Methodology for a preliminary building energy assessment

Inputs used by the assessment

The model accepts construction layers for foundations, floors, walls and roofs; areas for building elements; window and door areas and U-values; building area and storeys; selected climate data; heating and consumption information; and ventilation or blower-door inputs where provided. Missing information may invoke application defaults, so it should be treated as an uncertainty rather than a verified condition.

Thermal resistance and U-value

For a recognised material layer, the application calculates thermal resistance as layer thickness in metres divided by thermal conductivity. It sums recognised layer resistances and reports U-value as the reciprocal of that sum: U = 1 / Rtotal.

This is a simplified layer calculation. It does not include every surface resistance, junction, repeating bridge, air gap behaviour, moisture-dependent property or installation defect that a detailed construction assessment may require.

Transmission and ventilation heat-loss estimate

For each modelled envelope component, annual transmission contribution is calculated as U × area × heating degree days × 24 ÷ 1000, expressed in kWh per year. Window and door contributions use entered area and an area-weighted average U-value when values are available.

Ventilation loss is calculated from estimated building volume, air changes per hour, air density, specific heat, heating degree days and 24 hours per day. The default air-change assumption is 0.5 ACH; an entered blower-door value replaces it. The model then applies simplified internal gains and orientation-based solar gains before reporting non-negative theoretical heating demand.

Renovation comparisons, condensation and uncertainty

The report uses the entered record to present component contributions, renovation-oriented recommendations and comparison questions. Savings, costs and payback remain approximate because prices, scope, system efficiency, weather, occupancy and workmanship vary.

The condensation-related calculation looks for insulation layers and estimates an interface temperature from layer resistance with assumed 20°C indoor and -20°C outdoor temperatures; it uses a simplified 12°C dew-point threshold. It does not model relative humidity, transient moisture transport, rain, vapour diffusion or mould growth, and must not be used as detailed moisture design.

Appropriate use of the result

Use the output to document assumptions, prioritise evidence gathering and prepare renovation planning discussions. Do not use it as a site survey, legal determination, certified audit, EPC, guaranteed savings calculation or substitute for detailed engineering design.

Start with documented assumptions

Create a preliminary assessment and keep the source information available for review.