Building energy calculations
Energy and calculations answer different design questions
Choose calculations by building use, proposed work and the evidence being requested. A fabric calculation, a whole-building model and an operational certificate describe different things; commissioning one does not automatically supply the others.
| Calculation family | What it informs | When to begin |
|---|---|---|
| Whole-building energy | SAP assesses dwelling energy performance; NCM/SBEM provides the non-domestic methodology. Confirm the appropriate route for each part of a mixed-use project. | Start while fabric, glazing and services can still change; update the inputs to reflect the completed specification. |
| Construction details | U-values, junction heat losses and condensation analysis examine different aspects of the proposed fabric. Combine their findings before fixing insulation build-ups and connection details. | Develop these alongside drawings, before substitutions or concealed construction make changes difficult. |
| Comfort and resource use | Approved Document O, Approved Document F and Approved Document G address overheating, ventilation and water efficiency respectively. Each needs its own supporting inputs. | Review openings, ventilation provision and fittings while layouts and schedules remain open to revision. |
Energy and calculations with a shared project brief
Energy calculations are mostly desk-based, and their accuracy depends on the drawings and specifications supplied. Tell us the intended use, project stage and documents requested, and we identify the appropriate assessment work.
National Building Compliance arranges accredited assessors rather than asking you to coordinate separate enquiries. You have one contact for every compliance service, work carried out by accredited, qualified assessors, and reports written to be accepted by Building Control and planning officers first time.
For a house, glazing choices can affect both energy demand and summer comfort. In a commercial shell, uncertain tenant services can leave important inputs unresolved. Record assumptions openly, identify who will confirm them and revisit the model when the specification changes. Avoid circulating an early design result as if it describes the finished building. For a conversion, separate the evidence about retained fabric from assumptions about new work. Ask the assessor to flag missing construction information before options are compared, so a provisional input does not quietly become a procurement decision.
Energy and calculations from drawings to final inputs
Define the assessment boundary
Provide plans, sections, intended uses and the proposed works. Identify retained construction as well as new elements, and separate planning requests from Building Control evidence. Planning requirements differ by council; check the local information requirements described in planning application guidance.
Test connected choices
Review the fabric, junctions, services and ventilation strategy together. Compare proposed changes before procurement, and ask which other assessments need updating when an input changes. A better insulated wall alone does not tell you whether a room will remain comfortable in summer.
Close the evidence gaps
Keep a controlled specification with a named source for each input. Pass installed-product details and relevant test results to the assessor, then check the property description and document purpose before submission. Retain the final calculations alongside the drawings they describe for later queries.
Energy and calculations services by purpose
SAP Calculations
Use SAP calculations when developing the energy assessment for a dwelling. The government’s SAP guidance explains the methodology, linking fabric and building services within the assessment rather than judging either in isolation.
SBEM Calculations
Bring SBEM calculations into non-domestic design while zoning and services specifications are being established. The National Calculation Methodology explains the framework, making accurate activity and system inputs important to the result.
U-Value Calculations
Request U-value calculations when wall, roof or floor build-ups are being selected. They describe heat transfer through an element and provide fabric inputs for the wider energy work addressed by Approved Document L.
EPC
Arrange the appropriate EPC route once the property type and purpose of certification are clear. Official guidance treats domestic EPCs and commercial EPCs separately, so flag mixed-use buildings at enquiry stage.
Display Energy Certificates
Consider Display Energy Certificates when reviewing energy reporting for a public building. The government DEC guide explains operational ratings and applicability, making actual consumption records relevant to the assessment.
Water Calculations
Prepare water calculations while sanitary fittings and appliances are being scheduled. The dwelling water-efficiency guidance in Approved Document G connects the selected fittings with the assessment, so later product changes need attention.
Condensation Risk Analysis
Seek condensation risk analysis before fixing an insulation strategy or changing an existing construction. It informs moisture-related design decisions considered in Approved Document C, using the proposed layers and relevant environmental assumptions.
Home Energy Model
Discuss the Home Energy Model when the project brief raises questions about the government’s developing calculation approach. Check the published Home Energy Model service guidance before assuming it replaces the methodology applicable to your submission.
Thermal Bridging Calculations
Commission thermal bridging calculations as junction details become sufficiently developed for assessment. Their heat-loss information supports the energy approach in Approved Document L, connecting detail design with whole-building calculations.
Background Ventilation Calculations
Check background ventilation calculations while window schedules and room layouts are still being coordinated. Approved Document F sets out ventilation guidance, so assess provision in relation to the chosen strategy rather than selecting vents independently.
Overheating Assessments
Begin overheating assessments before glazing areas and opening arrangements are fixed, checking whether the simplified route is suitable. Approved Document O describes the assessment approaches; detailed thermal modelling belongs with our TM59 and TM52 service.
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