Ventilation: Approved Document F, Volume 1 — Dwellings

O N L I N E V E R S I O N B B10 For the purposes of this approved document, for all dwellings (both new and existing, where Part F applies), the moisture criteria are likely to be met if, during the colder months of the year, the moving average surface water activity of the internal surfaces of external walls is always less than the value in Table B2, evaluated over each moving average period. Table B2 Surface water activity Moving average period Surface water activity 1 month 0.75 1 week 0.85 1 day 0.95 B11 For new dwellings, for the purposes of this approved document, the moisture criteria in Table B2 is likely to be met if, during the colder months of the year, the moving average relative humidity in a room is always less than the value given in Table B3, evaluated over each moving average period. Table B3 Indoor air relative humidity Moving average period Indoor air relative humidity (%) 1 month 65 1 week 75 1 day 85 41 Building Regulations 2010 Approved Document F Volume 1, 2021 edition O N L I N E V E R S I O N