SAP, RdSAP and
U-values explained.

Which assessment do you need?

SAP stands for Standard Assessment Procedure. It models a dwelling’s energy performance using information about its construction and building services. It is used for new-home energy assessments and Building Regulations compliance.

RdSAP means Reduced Data SAP. It is a simplified approach for existing homes, combining inspection data with standard assumptions where full construction information is unavailable. It is not simply a less accurate version of the same survey: it is designed for a different evidence situation.

For an existing home’s certificate, start with our Domestic EPC service. For a new dwelling or residential building project, see SAP calculations. For an extension or conversion, confirm the assessment route before commissioning work.

Background: Government guidance on SAP and RdSAP.

What does a U-value tell you?

Developing a construction specification? Our U-value calculation service supports architects with project-specific wall, roof and floor build-ups.

A U-value describes how readily heat passes through a building element, such as a wall, roof, floor or window. Its unit is W/m²K: watts per square metre for each degree of temperature difference. Lower values mean less heat transfer under the same conditions.

Four values that shouldn’t be confused

RdSAP default / assumed value

A prescribed assessment input selected according to the method’s rules, including relevant construction and age information. It is not a measurement of the wall’s actual heat loss.

Calculated element U-value

A value derived from the particular construction build-up and material properties. It needs appropriate supporting information; an insulation product’s conductivity alone is not the whole wall’s U-value.

Notional-dwelling value

An input to the reference dwelling used to establish performance targets. It is a benchmark for the model, not an automatic requirement to copy every element of that specification.

Limiting value

An element-level performance limit under the applicable guidance. Meeting individual fabric limits alone does not demonstrate whole-dwelling compliance.

In RdSAP, replacing a default U-value requires property-specific documentary evidence that satisfies the methodology and conventions. A verbal description or a preferred design target is not enough. BRE’s RdSAP 10 specification, section 5, explains the evidence requirements.

For new homes, the notional dwelling and the proposed dwelling are distinct models. Fabric, services and the relevant whole-dwelling targets must be considered together. See Approved Document L for England.

What should you send your assessor?

Send what you already have. We can identify gaps before the assessment rather than asking you to guess missing specifications.

• The property address, project type and the reason you need an assessment.
• Plans, elevations, sections and the proposed or as-built specification, where available.
• Wall, roof and floor build-ups, including material names and thicknesses.
• Insulation installation records, relevant photographs and property-specific U-value calculations.
• Window and door specifications, plus heating, hot-water and ventilation details.
• For compliance work, the project’s building-control submission and relevant start dates.

Documents are reviewed for relevance and acceptability; providing them does not automatically mean a default can be replaced.

Will a better U-value guarantee a better EPC?

No. It changes one input. The rating also depends on the rest of the dwelling and the assessment method. Likewise, one improved element does not guarantee that a design passes its compliance checks.