Calculation could be on monthly basis and may take into account winter thermal mass and air conditioning

Proposed changes to the energy calculation tool for homes should make it easier for designers deliver low-carbon homes. The consultation on the Standard Assessment Procedure, which was launched this week has been updated so it takes account of a range of low-carbon technologies and an up to date understanding of building physics. The main changes are:

The calculation on energy use is now built up on a monthly basis rather than annually. This means the calculation will now take account of the effect of monthly variations in the weather on the building such as solar gain.

The benefits of thermal mass are taken into account in the winter as well as the summer. Currently the tool allows for the cooling effect of thermal mass in the summer but doesn’t allow for reduced heating demand in the winter when solar gain heats up the thermal mass of a building.

The energy used by air conditioning systems is now accounted for in SAP which means it will be harder to get homes fitted with air condioning to comply with Part L.

Heat loss through poorly constructed party walls will be taken into account. Currently the tool assumes there is no heat loss through a party wall but this has been changed so the SAP rating can be changed post completion if building control finds a party wall has been poorly built so heat can escape via the cavity in the wall.

Assumptions for the amount of hot water needed in a property has changed. This is calculated on the basis of the number of people for a given floor area. The tool now assumes there are fewer people in a property for a given floor area than before. The tool will now account for the reduced efficiency of a boiler when it is only used for heating water in the summer.

Brian Scannell, the managing director of National Energy Services welcomed the proposed changes. He said, “It’s an evolution not a revolution. They are making the changes necessary to deliver low carbon homes and are an improvement in terms of the technical calculation.” He added the real challenge for the industry would be contained in the proposed changes to Part L of the Building Regulations which are due imminently. The SAP consultation closes on 12 August.