The air­ sealing lay­er pro­tects the in­su­la­tion from mois­ture and con­dens­a­tion from the in­side, en­sures that the in­su­la­tion works ef­fect­ively and provides a healthy in­door liv­ing en­vir­on­ment.

The thermal in­su­la­tion sep­ar­ates the in­door from the out­door cli­mate. The tem­per­at­ure dif­fer­ence between the two cli­mates at­tempts to be­come equal­ized through the flow of air. This means in winter the warm air from the build­ing trans­fers through the struc­tur­al ele­ments to the out­side. The air­ sealing lay­er pre­vents this air flow, the so-called con­vec­tion, and there­fore the loss of hot air to the out­side. The in­teri­or space is here not her­met­ic­ally sealed off - like us­ing a plastic bag - from the out­side air. In­stead, the ex­change of air from the in­side to the out­side con­tin­ues via dif­fu­sion.

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 and air­ sealing lay­er on the in­side of the thermal in­su­la­tion helps to avoid such struc­tur­al dam­age.