The thermal insulation separates the indoor from the outdoor climate. The temperature difference between the two climates attempts to become equalised through the flow of air. This means in winter the warm air from the building transfers through the structural elements to the outside. The airproofing layer prevents this air flow, the so-called convection, and therefore the loss of hot air to the outside.
If indoor air were to flow unrestricted through the thermal insulation, it would increasingly become cooler the farther it penetrates towards the outside until it finally emerges as condensation. Condensation may cause considerable damage to the building and its components. Load-bearing structural elements may rot and lose their strength.
Similarly, moisture also promotes the development of harmful mildew and mould.
A vapour retarding and airproofing layer on the inside of the thermal insulation helps to avoid such structural damage.