‘The window area shall be at least one-tenth of the area of the room’: The origins of a daylight (and ventilation) requirement in modern building codes

Current New Zealand (New Zealand Building Code), Australian (National Construction Code) and American (International Building Code) building codes include deemed-to-satisfy requirements for a given proportion of a house’s floor area (e.g. 10%) to be in windows. These, plus the current English Building Regulations, also require a proportion of windows to be openable (e.g. one-half or 5% of the floor area). A detailed summary table supported by code extracts is used to trace the evolution of these requirements. Even the architects of antiquity provided only rules for window proportions but not for window area or size based on room dimensions. The paper demonstrates the evolution of this 21st century requirement follows a direct line to the English 1859 model ‘ Byelaws as to New Streets and Buildings’. Prior to that date, only two rules from 1842 (room volume based) and 1734 (height and volume based) have been found which gave a required window area based on room size. It is hypothesised that one of these led to the current rules. Although the current proportion rules may be appropriate, no empirical research-based origin has been identified.