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Getting Glazed Extensions Right

Getting glazed extensions right.

This is one of those thorny issues where a deeper understanding of SAP2005 can enable any Building Control Officer to offer a better service to clients and enhance the reputation of the profession.

One of the many ‘hard to see’ implications of the introduction of Part L is the way in which it affects the addition of glazed extensions* to existing properties. The Guidance Notes in Approved document L1b state that SAP 2005 may be used to show compliance for excessively glazed extensions. This approach provides a great deal more flexibility in the way in which the regulation is applied than most people assume.

However, interpretation does depend on there being a SAP calculation for a notional building, and a SAP calculation being done for the actual proposal. The notional calculation would consist of the full existing dwelling and a theoretical extension which has the same size, shape and orientation as the proposed extension, but is calculated assuming that the minimum standards set out in Approved document L1b for U Values and the maximum allowed glazing have been followed. Such a calculation will give the carbon dioxide emissions for the house and the proposed extension, as if it was meeting the minimum standards set out in the Approved Document.

The second calculation including the actual proposed extension is then undertaken, taking into account all the thermal elements for the proposal and the proposed area of glazing of the extension. If the carbon dioxide emission rate of the house plus the actual proposed extension is lower than the notional CO2 emissions, the proposal meets the energy efficiency requirements of the of the Building Regulations (Part L).

However, if the second emission calculation of the house and proposed extension has a higher carbon dioxide emission than the notional building then other works will have to be undertaken to further lower the CO2 emissions.  Significantly, these further works may be carried out elsewhere in the house, not just in the extension, ie extra loft insulation in the main house can be used to offset a shortfall in a highly glazed extension. Alternatively, this might involve changes to the boiler, the insulation, lighting and glazing; all of which it should be possible for trained Building Control officers or other specialists to comment on, and all of which it should be possible for the householder or builder to carry out and have inspected. The result is less breached regulations, more efficient homes, more building work and more satisfied customers all round so long as specialised SAP calculations are undertaken.

The building needs to be considered as a whole – it is not just the extension that is being judged. Elmhurst trains people in just such plan rating or can provide the service on request.

*Only non-thermally separated extensions are being considered here.Back to main news