Knowledge Base > Breathability Breathability and Insulation Breathability: A red herring? A study into the impact of breathabillity on condensation, mould growth, dust mite populations and health. the main findings were: If air-borne moisture is allowed to build up within a building it may lead to surface condensation, mould growth and exacerbated dust mite populations, which may in turn lead to an increase in health problems. Air-borne moisture is transported in and out of buildings by two mechanisms, water vapour diffusion through the roofs, walls and floors of a building (breathability) and bulk air-exchange (intended ventilation plus air-leakage) in and out of the building. Assuming a bulk air-exchange rate of 0.5 air changes an hour (the minimum considered to be healthy), ventilation accounts for 95% of the vapour transfer from a house with breathable walls. If the bulk air-exchange rate is increased to a level that maintains the internal relative humidity at 65% (the ideal level to control mould and dust mites), bulk air-exchange accounts for 96.7% of the vapour transfer from a house with breathable walls. Vapour diffusion (breathability) does not make a significant contribution to the rate of vapour transfer from a house. Breathable constructions and the breathability of insulation products are therefore at best a side show, in reality they are a complete red herring in the avoidance of surface condensation, mould growth and exacerbated dust mite populations. Bulk air-exchange (intended ventilation plus air-leakage) is at least 19 times more important than breathability in controlling air-borne moisture, surface condensation, mould growth, dust mites and consequent health problems. Download the full breathabilityTechnical Bulletin.