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Andrew J Ball

On Construction Domestic Energy Assessor (OCDEA) for new-build home energy SAP ratings, providing On Construction Energy Performance Certificates (OCEPCs)

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SAP Calculations & Existing Dwellings


For existing dwellings the majority of home energy rating work is now undertaken by suitably qualified Domestic Energy Assessors (DEAs) using a cut down version of the full SAP calculation known as Reduced Data SAP (RDSAP) and based upon site surveys rather than working off-plan. This type of calculation is now required for the majority of sales and rentals of existing residential properties.


RDSAP is similar to the full SAP calculation that is normally used where the work on a dwelling requires signoff by building control but makes more use of inferred data items where these are unknown, often based upon a known approximate date of construction.


A good example of this relates to floor insulation for a solid concrete slab on the ground. It will not normally be obvious from routine inspection whether any insulation is present, or if it is, what type and thickness has been used. However, knowing the date of construction and with knowledge of the prevailing building practices and legislation in force at that time it is possible to make an informed judgement as to what is likely to be present.


There are times when a full SAP calculation is useful to support proposed works for existing dwellings. In particular a full SAP calculation can be useful when extending a dwelling or when an existing dwelling undergoes a material alteration or a material change of use. In these cases the appropriate building control body may insist that a full SAP calculation is undertaken under part L1b of the Building Regulations (2010).


The greater depth and accuracy of a home energy rating based upon full SAP rather than RDSAP can also be useful when trying to model different options for improvements to an existing dwelling. In this way, for example, the impact of improving wall insulation, or providing a new boiler of a different type and performance, may be modelled. Those with an interest in using renewable energy can perform what-if modelling calculations - perhaps to consider the implications of different sizing and orientation of solar hot water collector panels.


Please contact us for further details of the opportunities for modelling improvements to existing dwellings using full SAP.