The air-tightness of the thermal envelope of buildings is one of the measures to reduce their
energy demands in order to achieve global warming reduction targets. To this end, airtight sheets
with different water vapour permeability characteristics are used. The different products studied are
highly dispersed in terms of equivalent air thickness values, leading to confusion. After the analysis
carried out, it is concluded that all airtight sheets are vapour barriers. To clarify whether or not
these sheets are necessary as vapour barriers, a condensation analysis was carried out on 13 different
facades for 3 climate zones with severe winters as defined in Spanish regulations. The results reveal
that interstitial condensation occurs in only 7 of the 39 case studies, with the traditional facades
of brickwork with render causing the greatest problems if the appropriate products are not used.
In these cases, airtight sheets with water vapour barrier characteristics must be applied on the interior
face of the insulating material. In all other cases (32), the airtight sheets must be permeable to water
vapour if it is looked for a more breathable wall to water vapour and a better control of the interior
humidity conditions