Part O aims to ensure that new residential buildings, including homes, care homes, student accommodation and children’s homes are designed to reduce overheating.
The regulation has two routes for compliance:
‘simplified method’ to compliance based on minimising solar gain and removing excess heat.
It splits England into areas of ‘moderate risk’ and ‘high risk’ of overheating, the latter including urban and some suburban parts of London. It sets standards based on whether the house or residential unit is cross-ventilated, (if it’s not possible to cross ventilate DSM is required) considers orientation and introduces a standard for the maximum amount of glazing allowed in a single room.
Cross ventilation is not 100% clear in its meaning within the regulations – ‘The ability to ventilate using openings on opposite facades of a dwelling’ (there is no mention of obstructions nor roof light) But it is a simplified method so any deviation would require the second compliance route:
‘Dynamic thermal analysis method’ which is a more complex assessment using the CIBSE TM59 methodology
The guidance includes strategies for limiting unwanted solar gain in the summer through:
Fixed shading, Glazing design, Building design / layout orientation and other means.
And then the removal of excess heat through:
Opening windows, Ventilation Louvres and Mechanical extract systems
When designing to Part O the measures implemented must ensure overheating strategies are safe and usable by occupants, taking into account noise and pollution near the home, as well as the safety and usability of the windows and security, which may affect occupant behaviour.
Information on overheating strategies must be passed to the building owner in the form of a Home User Guide.
Designers Compliance Checklist must also be provided to building control.
Also to things to think about are the interactions with:
Part B – openable glazing size may impact on the overheating calcs if your window area needs to be smaller than an escape window
Part F –the removal of excessive heat may exceed that of purge ventilation
Part J – open flued appliances gasses may be drawn into other areas (but who’s putting a open fire in?) and if your building is over heating why are you putting the fire on?
Part L – Limiting solar gains in of windows could have a detrimental effect on a buildings ability to absorb heat from the sun in winter, unless it’s designed with solar shading
Part K & M – increase protection from falling maybe required, and it could be problematic where restricting access or limiting the amount of opening is required
Part Q – Security could be an issue especially for ground floor dwellings as openings may create a security risk
It will also have to take into consideration any planning requirements for noise and pollution that have been set down as openable windows may not be acceptable or suitable.
So, when you are designing a new building keep the above in mind!