Building Energy Modelling

Building energy modelling maps the flow of energy in and out of a building using computer software to analyse thermal performance and energy efficiency.

Energy modeller working on a computer
Energy modelling typically involved using building simulation software to analyse the performance of a concept or design

What is building energy modelling?

Modelling building energy is an intrinsic part of designing new buildings, planning retrofit works to existing buildings and predicting how a building will perform in real life.

Building energy modelling involves using software to map the flow of energy in, through and out of a building. It can encompass everything from thermal energy used in heating and cooling, modelling weather conditions, daylight and artificial lighting as well as the airflow within and around buildings.

Building energy modelling is increasingly important as a design criterion given the significance of energy use and meeting net zero emissions targets. It may also be required for compliance purposes such as Part L or to meet certain standards and client expectations.

Modelling building energy has many benefits. It enables designers and architects to reduce the impact that a building will have on the environment during both construction and operation. For building owners, it will minimise energy use and CO2 emissions and can form the basis of full lifecycle analysis. Whilst for occupants, it will improve comfort and wellbeing whilst using the building.

Standard Assessment Procedure (SAP)

The Standard Assessment Procedure (SAP) is the Government approved method for modelling the energy usage of all new-build domestic dwellings. A SAP calculation requires knowledge of a dwelling's construction, building elements, ventilation system, heating system, lighting and any on-site renewable technologies.

The SAP assessment is a requirement for both building and planning compliance under Part L and must be carried out by all housebuilders prior to construction commencing. The calculation itself produces a rating typically indicated by a score from 1 to 100+, whereby higher scores indicate lower energy costs and associated CO2 emissions.

Within a SAP calculation, a Heat Transfer Coefficient (HTC) will be calculated based on the input data. It is possible to directly compare this calculated HTC with a measured HTC using SmartHTC energy performance software which enables a performance gap to be determined. A performance gap is the difference between the intended design and the actual performance of a building in operation which allows you to assess if a building is performing as expected.

Energy Performance Certificates (RdSAP)

RdSAP stands for Reduced data Standard Assessment Procedure and is the energy model used for Energy Performance Certificates (EPCs). An EPC is required when selling any domestic property that is over 10 years old as well as when letting out a property or applying for certain grants covered under Energy Company Obligation (ECO).

RdSAP is a simplified version of a SAP assessment but uses data collected during an assessment/survey carried out at the property by an accredited energy assessor, as opposed to original construction plans.

For new energy performance assessments, it is possible to input a measured airtightness value and measured U-values into the RdSAP software to produce a more accurate EPC. Providing evidence of measured values such as these can lead to a lower rating and EPC band. Airtightness can be measured using a Pulse air permeability system, whilst in-situ U-value can be measured using Heat3D infrared thermography or heat flux plates.

Measured energy performance software

It is possible to accurately measure the in-situ thermal performance of existing buildings and introduce the results as a input into energy models to make them more accurate.

Temperature sensors

SmartHTC Measured Thermal Performance

A low-cost and non-invasive way of measuring the true thermal performance of a house. It requires temperature and meter data to calculate an accurate heat loss rating over a 3-week period.

Learn more about SmartHTC Measured Thermal Performance
SmartHTC on an In Home Display with temperature sensors

SmartHTC Heat Loss API

Unique algorithm and software that calculates an accurate and reliable measurement of whole building fabric heat loss (also known as a Heat Transfer Coefficient or HTC).

Learn more about SmartHTC Heat Loss API

Simplified Building Energy Model (SBEM)

Whereas SAP and RdSAP are used for domestic buildings, SBEM is used for non-domestic buildings for both building compliance testing and EPCs. Examples of non-domestic buildings include offices, warehouses, retail units, restaurants, leisure centres and retirement homes.

SBEM stands for 'Simplified Building Energy Model' and models non-domestic properties over a 12-month period of normal operation using information about a building's type, layout (zones), usage, elemental U-values, HVAC system and lighting efficiency.

An SBEM calculation is required for building compliance by Part L (England & Wales), Section 6 (Scotland), Part F1 (Northern Ireland). It is also used to generate Energy Performance Certificates for non-domestic buildings during construction and at the point of sale or rental.

The results of an SBEM calculation are an energy performance rating and CO2 emission rate indicating how energy efficient a building is. To pass, the Building Emission Rate (BER) of a property must not exceed the Target Emission Rate (TER) for a 'notional' building of a similar type.

Building simulation software

Aside from the SAP, RdSAP and SBEM modelling methods, commercial energy modelling and building simulation software are used throughout the design process by architects and building engineers. This building modelling helps refine designs from concept through to completion and determine the predicted energy performance, thermal comfort, visual comfort and noise levels of a particular design pre-construction or retrofit.

Modelling a building requires using architectural plans, construction materials, HVAC system specification, temperature & weather data, intended occupancy and operational patterns to construct a virtual model of a building in its intended location and environment. Simulations are then run on the model using energy analysis algorithms based on the principles of building physics to calculate what heating/cooling loads, lighting levels and other building services are required whilst meeting the desired energy performance level.

The output from the simulation will be various metrics relating to internal temperatures, ventilation rates, energy loads and CO2 emissions. Popular software packages for building simulation include DesignBuilder, EnergyPlus and IES VE.