Air permeability design options for dwellings

The ventilation provisions recommended for new dwellings in this Technical Booklet have been specified according to two standard designs of air permeability – (a) in the default option, the guidance assumes no air permeability and therefore no infiltration, resulting in the building's ventilation being reliant only on the installed ventilation provisions. This should provide adequate ventilation provisions for dwellings at all levels of air permeability and be particularly suitable when intending to construct a more airtight dwelling or where the person carrying out the building work does not have previous experience of closely matching design with as-constructed air permeability levels and therefore may achieve a significantly better performance than designed. If the building's performance is likely to be substantially better than the design figure the default option should be used; and (b) in the alternative option, the guidance assumes an infiltration of 0.15 air changes per hour (ach). The recommended ventilation provisions are therefore less than in option (a) as infiltration contributes to the total amount of ventilation required. This option is most appropriate when designing to an air permeability greater than 5 m3/(h.m2) at 50 Pa.