Mould and damp in a dormer: 2026 guide
Mould in a dormer is almost always a symptom of an underlying building-physics issue, not poor cleaning. Four main causes: insufficient ventilation (50 percent of cases), thermal bridges at connections (25 percent), small leakage or condensation through wrong vapour barrier (15 percent), and old or damaged insulation holding moisture (10 percent). Fighting with mould cleaner without removing the cause works 3 to 6 months, then mould returns. A structural approach costs €350 to €2,500 depending on cause but prevents years of cleanup and health risks for occupants (airway issues, allergy, poor sleep).
- Share ventilation problems
- approx. 50 percent
- Share thermal bridges
- approx. 25 percent
- Share leakage/condensation
- approx. 15 percent
- Share aged insulation
- approx. 10 percent
- Optimal humidity
- 40 – 60 percent
- Mould risk threshold
- >70 percent for days
- Structural fix cost
- €350 – €2,500
Recognising mould: types and spots
Black dots or stains on cold surfaces (ceiling corners, behind furniture against cold walls, around window frames) are the most common dormer mould. Usually Cladosporium or Aspergillus, harmless in small amounts but with prolonged exposure causing complaints (airway irritation, headache, fatigue).
White or grey fluffy patches (surface looks like frayed cotton) indicate Aspergillus or Penicillium and appear on damp walls or in insulation packs. Heavier forms producing active spores that immediately cause complaints in allergic occupants.
Discolouration without fluff (yellow, brown) is usually moisture marks or nicotine absorption, not always mould. Test by lightly wiping with a damp cloth: mould wipes off slightly and leaves traces; moisture marks stay. When in doubt: mould test strip (€25-€50) or air quality survey by certified consultant (€350-€600).
Cause 1: insufficient ventilation
Most common cause is too low ventilation rate. A well-insulated dormer without or with under-sized vent sees humidity rise to 70-85 percent at night and in winter. At these values for several days mould forms within 4-8 weeks.
Diagnosis: measure humidity with a hygrometer (€15-€25). Above 65 percent in winter, structurally above 60 percent over a long period: ventilation shortfall. Check the vent is open, dust-free, and mechanical extraction (bathroom/WC in same home) runs.
Action: replace vent with larger capacity (€150-€350) or upgrade to acoustic model for quiet bedroom (€250-€550). For persistent issues: targeted mechanical extraction for the dormer room (€450-€900). Always combine with behaviour change: no drying clothes in room, bathroom door open while showering, regular airing.
Cause 2: thermal bridges
Thermal bridges are spots where insulation is broken (wooden rafters, connection to existing roof, wall-to-ceiling corner). Wall temperature is lower here, so indoor moisture condenses and mould forms — often visible as vertical strips or edges.
Diagnosis: infrared camera survey by building physics adviser (€350-€600) shows thermal bridges clearly. DIY: feel whether certain spots on wall or ceiling feel cooler than others; possibly measure with laser thermometer (€20-€40) on a cold winter day.
Action: local re-insulation inside or outside. Inside: 40-60 mm PIR insulation panel with vapour barrier on the problem zone, with finishing plasterboard (€350-€800 for 2-3 m² zone). Outside: thermal bridge strip in the facade construction — requires dormer opening, usually only on wider renovation (€800-€2,000).
Mould remediation: do and don't
Do: small mould patch (<0.5 m²) DIY with household chlorine 1:10 diluted (€2 per litre) or specialist mould cleaner (HG Mould Stain Remover, €8-€12). Work with gloves, windows open, rinse with clean water after 15 minutes. Then remove the cause (see above), else mould returns.
Don't: paint over mould without first cleaning. Mould grows through new paint within 2-6 months. Anti-mould paint (Sigma Sigmaplane Hydra Antimould, €45-€70 per tin) only works after thorough cleanup and simultaneous cause action.
Call a pro for: stains larger than 1 m², mould in insulation pack (often behind wall finish), or with health complaints from occupants. A specialist remediation firm charges €450-€1,200 for full dormer-room cleanup, including HEPA-filter extraction and warranty.
Keep photos of mould spots and all invoices. In rented housing the landlord is responsible for structural fix if cause is in the construction. On purchase within 1 year of reported mould: notify the seller for possible liability.
FAQ
- Why does mould keep coming back?
- Because only visible mould is cleaned, not the cause. Mould regrows within 3-6 months if humidity stays above 60 percent or thermal bridges aren't addressed.
- What's the optimal humidity to prevent mould?
- 40-60 percent. Above 65 percent for days mould forms; below 30 percent occupants get dry mucosa. Measure with hygrometer (€15-€25).
- Is mould harmful to health?
- Yes, especially for allergic occupants, children and elderly. Complaints: airway irritation, headache, fatigue, poor sleep. Long exposure can trigger asthma.
- Can I clean up mould myself?
- Patches under 0.5 m²: yes, with chlorine solution or mould cleaner. Larger patches or insulation mould: call specialist (€450-€1,200).
- Does anti-mould paint work?
- Only as aftercare after thorough cleanup and cause action. On a wrongly maintained construction mould grows through the paint within months.