Part 3: ‘Limits on Design Flexibility’ (see related articles for parts 1-5) In our previous blogs in this series, we have already looked at the ‘elemental recipe’ for compliance with Part L. It’s very likely that your own specification is going to be different to this recipe, and providing you achieve the TER and TFEE you are given ‘design flexibility’ under Part L to have worse U-Values than those which are used in the notional calculation.
But how high can you go when creating your U-Values? Providing you achieve the two mandatory targets, you are allowed to use what is called ‘limiting U-Values’ for some elements. If the average U-Value for your walls, roofs, floor or openings are higher than these limiting U-Values; that will give you an automatic fail under Part L.
These maximum figures are to be used for new domestic buildings in England (there are different figures for both commercial and existing domestic buildings): 0.25 for heat loss floors, 0.30 for walls, 0.20 for roofs and 2.0 for external doors, windows and rooflights. Party walls should achieve 0.20 as a maximum, and for any houses lucky enough to have indoor pools, the basin should be insulated to a maximum of 0.25. Where you have an unheated communal area, such as a corridor in a block of flats, this section of your building isn’t covered by SAP, however it is considered that ‘reasonable provision’ is to insulate this part of the building to meet these limiting U-Values or better.
We can’t stress enough that you’ll need a significantly better specification than these figures to meet the TER and TFEE targets – these limiting U-Values are there to allow design flexibility, and should be treated as a last option.
Also covered in this section is the maximum air test rate. This section hasn’t changed from the 2010 version of Part L. If you are having an air test completed on a dwelling, it needs to achieve a result no higher than 10. If you’re having a sample of plots on a site tested, you need to make sure the average result per house type is no higher than 8. If your site is less than three dwellings, you can seek an exemption which will assume a worst-case value of 15. But, remember the ‘elemental recipe’ is set at 5 for the assumed air test result.
This means any result higher than 5 will need to be compensated for by making improvements elsewhere. The Design Flexibility section of Part L also includes minimum efficiencies of fixed building services. For example, at least 75% of light bulbs must be low energy, a gas boiler should be at least 88% efficient, ventilation systems with heat recovery should be at least 70% efficiency. A full list of these recommended requirements are listed in the 2013 Domestic Building Services Compliance Guide.