Metal garden sheds can suffer from condensation on the roof & walls due to trapped water. Taking steps to increase ventilation and keep the interior dry can prevent condensation from coming back.
While most garden buildings are constructed from timber, metal sheds are becoming more common, particularly for storing tools and garden equipment. There’s plenty of reasons why. Metal sheds are often more cost-effective and much easier to assemble. Due to the increased durability of metal, no maintenance is required and there are no wood treatments to apply as would be needed with a timber shed.
It’s not all upsides when choosing a metal shed and there are some additional challenges to consider. One of those is condensation which is much more common in metal garden buildings. In humid conditions, the ceiling of a metal garden shed can often be covered in water. Leaving condensation for an extended period can result in damage to the items inside the shed and may even lead to mould growth on some materials.
Thankfully, condensation and damp issues in a metal shed can be resolved easily by taking practical steps to improve air flow and reduce moisture.
What is condensation?
I’m sure we’ve all seen condensation before, typically near a cold window during the winter, resulting in water collecting on the glass and around the frame. Steam from a hot drink placed next to a window and leading to water on the glass is another common example.
Condensation occurs when water vapour in the air turns back into liquid water. The water droplets then sit on the surface and can build up as more condensation occurs.
In garden buildings, condensation usually happens due to the air being cooled to its ‘dew point’. In some environments (e.g. a hot tub room), condensation can occur due to their air being so saturated with water vapour that it cannot hold onto any more water.
The dew point is the temperature at which condensation takes place. This is a colder temperature than the current room temperature, causing the vapour in the air to cool as it comes into contact with the colder temperature item and leading to water droplets.
Condensation should not be confused with evaporation, which is the opposite process in the water cycle. During evaporation, water droplets on the surface turn to water vapour in the air as the temperature heats up. Condensation takes place later on as the temperature on surfaces begins to cool again.
Why does a metal shed get condensation?
Even though condensation can occur in any garden building, it’s most common in metal sheds due to their structure.
During the day, as the outside temperature rises, so does the temperature inside a metal shed. This leads to warm moist air beginning to rise and the temperature will be similar to the conditions outside. During the evening, the temperature outside quickly cools, with warm air rising. The warm moist air on the inside of the shed has nowhere to go as it is stopped by the roof & walls.
Metal is a very good conductor of heat, so as the outside temperature cools, so does the surface temperature of the roof and walls of the shed. This results in the ‘dew point’ being reached on the metal surface, and the moisture in the warm trapped air forms condensation leading to water droplets appearing. The condensation is most noticeable on the shed roof and in extreme cases can also be seen on the walls.
Where does the water come from?
Condensation in a metal shed is a sign of too much moisture inside and it can come from a number of different sources. The most common reason is using a concrete base underneath with nothing else on the floor. Moisture from the ground below can slowly work its way through the concrete and rise from the surface into a metal shed. Without a vapour barrier in place, a concrete base can be a constant source of damp issues.
The other common reason is placing slightly damp items inside the shed and then locking the doors and windows. Items such as lawnmowers after cutting wet grass or garden furniture caught in the rain can hold onto water when returned to the shed. As the temperature inside rises, the water will evaporate into the air and eventually form condensation as the temperature inside cools back down.
What about timber sheds?
Timber sheds are less prone to condensation when compared to metal sheds. This is due to the thicker (at least 8mm) wood acting as insulation between the changing temperatures. Metal sheds are often much thinner and also conduct heat better.
The thicker walls on a timber shed won’t hold onto a huge amount of heat but are thick enough to stop the interior side of the wall from cooling down as quickly. Timber construction also has less airtightness, allowing air to move around more freely and fresh air to replace the escaping warm moist air inside.
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How to stop condensation in a metal shed
To prevent condensation in a metal shed we need to consider both ventilation and keeping the interior dry. A combination of both is enough to remove condensation and stop it from coming back.
Plenty of ventilation allows air to escape the shed and prevent the build-up of humidity. Air circulating correctly results in a constant supply of fresh air into the shed.
Installing air vents is one of the lowest cost and most effective solutions to reduce condensation. If the shed already has air vents installed, they should first be inspected to ensure they are not blocked and can open correctly. If the shed does not have any air vents already, they can be installed by cutting through a small section of the metal and attaching them to the wall.
Passive air vents allow fresh air to naturally flow into the building. For a medium to a large shed, we recommend placing two vents on opposite sides to each other. One should be placed at the bottom of the wall and the other near to the top of the opposite wall. This system will create a natural wind tunnel, allowing cool fresh air to be pulled in from the bottom and warm moist air to exit on the opposite side.
With several air vents in place, it will be difficult for the atmosphere inside the shed to hold onto enough moisture for condensation to form. Electrical fan-operated vents can be used to increase airflow further, but these are usually overkill for a metal shed.
Metal shed roofs do not take on water, instead, the droplets remain on the surface. A number of anti-condensation products are available to prevent sitting water. One option is to retrofit anti-condensation roofing sheets. The sheets contain a membrane that will absorb water and store it while the weather is cold. Once the temperature warms up, the water evaporates and the membrane drys out.
Anti-condensation paint is common in basements to reduce the feeling of damp. The paint is much thicker than regular paint and works by providing an extra layer of insulation to stop the surface temperature from getting as cold and also blocking mould growth. The reality is that it’s likely to do little to prevent significant shed condensation but may help for smaller condensation issues.
Insulating a garden building is always a great option to provide a more comfortable atmosphere inside. The insulation reduces condensation by stopping the warm moist air inside the shed from touching the cold metal surface. Insulation in between prevents heat transfer and stops the air from touching a surface cold enough for the ‘dew point’ to occur.
We usually recommend rigid foam insulation boards in a wooden shed, but as a metal shed does not have the timber framing/battens on the inside, there’s nowhere for them to sit in between. Instead, a metal shed uses a rail system with nowhere to attach to.
Spray foam insulation is the best solution for a metal shed. The insulation is applied by spraying it against the walls and ceiling, where it will dry into a hard foam. Polyurethane spray foam provides excellent insulating properties and is also airtight to ensure there’s nowhere the warm and cold air can touch.
Using insulation to resolve condensation can be an expensive solution. Insulation is best suited to when the metal shed will get other benefits from the heat insulation such as if it is being used as a workshop.
Keeping the interior of a metal shed dry can prevent moisture from building up. There are plenty of places that water can get in including from wet garden furniture and through the base.
A common reason for condensation in a metal shed is not because of the shed itself, but instead the design of the base. A concrete base is great for a garden shed, but if there’s nothing to stop moisture rising from the base it can lead to condensation. When a concrete base is first installed, it should be left for several weeks to completely dry out before installing a shed.
The challenge with concrete is that it is partially porous and can allow moisture to work its way through. When used as a base, the concrete will absorb water from the ground below. The moisture will leave through the top of the base as it evaporates, resulting in water inside the shed. As the water cools again on cold surfaces, condensation is formed.
To stop a concrete base from causing damp issues, a vapour barrier should be installed in the base just above the height of the surrounding groundworks. A vapour barrier is a polythene sheet that prevents water from passing through it, eliminating any water from rising into the shed. If the shed is already installed on a concrete base, consider a new floor that allows airflow underneath the shed and prevents water from rising.
Seal the bottom
Water could be getting into the shed from the floor and causing condensation. The shed should be positioned in a location slightly higher than the ground around it. This prevents water from puddling in heavy rain and finding its way inside. Even small gaps in the floor can lead to water getting in and causing issues.
If the shed is placed directly onto a concrete base, silicone sealant can be used around the edges of the shed to stop water from getting in from anywhere. We don’t have to worry about blocking air flow if we have sufficient air vents further up the wall, where water can’t get in at the same time. The edges of the shed should be sealed enough to stop water marks from forming on the sides of the floor if it’s raining.
If condensation and moisture are causing a significant issue in a metal shed, a dehumidifier can be used. When a dehumidifier is turned on, the air is passed through the system and some of the moisture is removed. This will be contained inside a collection tank in the dehumidifier that will need emptying periodically. The metal shed will require electricity to use a dehumidifier so it would not be an option for everyone.
On its own, a dehumidifier can be expensive to run for long periods. Instead, it should be used periodically or on a timer and combined with other methods to reduce condensation. We recommend trying other options before using a dehumidifier to remove condensation.
Overall, condensation in a metal shed is a common problem that can be easily reduced by taking practical steps. Metal sheds are prone to condensation due to the surface temperature of the metal structure cooling fast, reaching the dew point for condensation. Left untreated for an extended period, condensation could lead to damp issues or damage to the equipment stored inside.
Condensation is easy to resolve with a focus on increased ventilation and keeping interior items dry. A simple solution is to add more air vents to allow air to circulate easily and reduce the build-up of humid air. To keep interior items dry, a concrete base should use a vapour barrier to prevent moisture from getting into the shed from underneath.