the air, sometimes there is more of it than at other times. When warm moisture laden air meets a cold surface it condenses to form the water droplets that you see on the underside of your metal shed roof.

Let's take a typical clear warm autumn evening. At the start of the evening the temperature in your metal shed will be about the same as the outside air temperature. As the evening progresses it starts to get a bit chilly as the sun goes in and the warm air at ground level rises. However the air in your shed can't rise it is stuck there.

As the temperature outside gets colder it affects the fabric of the shed and this cools too. The metal skin of the shed is only a millimetre or so thick and metal is a good conductor of heat (as well as of electricity). Quite quickly the metal becomes cold enough so that it reaches what is known as the 'dew point'. The cold metal in contact with the warm air means that water droplets/condensation forms on the inside surface of the shed roof. The condensation occurs most commonly on the roof as the warmest, most moisture laden air rises to roof level.

Ok, now we understand a bit how the condensation in metal sheds happens.

How can we stop condensation in metal sheds forming in the first place?

The answer here comes on two fronts.

The first is to get the inside of the shed as dry as possible. Often metal sheds are not supplied with a raised floor. The metal shed is built and installed on to a concrete slab foundation. If this concrete foundation does not have an effective moisture barrier beneath it then it will act as a store of water and also transmit water from the ground to the inside of the shed. If there is a problem with condensation then cutting off the source of that water vapour will significantly help the condensation problem.

It is a bit like that cup of coffee on the kitchen window sill. Once the source of water vapour was removed the condensation on the window gradually evaporated and disappeared.

The second front is to improve the ventilation of the shed at the eaves level. By making some small holes in each side of the shed at eaves level the warm moisture laden air inside the shed can escape to the surrounding atmosphere. This then reduces the moisture level inside the shed and reduces the possibility of condensation occurring. Having ventilation in opposite walls of the shed means that if there is any breeze it will help to clear the warm air.

This combination of blocking the source of moisture and also providing a means for it to escape means that over time the shed will dry out.

Why doesn't condensation happen in timber sheds?

The main reason is that the timber (typically 12mm thick) acts as insulation, it doesn't get as cold as quickly as the thin conductive metal. Of course 12mm isn't good enough insulation to keep the building really warm. But it is enough to act as a buffer and allow the internal and external temperature to equalise before condensation can occur.

Also many timber sheds are a bit less air tight than metal sheds and this allows the warmer air to escape to the outside, equalising that important temperature difference.

What if I my shed is built on a concrete slab that doesn't have a damp proof membrane?

The best way to solve the problem of condensation in metal sheds, for a garden shed at least, is to build a timber sub-floor. Building timber floor to support the metal shed frame will form a barrier to block future sources of damp. An additional measure that you should incorporate into this floor is ventilation to allow air to flow beneath the shed which once again will stop moist air from entering the shed.

Coffee and garden sheds have the same condensation principles at play

To solve the problem of condensation in metal sheds its easiest to block the water vapour at source and provide ways to let it escape to the atmosphere.

And that way you get to keep the inside of your metal garden shed and the contents nice and dry.

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