Do you wake up to wet windows, a dark patch behind the headboard or a musty smell in the wardrobe? It is almost always condensation: water vapour in the indoor air turning to liquid on a cold surface. This guide brings together what Portugal’s health directorate (DGS), the housing energy regulation, the Swiss public health office and the consumer body DECO PROteste say about ventilating, heating, moving furniture and using (or not using) a dehumidifier.
Updated 3 November 2026. General information, not a substitute for an assessment by a qualified specialist.
The essentials
- The DGS writes that excess humidity can cause condensation on cold surfaces, especially walls, and that this can lead to mould.
- The Swiss Federal Office of Public Health (FOPH) recommends 30% to 50% relative humidity in the heating season (no more than 40% on very cold days) and airing at least three times a day, with windows open for 5 to 10 minutes.
- On heating, the FOPH asks for 20 ºC in old, poorly insulated buildings and accepts 18 ºC in bedrooms only in well-insulated, well-ventilated ones.
- A dehumidifier is a complement (DECO PROteste). The FOPH says to never switch one on before the mould has been removed.
- If mould shows in corners or by windows, DECO PROteste points to thermal bridges or too little insulation and suggests better insulation and window frames.
Where the vapour comes from
The WHO (2009) lists the sources of moisture indoors: cooking, cleaning, bathing and showering, dishwashing, washing and drying laundry, breathing and perspiration, animals and plants. Someone sleeping in a closed bedroom gives off vapour all night. DECO PROteste adds poor ventilation, thermal bridges and poor insulation as causes of condensation.
Vapour only turns to water where a surface is cold: corners of outside walls, glass, window frames. To tell condensation from rising damp or a leak, see damp walls: the three types.
Ventilation: what the sources say
| Measure | What the source says | Source |
|---|---|---|
| Airing | At least 3 times a day, with a through-draught, all doors and windows open for 5 to 10 minutes. If you are out all day: in the morning, on return and before bed | FOPH, 2021 |
| Tilted window | Avoid leaving tilted windows open permanently while the heating is on | FOPH, 2021 |
| Cooking, bathing, ironing | Ventilate or switch on the extractor; repair broken extractors and clean them | FOPH, 2021 |
| After bathing | Open the window wide, close it once the air has changed; keep the room door shut while showering or cooking | DHSC (UK), 2026 |
| Window vents | Should be controllable by the occupant and kept unblocked | DHSC, 2026 |
| Laundry | Avoid drying in rooms with damp problems; if unavoidable, ventilate | FOPH, 2021; DECO PROteste, 2026 |
The FOPH warns that new windows in an old, poorly insulated building call for airing more often. The DHSC notes that energy-efficiency works can reduce ventilation, so it should be checked afterwards.
And the law?
The energy-performance regulation for homes (Portaria 138-I/2021 of 1 July, annex point 1.2.1) requires a minimum hourly air renewal rate of 0.50 per hour, calculated with the SCE Manual method. The table covers new buildings, major renovations and renovations of the ventilation system. Where only the windows of the main rooms are replaced, it requires air inlets in the façade (60 cm² free area) or self-regulating vents with a stated minimum flow. This is a design requirement, not a rule about how often to open a window.
Heating
The FOPH says old buildings with cold, poorly insulated walls are the most delicate: do not delay switching the heating on, and keep 20 ºC. In well-insulated, well-ventilated buildings, lowering bedroom temperature to 18 ºC is not a problem. The DHSC says radiators work best on outside walls and under windows. SPAB (a British conservation charity) advises avoiding portable gas heaters, which release vapour.
We did not find an official Portuguese source giving a minimum bedroom temperature.
Furniture and curtains
According to the FOPH, leave 10 cm between furniture and outside walls, do not push large wardrobes against cold outside walls, and keep curtains out of cold window reveals. The DHSC gives the same advice about a gap between furniture and outside walls.
Dehumidifier: when it helps
DECO PROteste (21 March 2026) presents it as a complement to ventilation, to keep relative humidity under control. In an 8 April 2026 guide the same body recommends an extraction capacity above 16 litres a day for areas over 20 m², and notes that a unit too small for a large room runs longer for the same result, which uses more electricity. Shop prices quoted in that guide are around 160 to 190 euros (DECO PROteste’s figure, not ours). The detailed results are paywalled and we did not read them.
DECO PROteste tests units in a laboratory (page of 6 December 2024): water extracted over 24 hours under four temperature and humidity conditions, how fast humidity falls from 80% to 60%, efficiency, noise and humidistat accuracy.
The FOPH adds two cautions: never run dehumidifiers before the mould is removed, and do not use humidifiers in problem homes. For mould, see mould on walls.
A step-by-step plan for a bedroom with condensation
- Measure with a hygrometer. Aim for 30% to 50% (FOPH).
- Air the room for 5 to 10 minutes, 3 times a day, with a draught. Avoid leaving a tilted window open with the heating on.
- Cut the vapour: bedroom door shut while showering, extractor on, lids on pans, laundry out of the bedroom.
- Move furniture 10 cm off outside walls.
- Heat steadily, without letting outside walls get very cold (FOPH).
- Clean the mould before running a dehumidifier, with the precautions in the mould guide.
- Watch for at least 6 weeks (the period the DHSC uses to check whether a problem has returned). If the stain comes back, get a diagnosis: it may be a thermal bridge, a leak or rising damp, as explained in rising damp treatment and the guide to the three types.
- Only then paint. Paint does not fix a cause: see anti-damp and anti-mould paint: does it work? and which paint for each room. For quantities, use the calculator.
If you rent and the problem comes from the building, see also painting before renting or selling.
What we could not confirm
- A number of air changes per hour for existing homes with no works. The Portaria we read sets 0.50 h⁻¹ for the contexts in its table; we did not check how it applies to a home already built and not being renovated.
- ADENE or LNEC guidance for occupants on bedroom condensation: we found no specific text.
- A recommended minimum bedroom temperature in Portugal.
- Which dehumidifier is worth buying: the full DECO PROteste results are behind a paywall.
- How long humidity takes to fall with these measures: no source we read gives a timescale.
Sources (read on 3 November 2026)
- Direção-Geral da Saúde, Qualidade do Ar Interior (sentence on humidity and condensation on walls). Undated page.
- Diário da República, series 1, no. 126, 1 July 2021, Portaria 138-I/2021, point 1.2.1 and Table 10 (read in the PDF).
- FOPH (Switzerland), Bolor em espaços habitáveis, Portuguese version, February 2021 (sections on room humidity, ventilating, heating, furnishing; warnings on dehumidifiers).
- DECO PROteste, Que tipo de humidade tenho em casa?, 21 March 2026.
- DECO PROteste, Desumidificadores grandes: melhores modelos, 8 April 2026 (open part; paywalled results not read), and Como testamos, 6 December 2024.
- World Health Organization (Europe), WHO guidelines for indoor air quality: dampness and mould, 2009 (page; moisture-sources figure and text on moisture management).
- Department of Health and Social Care (UK), Understanding and addressing the health risks of damp and mould in the home, updated 1 April 2026 (first 100,000 characters of the page; annexes not read).
- SPAB, Control of Dampness, technical advice note by Douglas Kent, PDF, no printed date (file modified 11 August 2020); only the sentence on gas heaters, in the context of British pre-1919 buildings.