It´s almost the same principle as seen around the windows & doors: Air Tight Membranes prevent draught and heatloss and protect against structural damage, caused by condensation.
Air Tight Membranes have to be installed on the shell of the building - mostly in pitch roof areas or at the highest ceiling. Timber framed houses should get air tight membranes on the entire internal building envelope.
Some facts about air tight membranes:
We´ve been often asked regarding an air tightness strategy for structural boards like OSB or plywood. Due to the high density of such structural boards, these boards will have the required air tightness performance in the most cases, but attention is needed at all junctions, joints and penetrations. They definitely need to be taped or sealed with suitable air tightness products like air tight tape, air tight sealant, air tight foam or air tight paint.
The official declination for an air tight product means, that the product must have an air permeability of less than 0.1m³/m²h at 50Pa differential pressure. Some thinner OSB boards of certain manufacturers may perform worse in reality. Please check the specs prior the installation and allow for an additional coat with air tight paint if the air tight performance of the board is not sufficient.
We always recommend a preliminary air tightness test (Blower Door Test) during the construction stage.
Based on our experience, the test should be done, once the air tight layers have been completed (e.g. all windows & doors taped or sealed plus air tight membranes completely installed). The test should be treated rather as a quality control than an official air tightness test with values. It´s more important to take the time to detect existing leakages instead of gambling with current values for air permeability and air change. Such values will be always much higher in comparison to a final air tightness test result.