Dehumidifiers: do they really help with damp?

Yes, a dehumidifier helps lower the humidity of the air and reduces condensation and mould. But it does not fix a leak or damp rising from the ground. It works well as support, not as the only solution.

What a dehumidifier does

The appliance draws in the air of the room, removes water from it and returns drier air. The water collects in a tank, which you have to empty, or drains through a tube into a drain.

Less humid air means less vapour condensing on cold walls, windows and corners. That makes it harder for mould to grow.

When it helps

in homes with condensation in winter, especially in bedrooms and living rooms with little ventilation;
when you have to dry laundry indoors;
in bathrooms with no window;
in homes that are little used, such as second homes, where the air stays still;
after cleaning mould, to help the wall dry.

In damp areas, such as the Sintra mountains, or near the sea, in Cascais and Oeiras, it can be a good support during the dampest months.

What it does not solve

rain leaking in through a roof, a façade or a window;
a water leak from a pipe;
damp rising from the ground up the walls;
mould that is already established. You have to clean it.

If the cause is water getting into the wall, the appliance keeps drying the air, but the wall keeps taking in water. In that case, the source has to be repaired.

How to choose one

You do not need to know much. Look at these points:

Tank size: larger tanks need emptying less often.
Capacity: it should suit the size of your home. Read the guidance for each model.
Noise: if it is for the bedroom, choose a quiet model.
Continuous drainage: lets you connect a tube so you do not have to empty the tank.
Low-temperature operation: not all of them work well in very cold rooms.
Energy use: an appliance running for many hours uses electricity. Check the manufacturer's information.

How to use it well

Use it in one room at a time and keep doors and windows closed while it runs.
Leave space around the appliance so air can circulate.
Empty and wash the tank regularly, to avoid bad smells and mould.
Clean the filter, following the manufacturer's instructions.
Open the windows afterwards to renew the air.

Tip: A hygrometer shows the humidity of the air. Many appliances already have one built in. That way you know when to switch it off and do not waste energy.

Salt sachets and tablets

Small absorbent products, made with salt or another material, are meant for very small spaces, such as a cupboard or a car. In a living room or bedroom, their capacity is limited. They do not replace ventilation or a dehumidifier.

In summary

Use the dehumidifier as an aid against condensation, together with daily ventilation and a steady temperature. If the damp comes from the wall, a leak or the ground, the appliance is not enough. If the mould keeps coming back, ask a professional to assess the cause before painting.