Windows are, in terms of energy, the weakest point of a building. This applies to uninsulated existing buildings as well as to new-builds, even though in this case, the energy standards for solid and transparent building elements are considerably higher.If, in a renovation, only the windows are replaced (that is, new windows with a lower U-value and more airtight joint seals are installed), this can result in the exterior walls, the reveals or other cold bridges being less insulated than the windows themselves. Without changing the user’s ventilation habits, the growth of mould around these building elements is not uncommon. In these cases, it is worth considering the installation of ventilation systems, or vents in the window reveals.