Approved Document O

Approved Document O uses the CIBSE TM59 methodology as a basis for assessing overheating risk for domestic properties. For full details, see Approved Document O


For Part O, the occupant control of apertures depends on the time of day.

Day Operation

Approved Document O specifies that, during the day (8am to 11pm), openings should be modelled to:

  • Start to open when the internal temperature exceeds 22°C

  • Be fully open at 26°C

The apertures should close using the same profile when the temperature falls.

In Tas, this can be achieved in the Building Simulator using the ZDWNN function:


This function should be used if the apertures stay closed during the night.

Night Operation

Approved Document O specifies that, during the night (11pm to 8am), the openings should be modelled to be fully open if:

  • The opening is not on the ground floor and is not easily accessible

  • The internal temperature exceeds 23°C at 11pm

This can be achieved in Tas using the Building Simulator and the ZDWNO function:


This function behaves like zdwnn during the day, but if the dry bulb temperature of the zone at 11pm exceeds 23°C, the aperture will remain open until 8am.


This function should not be used with schedules.


ZDWNO is an aperture function introduced in Tas v9.5.4