How to damp proof a shed

Garden sheds, just like any other building, are susceptible to the growth of damp and mould if not cared for properly. With it being one of the most Googled shed questions on the internet, we thought we’d make this comprehensive guide on how to damp proof your shed.

What is damp?

Damp itself is just the unwanted presence of moisture within a building. However, the term damp is commonly used to refer to the mildew and mould that can grow as a result.


hygrometer slash humidity monitor for measuring humidity levels to prevent damp in a shed
Purchasing a device like this, known as a hygrometer or a humidity monitor, can be useful for knowing how much moisture there is in your air.

We tend to give humidity a bad rap. Oftentimes, we don’t have much good to say about moisture in the air. But the truth is we need moisture to survive.

As humans, certain humidity levels are ideal and others are not. Humidity levels between 30 and 60% are ideal. Anything below 30% can cause chapped lips and dry skin, for example, such as in winter. The reason why we get dry skin in winter and have to apply moisturiser is because the air is too dry. Anything above 60% causes us to sweat and provides perfect conditions for mould and mildew growth.

Relative humidity

Despite condensation being able to happen between any temperature difference, it’s more likely to happen when a surface is cold. This is because colder air can hold less moisture than warmer air. This introduces a concept we call relative humidity.

For example, a cold room might have a relative humidity level of 30%. If that room is warmed up in temperature, the amount of moisture in the air won’t change, but because the warmer air can hold more moisture, the relative humidity level will go down. It is less humid, as less of the air is holding water. Colder air has a much lower ability to hold moisture, so it’s easier for water vapour (moisture) to condense when it nears a cold surface, as the air around it does not have the capacity to hold it.


As seen here, condensation on glass is just water vapour that has reverted to its liquid form.

Condensation is the process of water in a gas state reverting to a liquid state. The air always has moisture in it, in gas form. This is known as water vapour. The concentration of water in the air is known as humidity.

When this water vapour hits a surface like a wall or a window, it turns back into water droplets. Intriguingly, condensation always occurs on surfaces that divide two places with a difference in temperature. This could include a warm beaker of water where the lid becomes steamed up as the water evaporates (turns into water vapour) and then condenses (turns int liquid) when it hits a cooler surface.

Temperature difference

As mentioned, condensation always occurs with a temperature difference. It is always warm water vapour coming into contact with a cooler surface. That’s why on a rainy day, your car can steam up inside. This happens because your warmer breath is colliding with the windows or windscreen, which have been cooled down (on the inside and out) by the rain outside.

It’s also a phenomenon that occurs when you breathe onto glass on a freezing cold day. Your warm breath hits the cold glass and condenses back into liquid form. Subsequently, you can see it on the glass. This can’t happen on a warm day because there isn’t enough of a temperature difference between your breath and the window.

In the same way, the temperature difference doesn’t have to be warm (breath) to freezing (windows). It can also occur in a situation where the difference is boiling hot (vapour evaporating from a pan where you’re boiling food, or a hot shower) to warm. There is still enough of a temperature difference between hot and warm as there is between warm and freezing. That’s why the bathroom steams up when you’ve had a shower, or the kitchen when you’re cooking.

It could also occur, of course, in a shed, where the outside temperature is colder than inside, and the warm air meets the colder wooden structure of your building. The water vapour in the air will condense into liquid form and settle on the windows. This is what damp is.

Will a dehumidifier work in a shed?

A dehumidifier is a device that extracts moisture, along with dust particles and other unwanted air pollution, from the air.

Dehumidifiers are great devices. While they use complex electronics, the concept of them is actually quite simple. They suck in warm air full of moisture and run it over freezing cold pipes within the machine. This causes the moisture (water vapour) in the air to evaporate and turn back into liquid. This liquid then collects in the dehumidifier, meaning you’ll need to empty it out once in a while. This is great for any appliances or circumstances in which you might need some spare water. A lot of people use their excess water for their iron – and can save money in the process.

It is entirely possible to use a dehumidifier in a shed, but you’ll have to make sure you get a battery powered one, unless you have a plug socket in your shed.

Health issues

Damp and moisture can actually cause a host of health problems. As per the NHS website, mould growth can be responsible for asthma, allergic reactions and other infections. This is because the mould releases spores into the air which can be dangerous if inhaled.

Why is my shed damp?

Single glazed windows

Single glazed windows are in direct contact with the outside air. Whereas double glazing protects against condensation due to the barrier between the world and the indoors, one pane of single glazing is in contact with both the outdoors and indoors. This means that the internal temperature will be directly impacted by the external temperature.

With double glazing, two thin panels of glass are separated by a funnel of compressed air, inert gas or a vacuum (where all the air has been removed). For scientific reasons that have failed us since we left school, the cold outdoor air cannot travel as well between these two panels of glass as they can through a solid material.

The time of year

winter damp condensation robin on branch of frosty tree
Winter is prime time for damp and condensation problems.

Damp, condensation and moisture build up is a bigger problem in winter because the air outside is colder, and the air inside is not being refreshed as much due to windows being kept closed to keep heat in.

Gaps in the wood

As a natural material, wood is prone to developing gaps, to warping and bending in and out of shape, and to being generally unpredictable. This means that tiny gaps, some of which may be invisible to the naked eye, can appear in

Should you treat the inside of a shed?

No. Treating your shed manually by hand mainly serves the purpose of protecting the shed against rot and bug infestation, which cannot happen from the inside.

With that being said, we offer pressure treatment on many of our garden buildings. This process, where liquid preservatives are soaked deep into the grain of the wood, takes place before the building is constructed. The entirety of a wood panel is soaked in the solution, meaning that when you do go to build your shed or cabin, the inside will be protected too.

How to damp proof a shed

Keep your shed ventilated

Even in winter, it’s good to make sure your shed doors or windows are opened once in a while to allow fresh air in. This will lead to less of a temperature difference between the outside and inside but will also ensure moisture doesn’t build up in the shed without anywhere to go.

Moisture in the air will naturally diffuse from an area of high concentration to an area of low concentration. Outside, there will be less moisture in the air in the colder months, so opening the doors will allow all of the air out. Circulation of air is key to preventing damp in your shed.

You could even install air vents within the walls of your shed.

Insulating your shed

It sounds like a bit of a paradox, because the last step was all about getting rid of musty, old air. Insulation, is of course, about keeping air in. But remember when we spoke about relative humidity? Insulation maintains the temperature of the inside of the shed, meaning that the relative humidity is decreased. The warmer shed and the subsequent warmer windows can hold more moisture, meaning it is less likely to condensate on the windows.

Seal Gaps

While moisture will build up anyway, sealing gaps can prevent any water ingress from outside sources such as rainfall. It’d be a real frustration if you’d spent all that time trying to dehumidify, only for mould and damp to grow from the water from the rain. Our article on preventing rot on your wooden building explains a bit more


At Garden buildings Direct, we offer guttering on most of our garden sheds. Guttering can prevent moisture from leaking into your building and subsequently causing damp by redirecting rainwater away from the shed walls.

When it rains, water that trickles down the roof is much better deposited onto the grass or concrete away from your shed to prevent any from getting into your shed through any nooks and crannies. Even with a fully treated shed with no gaps, water and wood is never a good combination. If you can direct water away from it, that’s always the better option.


Another less-practised method for keeping water out is a waterproof membrane. While we offer a 10-year anti-rot policy on most of our buildings, we cannot guarantee they will be completely waterproof as wood is a natural material. A waterproof membrane will be a pricey investment but certainly one that will help if you think your shed might in danger of condensation.

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