Using SAP and SBEM calculations to measure building compliance

How is building performance assessed?

For new-build construction projects, building regulations take an inter-connected ‘package’ of measures into consideration when determining whether a building meets the targets for carbon dioxide emissions and/or energy consumption.

A ‘good’ building relies on low U-values for the main building elements, and minimal heat transfer at junctions where those elements meet (to avoid cold bridging). It also reduces uncontrolled ventilation to avoid the excessive loss of heated air while making sure occupants still have a supply of fresh air to breathe.

This is typically referred to as a fabric first approach. Renewable technology can assist with heating or hot water demand, but the key word is ‘assist’. Reduce the building’s energy demand in the first place and technology doesn’t have to work miracles to create a comfortable living or working environment!

There are two calculation methods: the Standard Assessment Procedure (SAP) for new domestic buildings, and the Simplified Building Energy Model (SBEM) for non-domestic buildings.

How do compliance calculations work?

Under both SAP and SBEM, the proposed specification is assessed against the ‘notional building’ – a theoretical identical building that meets a defined level of performance – and must better it to be deemed as compliant.

As well as U-values, linear thermal bridging (the junctions where building elements meet), airtightness and any renewables, the calculations take into account:

– The type of dwelling (semi-detached, detached etc.).
– The primary space heating system.
– Any secondary heating provision.
– Hot water generation.
– Ventilation systems.
– Efficiency of lighting.
– Size, composition and orientation of doors and glazing.
– Solar gains.

SBEM calculations are understandably more complex than SAP calculations as they have to take into account a broader range of building types and uses, but the underlying principles are similar.

While the SAP and SBEM methodologies are intended to be a reasonable approximation of how the building will perform, they do not take into account detailed occupant behaviour. The calculation results are a means of establishing compliance and comparing predicted energy efficiency and running costs of buildings.

The actual running costs will vary depending on the way the building is used. For example, a four bedroom house will cost more to run with a family of five living in it than if just one person lived there. No calculation method can predict how a building’s occupancy and use will change over time!

When should compliance calculations be done?

Given the array of design and specification features covered, it’s beneficial to undertake some compliance calculations from the early stages of a project to have an idea where the outline specification might be falling short.

The process of modelling the building design means values can be tweaked and changed, assessing different iterations as the specification develops. In truth, however, SAP and SBEM calculations are rarely carried out early enough in the process. That can lead to last minute design and specification changes, or issues on site such as improvising the construction to include extra insulation or products with better performance.

At the end of a project, as-built SAP or SBEM calculations are carried out to reflect what was built. One thing they cannot do is account for the quality of installation.

What is the performance gap?

This system of compliance rarely delivers buildings that, once constructed, meet their designed performance levels. Studies show that carbon emissions and energy use are routinely two to three times greater than predicted – a disparity between design intent and constructed reality known as the performance gap.

Awareness of the issue, and good communication that addresses potential problems before they occur, are excellent places to start, however. Clients have to want more efficient buildings (or be guided as to the benefits of aiming for them); architects and design professionals have to plan, design and specify to achieve better performance; and contractors have to be able to deliver it – and not be afraid to ask if they don’t know how.

This topic is explored in our ‘Thermal Performance of Roof Glazing’ white paper, available to download for free here.