What’s involved in producing a SAP calculation?

Passivhaus Building

All new dwellings must be assessed to determine their energy efficiency (SAP rating), their Environmental Impact (EI) and whether they comply with standards. This is initially carried out at the design stage, before work starts on site. The whys and the wherefores were covered in my previous post. This one covers what’s actually involved in producing a calculation…


SAP is the ‘Standard Assessment Procedure’ methodology used to demonstrate to a Building Control Body (BCB) that a building will be constructed in compliance with appropriate regulatory guidance and to compare performance of two or more dwellings.

Design stage calculations have to be undertaken by ‘competent persons’, whilst for an EPC, an ‘accredited assessor’ is required. The reality is, someone who is both competent and accredited is needed to ensure that the specified design is correctly transferred into a compliant dwelling’s energy assessment.

The software used to produce a calculation requires approval by the Building Research Establishment (BRE), who ensure that the calculation software gives the correct results. Approved software is listed on the BRE’s website for each version of SAP.

The BRE also produce and host the SAP documents and technical papers used to develop SAP, which assessors need to be familiar with, together with the latest SAP conventions and the appropriate building standards guidance.

The version of SAP used for building regulations compliance depends on when building control approval was given, whilst for production of the EPC, the most recent version of SAP appropriate to the building standards in force is used to update calculations before production. For example a site having approval in England dating back to 2008 would need to show compliance against SAP 2005 and Approved Document L1A (2006); but once built, SAP 2012 would be used for generating the EPC.


What inputs are needed for calculations?

The first stage is all about gathering information; plans, sections and elevation drawings for the dwelling are required, from which dimensions can be taken in accordance with measurement conventions. Ideally CAD drawings are usually preferred as it significantly improves accuracy and speed of take-off’s.

The assessor will also ask for full details of the proposed building fabric, from which U-value calculations will be undertaken; ideally these should be undertaken by someone competent to do so, following correct conventions, taking correct fixing and bridging assumptions and considering condensation risk as part of the process.

A range of factors contribute to the energy efficiency and carbon emissions associated with a dwelling and any of them can have significant effects on the calculations. Information for each of these elements will be required by an assessor for use in the calculations, taken either from supplied drawings, from a specification or from a form that the assessor might provide for completion by the specifier.

Important information required includes:

  • The building’s form, volume and area of all heat-loss elements
  • The thermal insulation of the building fabric
  • Opening sizes, type and orientation (U-window for losses, g-value for solar gains)
  • Proposed heating and hot water system and controls
  • Materials used in the construction (thermal mass)
  • Detailing at junctions (linear thermal bridging)
  • Proposed level of air tightness of the building
  • The ventilation system proposed to be used in the building
  • The amount of low energy lighting
  • The fuels used to provide heating, hot water, ventilation and lighting
  • Any additional energy efficiency affecting technologies

Some of this comes from the drawings, some from the specification or a form, but together these form the basis of the SAP assessment.


What happens then?

All of the required information is entered into the software by the assessor, with some assumptions unfortunately usually necessary at this stage. The old adage goes “to assume is to make an ass out of u and me”, but usually at design stage, some important design decisions will not have been fully mapped out. Conventions dictate what should be taken where this happens, but minimum standards also dictate what will need to be achieved if a dwelling is going to comply.

A specifier might know for example, that they want to use a regular gas boiler, just not which one yet, so a sense check says enter it with minimum compliant values and worst case performances that will comply until a firm decision is made – then check that still works via the assessor.

The important thing is to replace assumptions at the earliest possible stage and to do this all through the build with correct information once known. Assumptions contribute to a disparity between design and as-built performance.

Changes during the build that are not communicated to the assessor can cause the dwelling to fail to comply and might significantly worsen the dwellings performance.

Once all the parameters have been entered, the software will give the achieved performance and comparison of outputs against Building Standards requirements (which will vary depending on where the building is located).

If the proposed building does not meet the requirements then the assessor will have a look at the areas that are failing or poorer performing and depending on the assessor, may offer potential options that might be considered that would help to make it compliant, or that might improve performance. There is also a requirement to consider the technical, environmental and economic viability of high efficiency alternative systems.

Not all carbon reducing solutions lead to more efficient dwellings, quite the opposite can occur, so proposed measures should be considered carefully by the building designer, sometimes weighing up easier compliance against measures that may produce better, cheaper to run dwellings for the eventual occupants.

Once the specifier / builder settles on a preferred compliant solution, the design stage SAP assessment is then produced. The assessor will keep all of the information used to produce the calculations ready for the as-built stage and possible future audit by their accreditation scheme.

Outputs for the customer and for the Building Control Body (BCB) will include the SAP Worksheets, SAP Input Data and Building Regulations Compliance Checklist including key features for BCB to check and an indication of possible summer overheating is also undertaken as part of the assessment. Predicted Energy Assessments are also sometimes requested for dwellings marketed from plans .

A clear connection should be evident in the outputs between product specifications and the data inputs into the compliance software used (following the SAP calculation procedure and conventions).

Communication is key throughout this process, to ensure that what is designed and assessed, is then specified and built, with good feedback helping to ensure that calculated performance is closer to actual and that better buildings come out the other side of the process. 

Once Building Control receives a copy of the required outputs and confirm their approval, the building can start.


After it’s built

This calculation is updated once the house is physically complete, to produce an As-Built energy assessment and Energy Performance Certificate (EPC) for the finished dwelling and to show that the completed property complies with the regulatory requirements, but I’ll cover what’s involved in that process in a separate post.


Kingspan Insulation’s technical department offers a SAP assessment service, which includes analysis of all of the information provided by a customer for energy assessments for developments and guides the customer through the various relevant requirements, offering advice as part of the service where a specification may fall short of compliance.  More details are available on the SAP pages of our website.

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Jon Ducker is a qualified energy assessor working for Kingspan Insulation Ltd. He has an extensive knowledge of energy efficiency, renewable energy systems and sustainability in buildings with an expert knowledge of the relevant sections of buildings regulations and standards and their interactions with SAP. He provides authoritative advice regarding energy assessments for a wide range of public and private sector clients.

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