Black mould in a bedroom corner, where the wall meets the ceiling or behind the wardrobe, is rarely chance. These are the coldest spots on the wall, and water vapour in the air meets the cold and turns to liquid. This guide explains what a thermal bridge is, why corners suffer first, what you can do without building work and what only insulation work can fix.
Updated 4 November 2026. General information, not a substitute for an assessment by a qualified specialist.
The essentials
- The SCE guide (Portugal’s energy certification scheme) defines a linear thermal bridge as the junction of two external building elements, a point in the building envelope where heat flow is no longer simple. It says these cases are impossible to eliminate, but their effect can be reduced with better construction solutions.
- DECO PROteste (21 March 2026) writes that mould in corners and near windows may point to thermal bridges and insufficient insulation.
- The UK health guidance (DHSC) names corners and low points on walls behind sofas or wardrobes as places where condensation occurs.
- The FOPH (Switzerland) asks for 10 cm between furniture and external walls and does not want large cupboards pushed against cold outside walls.
- Moving furniture, ventilating and heating reduce the problem. If the wall stays cold, only more insulation changes that. Paint does not fix it.
What a thermal bridge is
The SCE guide (13 March 2020, point 3.6) explains that a linear thermal bridge joins two external elements and is measured by a coefficient ψ per metre of length. In the guide’s worked example, the linear bridges include the junction of the façade with the floor and with the roof, two walls meeting at an outward corner, the window frames and the roller-shutter box.
Portaria no. 138-I/2021 also defines “flat thermal bridges”: irregularities in the main area of the wall, such as beams, pillars, gutters or shutter boxes. Picture a concrete beam inside a brick wall: it is colder than the brick around it, and the stain follows the shape of the beam. That example is ours, as an illustration.
The SCE guide dates from 2020 and refers to 2013 orders for the ψ values. We did not check the regime now in force for those values.
Why it shows up in corners
The EPA (United States) explains the mechanism: in cold weather, moisture condenses on cold surfaces, and that can encourage mould. The DHSC adds that moisture produced indoors cools and condenses on the colder parts of the building, and that external walls and windows are more prone to moisture and condensation when a home is poorly heated or poorly insulated.
The FOPH says mould appears in slightly colder places such as windows, window frames and external walls, and that the cause is not always a lack of ventilation. A corner of two external walls has two faces losing heat, and still air behind a piece of furniture carries no warmth to it. That explanation is ours: the sources we read say where mould appears, they do not calculate the corner’s temperature.
To tell a thermal bridge from a leak or rising damp, see damp walls: the three types. For vapour in general, see bedroom condensation.
What you can do without building work
| Measure | What the source says |
|---|---|
| Move furniture | FOPH: 10 cm from external walls; no large cupboards against cold walls. DHSC: leave a gap between furniture and external walls |
| Curtains | FOPH: no curtains in cold window reveals |
| Ventilate | FOPH: air for 5 to 10 minutes with a through-draught, at least three times a day; more often in old buildings with new windows and poor insulation |
| Heat | FOPH: in old, poorly insulated buildings, 20 ºC, without delaying the start of heating; 18 ºC in bedrooms only in well-insulated, well-ventilated buildings |
| Cut the vapour | DECO PROteste: ventilate often, switch on the extractor when cooking, reduce steam |
The guide to the bathroom and extractor fans covers shower steam. For dehumidifiers, see dehumidifier: when it helps.
Cleaning: existing mould is cleaned before any painting, with the precautions in mould on walls. The FOPH gives an example of a wrong repair: painting a damp area with anti-mould paint was not a proper repair, because the mould and the damp remained. See anti-damp and anti-mould paint: does it work?.
What needs building work
DECO PROteste says that where insulation is insufficient, it may be necessary to reinforce wall insulation and improve the window frames. The FOPH adds that before replacing windows or doing any insulation work, it is wise to have a building physics specialist assess the situation, considering the risk of damage at thermal bridges and checking the ventilation. Only then can you be sure the mould will not come back a year or two later.
On external insulation (ETICS), a review by LNEC researchers (2023) states that these systems reduce the thermal bridge effect and the risk of internal condensation. The same review notes that highly insulating, light-coloured systems tend to have more biological growth on the outer face, and that anomalies need watching. That is not an argument against insulating, but a reason to demand good design and good execution.
In a 2009 paper, LNEC already listed surface condensation among the anomalies caused by poor thermal quality of the building envelope, and noted that more than 70% of the housing stock predates 1990, the year of the first thermal regulation (RCCTE). That is a 2009 figure and we have not updated it.
If you live in a flat, the façade and external walls are common parts. See damp in the building: who decides and pays. If you rent, see painting before renting or selling.
Step by step
- Measure indoor humidity with a hygrometer; the FOPH gives 30% to 50% during the heating season.
- Move furniture and curtains away from external walls.
- Ventilate and heat steadily.
- Clean the mould with protection and ventilation.
- Watch through the winter whether the mould returns to the same corners.
- If it does, ask a building physics specialist for an assessment before paying for works.
- Only then paint; work out the quantity in the calculator.
What we could not confirm
- A minimum surface temperature or a limit value of ψ for existing homes: we did not read the regulation in force.
- The ITeCons/ADENE catalogue of thermal bridges: we did not open it.
- What it costs to reinforce insulation and which aid exists today for it.
- Whether a 10 cm gap is enough in each case: the sources give the rule, they do not measure it on site.
- Official Portuguese guidance (ADENE, LNEC) addressed to occupants on mould in corners: we found none.
Sources (read on 4 November 2026)
- ADENE/SCE, Guia SCE: Parâmetros de Cálculo, 13 March 2020, point 3.6 and the linear thermal bridge example.
- Diário da República, Portaria no. 138-I/2021, 1 July 2021, definition of “pontes térmicas planas”.
- DECO PROteste, Que tipo de humidade tenho em casa?, 21 March 2026.
- FOPH (Switzerland), Bolor em espaços habitáveis, February 2021.
- 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).
- EPA (USA), Mold and Moisture (archive of 19 January 2025; only the sentence on condensation on cold surfaces).
- Parracha, Veiga, Flores-Colen and Nunes (LNEC/IST), Toward the Sustainable and Efficient Use of ETICS, Buildings 13, 1664, 2023 (introduction and the section on surface condensation).
- Pina dos Santos, Rodrigues and Vilhena (LNEC), Reabilitação térmica no sector residencial, 2009 (introduction).