[MEP Center Engineering]

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An apartment building was designed to the national thermal insulation code and the external walls meet the required U-value. After the first winter, occupants report condensation and black mould on the inside of external walls, mainly at the junctions with concrete columns and beams, around window reveals and in the corners of bedrooms.

If the U-value complies, why does condensation still appear? How should I check the risk properly, which values should the surface reach, and what are the practical remedies on the building side and on the ventilation side?

[MEP Center Engineering]

A compliant U-value describes the average or clear-field wall, but surface condensation and mould start at the coldest point: thermal bridges, corners and reveals, combined with high indoor humidity. Check the temperature factor f_Rsi of every critical junction against the value needed for your indoor humidity, then fix the bridges and control humidity with ventilation.

1. The criterion: temperature factor f_Rsi

f_Rsi = (θ_si − θ_e) / (θ_i − θ_e), where θ_si is the internal surface temperature (EN ISO 13788, with R_si = 0.25 m²K/W for this check). Mould is assessed at 80 % relative humidity at the surface, visible condensation at 100 %.

2. Worked example

Indoor 20 °C and 50 % RH, outdoor −5 °C.

• Vapour pressure indoors: 0.50 × 2337 Pa = 1169 Pa → dew point ≈ 9.3 °C.
• Mould limit: the surface may not be colder than the temperature at which 1169 Pa is 80 % of saturation (1461 Pa) → ≈ 12.6 °C.
• Required f_Rsi,min = (12.6 − (−5)) / (20 − (−5)) = 17.6 / 25 ≈ 0.70.

Clear wall with U = 0.40 W/m²K: f_Rsi = 1 − U·R_si = 1 − 0.40 × 0.25 = 0.90, θ_si = −5 + 0.90 × 25 = 17.5 °C. No problem.

Uninsulated column or beam face with f_Rsi = 0.60: θ_si = −5 + 0.60 × 25 = 10.0 °C. Above the 9.3 °C dew point, but below the 12.6 °C mould limit: mould, without visible water.

If the indoor humidity rises to 60 % RH, the dew point becomes ≈ 12.0 °C and the mould limit ≈ 15.4 °C: the same junction now runs wet. Humidity matters as much as insulation.

3. Remedies on the building side

• Insulate column, beam and slab-edge faces continuously with the wall; on existing buildings external insulation is the most effective cure.
• Insulate window reveals and set frames in the insulation plane.
• Treat balcony and canopy slabs with a thermal break or wrap them.
• Model critical junctions to EN ISO 10211 to get f_Rsi and the ψ-value; many national guides use f_Rsi ≥ 0.70 to 0.75 for dwellings.

4. Remedies on the heating and ventilation side

• Ventilate to remove moisture. Example: a household releasing about 10 kg/day (≈ 420 g/h), outdoor air at −5 °C and 90 % RH (≈ 2.9 g/m³). To hold 20 °C and 50 % RH (≈ 8.6 g/m³) requires 420 / (8.6 − 2.9) ≈ 73 m³/h of outdoor air; at 60 % RH about 56 m³/h. Continuous extract from kitchen and bathrooms with trickle vents in habitable rooms, or balanced ventilation with heat recovery, provides this reliably. Rates per EN 16798-1 or ASHRAE 62.2 as your code requires.
• Place emitters under windows, and keep furniture and curtains a few centimetres off cold external walls.
• Avoid deep night set-back or unheated bedrooms; a cold room with doors closed collects moisture from the rest of the flat.

Common mistakes

• Checking only interstitial condensation (Glaser) and never the surface factor.
• Accepting the code U-value for the wall and ignoring columns, beams and reveals.
• Painting over mould without changing ventilation or insulation.
• Sealing windows in a refurbishment without adding any controlled ventilation.

Related: ASHRAE 62.1 vs EN 16798: Ventilation Rates Compared, Radiator Sizing: Correction Factors. For deeper study: HVAC Systems Design Course.

Check the edition of the code that applies to your project.

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