The new 2010 Part L2a - Criterion 3 - Solar Gain.

With the new part L2 in use for a few months now it’s clear that the new criterion 3 is asking more of designers than ever before. Solar gains must be considered at an early design stage; the days of writing employers requirements and pushing this element onto the Contractor to sort out have gone, and the attitude that a cooling system will get you out of jail is not there anymore!

Solar gains must be considered on all building types including mechanically cooled buildings. These diagrams show the benchmark ‘notional’ models which your design is measured against. If you have more glazing than that indicated in these diagrams you could end up with a problem.

The ‘spirit’ of Criterion 3 is to reduce the loading on air conditioning and mechanical ventilation systems and encourage naturally ventilated buildings. If your building is mechanically cooled with lots of glazing you are going to have a hard time achieving compliance! Even naturally ventilated buildings are not exempt from the assessment, however if your building envelope fails the criterion 3 assessment there is an alternative way of demonstrating compliance. Generating a thermal model which follows the guidance in CIBSE TM37 and Design Guide A is an alternative way to get naturally ventilated buildings approved.