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“The sweating house”
Condensation damp: how to identify and fix it
It is the most common cause in Portugal. Warm, humid air touches a cold surface and the water condenses — in the corners, behind furniture, on the windows. In most cases it can be fixed without building work.
Signs
- Black mould spots in the upper corners and behind furniture
- Droplets on the windows in the morning
- Worse in winter and at night
- Strongest in bedrooms and bathrooms
cold zone: corners and window
Most common causes
Poor ventilation, laundry dried indoors, gas heaters, aluminium windows without a thermal break and uninsulated walls (about 70% of buildings date from before 1990).
Treatments and observed prices
- Airing, dehumidifier, moisture absorbers€20–300
- Positive input ventilation (PIV / VMI) in an apartment (estimate)€800–2,500
- Mechanical ventilation (MVHR / VMC)≈ €2,500
Sources: Habitissimo, Zaask, cume.pt, customer reports. Collected on 02.10.2026.
Read next
Mould in the bedroom: why it appears and how to get rid of it
Dehumidifiers: which one to buy and how many litres you need- Drying laundry indoors without causing mould
Written by the humidade.pt team · Reviewed by a civil engineer (to be confirmed) · Updated October 2026