Part 4: Limiting heat gain and striving for consistent building performance (see related articles for parts 1-5).
Limiting the effects of solar gains in summer
Part L isn’t just about setting Emission and Energy Targets, it also does it’s bit to make sure new homes don’t overheat in the summer. This can occur if you have lots of glazing in a small dwelling, most commonly seen on top floor, penthouse-type apartments. When a SAP assessment is completed, a separate calculation is worked out in the background – it looks at the amount and orientation of glazing, the ventilation system, and whether the windows can be left open. (eg. A ground floor flat can’t have windows open all day for security reasons). If the average internal temperature is calculated to be too high for the summer months (23.5 Celsius), the dwelling will not comply with Part L. Ways of combatting this are to use glazing with a different transmittance factor (to let less light / heat in), to reduce the area of windows or to change the ventilation system to increase the air flow through the dwelling. Installing a cooling system would help in reality, but flies in the face of Part L’s main objective of reducing energy use, so the use of cooling systems is ignored by this section of the calculation. Part L does not call for minimum daylight levels, however the less glazing you have, the darker the dwelling, so the more energy will be required for lighting.
Building performance consistent with DER and DFEE rate
It’s usual for slight adjustments to be made between the details which are shown on a building plan, and what actually gets built on site. This means a SAP assessment is never 100% reflective of the dwelling in question. Criterion 4 of Part L does it’s best to make sure the SAP ratings are as accurate as possible. Developers are required to confirm, to the best of their knowledge, that the information which has been fed into the SAP assessment is correct. It’s also a requirement to have the heating, ventilation and hot water systems of a new home suitably commissioned, and a sample of new plots need to have air permeability tests carried out to prove the quality of the buildings. In Part L 2013, the requirements for an air test are not changing from the previous version. The absolute maximum air test result is 10 for a newly built dwelling (but bear in mind the Elemental Recipe is based on 5, so this really needs to be your target). For any plots which are not tested, the average result for that house type must be used, with a ‘Confidence Factor’ of +2 added. This means plots which are not tested will not perform as well in the As Built SAP assessments as those which are tested. The developer needs to decide whether it’s cheaper to make sufficient improvements over the Target Emission Rate to allow for this ‘Confidence Factor’, or whether air testing all plots is more financially sensible. All commissioning certificates, including the As Built Part L reports and Energy Performance Certificate, must be received by Building Control (or the Approved Inspector) before final sign off will be given. Join the UK Building Regulations group on LinkedIn.