SAP Calculations explained

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SAP Calculations explained

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SAP calculations explained
A SAP rating is the calculation that is required in order to produce a Predicted Energy Assessment and an On Construction Energy Performance Certificate.
Building Regulations require that a SAP calculation and a Predicted EPC is submitted for new dwellings prior to the commencement of work.
A SAP calculation indicates a score from 1 to 100+ for the annual energy cost based on:
The elements of structure
The heating and hot water system
The internal lighting
The renewable technologies used in the home.
The higher the score the lower the running costs, with 100 representing zero energy cost. Dwellings with a rating in excess of 100 are net exporters of energy.

SAP calculations allow comparison to be made of the energy running costs of dwellings anywhere in the UK. This is achieved because the calculations are predominantly location independent and are based upon a notional standard occupancy that overcomes variations associated with physical location and the differing ways in which people utilise their homes.

A SAP calculation for a new dwellings is a desktop exercise, the client or their designer submits drawings, plans and specifications of the development to the assessor. It is recommended that the SAP calculation is undertaken early in the design process, this will help to prevent any costly redesign of the dwelling.

SAP calculations usually involve 4 stages:

Design - draft stage

From the plans and drawings provided by the designer, the assessor prepares summary numerical information which includes the total floor area of the dwelling; the floor area of the lounge or living room; the areas of the heat loss floors, heat loss walls and heat loss roofs; dimensions of external windows and doors; storey heights and so on.

From the specification provided the assessor calculates the performance of the thermal elements. These are expressed as 'U' values (the rate at which heat passes through the fabric of the building), the higher the 'U' value the greater the rate of heat loss. The assessor then inputs this data into the SAP calculation. Data is entered relating to:

Type of dwelling
Openings (windows, doors, roof lights)
Main and secondary space heating
Hot water generation
Renewable technologies, including photovoltaic panels and solar water heating
Energy efficient lighting
The software determines whether the proposed dwelling will comply with the requirements of the Building Regulations with regards to the conservation of fuel and power. The assessor is able to use the software to model different variations of the design if the initial specification doesn't show compliance. The assessor should then advise the designer of the shortfalls and recommend solutions as required.

Design - final stage

The client, designer and the assessor agree the finalised version of the design, this may involve amendments to the initial design in order to achieve SAP compliance. Data from the finalised design is input into the computer program.

From this the assessor produces reports that the client or designer need to submit to Building Control, this will include a Predicted Energy Assessment (this provides a rating of energy performance based upon the specified design).

Built - draft stage

For the majority of new dwellings an air pressure test will normally be required (for small sites of 2 dwellings or less an air leakage value of 15 m3/(h.m2) may be used in the calculation and no test is required). The client or designer provides the results of the air pressure test (if required) to the assessor and also advise of any variations from the specification.

The assessor will edit the SAP calculation to reflect the results of the air pressure test and any variations to the specification. The software is used to check that the completed dwelling still meets the requirements of the Building Regulations with regards to the conservation of fuel and power. If it does not the assessor recommends remedial action.

For new build dwellings the assessor checks to ensure that the dwelling is registered on the Government's central database register of national property addresses. If it is not the assessor arranges for the address record to be created.

Built - final stage

The assessor finalises the SAP calculation and creates the Energy Performance Certificate (this provides a rating of energy performance based upon the dwelling as built). The EPC must, by law, be displayed in a new dwelling put up for sale on the open market.

In addition there are other documents that are required by Building Control such as the SAP worksheet report and the SAP data input report.

The assessor provides the client with the reference of the EPC so that the client is able to obtain a copy from the Government's central registry website at: www.epcregister.com/.

Building Regulations require that an Energy Performance Certificate is provided prior to a completion certificate being issued.


Since SAP calculations are based upon a desktop exercise, and not a site survey, it is critical, that the correct information is submitted, the following is a suggested list of the information sources and data items required.

Information required

Site address and postcode.
Site plan to include orientation of the dwelling(s).
Plans of each storey, normally at 1:100 scale.
Elevations drawings for each elevation.
Sectional drawings of the dwelling.
A written specification which must include:
Details of the principal heating and hot water system to include make and model of boiler, details of heating emitters, hot water cylinder size (if applicable) and the system controls.
Details of any secondary heating system present.
Details of ventilation systems.
Details of the internal and external lighting.
Details of the construction of all different floors to the property to include type and thicknesses of insulation and any other building products used.
Details of the construction of all different external walls to the property to include type and thicknesses of insulation and other building products used.
Details of the construction of all different roofs to the property to include type and thicknesses of insulation and other building products used.
Details of the doors and windows to include sizes, type of frame, type of glazing, thickness of glazing, any low emissivity applications.
Details of any renewable technologies utilised such as ground source heat pumps, solar water heating or photovoltaics


Andrew R Williams & Associates Ltd are Independent Chartered Surveyors and have been established since 1979 providing a full range of surveying services to the Commercial and Residential Sectors.…



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