Internal insulation of a solid brick wall: U-value, dew point and Klimabonus
A 1965 detached house in Luxembourg-Ville has a 30 cm solid brick wall (lambda 0.58 W/mK), 15 mm cement render outside and 10 mm gypsum plaster inside. The owner wants internal insulation to reach a U-value of about 0.24 W/m2K using 100 mm mineral wool (lambda 0.035) on a stud frame, without a vapour barrier, and to keep the existing natural ventilation by opening windows. Winter design conditions are 20 C indoor, 50% RH and -5 C outdoor. The house is heated by a gas boiler and the owner applies for Klimabonus. The architect worries about interstitial condensation at the masonry/insulation interface and thermal bridging at the floor slab. The owner says the wall has always been dry and the cement render is rain-tight.
1. Calculate the U-value of the proposed wall and state whether it meets the 0.24 W/m2K target.
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Using standard surface resistances Rsi = 0.13 and Rse = 0.04 m2K/W: gypsum plaster 0.010/0.25 = 0.040; brick 0.30/0.58 = 0.517; cement render 0.015/0.80 = 0.019; mineral wool 0.100/0.035 = 2.857. Sum = 0.13 + 0.040 + 0.517 + 0.019 + 2.857 + 0.04 = 3.603 m2K/W. U = 1/3.603 = 0.278 W/m2K.
The proposal does not meet 0.24 W/m2K. To reach 0.24, the required total R is 1/0.24 = 4.167 m2K/W, i.e. about 0.56 m2K/W more. With the same mineral wool that means roughly 120 mm rather than 100 mm, or a lower-conductivity insulation. The architect should also count repeat thermal bridges at studs and floor slabs; a nominal 0.24 calculated in the field may be worse in practice. The target should be treated as a whole-wall value, not a nominal insulation value.
2. Assess the interstitial condensation risk with numbers, not just a general warning.
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The temperature at the cold side of the insulation can be estimated from the resistance from indoor air to that point: 0.13 + 0.040 + 2.857 = 3.027 m2K/W. The fraction of the total temperature drop is 3.027/3.603 = 0.84. With 20 C inside and -5 C outside, the interface temperature is 20 - 0.84 x 25 = -1.0 C. The dew point of indoor air at 20 C and 50% RH is about 9.3 C. The interface is therefore far below the dew point, and below freezing.
With no vapour barrier, water vapour diffusing from the warm side will reach that cold interface and condense or freeze in the mineral wool. The cement render limits outward drying, so the wall cannot be assumed to dry safely to the outside. The owner's 'always dry' observation is not a moisture analysis: the wall was previously warm on the inside. The proposal needs a continuous airtight layer and a vapour-retarder strategy on the warm side, or a change to external insulation. Convection through gaps in the stud frame would make the problem worse than diffusion alone.
3. What assembly and ventilation strategy would you recommend, and what does Klimabonus require in principle?
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Assuming external insulation is not possible, I would use a continuous airtight layer on the warm side, a variable-resistance vapour retarder behind the lining, carefully taped joints and service penetrations, and mineral wool or wood-fibre insulation with no unventilated air gaps. The floor-slab junction should be broken with perimeter insulation and an airtight membrane; the studs should be minimised or thermally broken. The wall should be detailed to dry inward, but the first defence is to keep indoor moisture out of the insulation. I would also check whether the cement render should be replaced with a vapour-open render if exterior work is later done.
Because the retrofit reduces natural infiltration, I would add controlled mechanical ventilation with heat recovery, with a moisture-sensitive boost in bathrooms and kitchen. Klimabonus support is conditional on meeting the programme's U-value, airtightness and installer-certification requirements and on producing the required energy certificate; the exact rates change, so the owner should check the current conditions before ordering. The key point is that the subsidy does not make a risky assembly acceptable: the moisture-safe detail comes first.