Building envelope unsealed: High heating costs
Even very small leaks in the vapour retarder layer – such as those that arise due to faulty adhesion between membrane overlaps or joints – have far-reaching consequences. This type of weakness has the same effect as a continuous gap between the window frame and the walls – and of course nobody would tolerate such a gap!
Accordingly, gaps in the vapour retarder should be given the same attention.
Sealed building envelope: Low costs and living comfort
The higher heating costs caused by faulty seals lead to reduced cost-effectiveness of the thermal insulation for the building owner.
A study by the Institute for Building Physics in Stuttgart (D) has shown that the U-value of a thermal insulation structure is reduced by a factor of 4.8. ¹⁾
When applied to a practical case, this means that the same amount of energy is required for heating a house with a living space of 80 m² (96 yd²) where airtightness leaks are present as would be required for an airtight house with a floor area of approx. 400 m² (480 yd²).
The uncontrolled entry of air can also have a negative impact on the indoor environment, e.g. due to draughts, excessively dry indoor air in wintertime, or fast, strong heating-up in summertime.
Only a gap-free thermal insulation structure provides the full insulation value
According to a survey in the year 2000, buildings in Central Europe consume 22 l of oil/m² (220 kWh/m²) of living space for room heating on average; a passive house requires only 1 l, while a 3-litre house uses 3 l of oil/m², as the name suggests – assuming that the airtightness is perfect. Gaps in the airtightness layer of buildings lead to an increase in the energy requirement per square metre of living space.
The Institute for Building Physics in Stuttgart (D) has studied a 1 x 1 m sized structure with a thermal insulation thickness of 14 cm.
With a joint-free, airtight design, the previously calculated thermal performance of 0.30 W/(m²·K) was confirmed. However, if the same structure features only a 1 mm wide gap in the airproofing layer, the U-value deteriorates to 1.44 W/(m²·K). This means almost 5 times more heat is lost than with the airtight construction.