Achieving Thermal Compliance - A Complicated Process

6 August 2019

Stuart Fairlie MCABE
Operations and Technical Director of Elmhurst Energy

The energy costs of new dwellings are now seen as one of the most important considerations by purchasers.

Coupled with ever-increasing standards to reduce CO2 emissions on new build, the ability to achieve thermal compliance can be a complicated process.
Elmhurst Energy looks at some of the most common areas where residential buildings face challenges, when complying with new build energy regulations:

1. Air pressure Test (APT) results

Air Pressure testing is one of the most crucial elements required for Part L1a compliance, this test shows how much energy/air is escaping through gaps, holes or cracks in the building fabric, with excessive air leakage indicating poor build quality and lower energy efficiency. Essentially, the lower the APT, the better the overall score for the energy calculation. Very rarely can you adopt the no air test route and compensate with a highly efficient heating and/or renewable system.

2. Mechanical ventilation systems

A building which is too air tight (has very low air pressure) can cause problems for natural ventilation within the building and in most of these cases it would be recommended to install a mechanical ventilation system. Efficient mechanical ventilation would largely help the building comply with new build Part L energy calculations.

3. Poor construction U-values

Getting the right U-values for all thermal external envelopes, such as walls, roofs and floors is another important factor when attempting to achieve Part L compliance. In the past it was possible to have poor U-values compensated with a highly efficient heating system, or renewable technologies as a trade-off method.
However, this is now a thing of the past and the focus in the current Part L1A Building Regulations is on Fabric Energy Efficiency. It’s, therefore, important to achieve good U-values with the products specified.

4. Heating systems

A natural gas system still appears to be the most preferred choice for many house builders with some form of heat pump used as an alternative option. However, Oil and LPG are still adopted by many and it is difficult to gain compliance with these fuel types due to the higher fuel factors in place within the SAP methodology.
In many scenarios renewable technology such as solar panels or photovoltaic panels are also implemented alongside oil or LPG gas to help a dwelling comply. How you choose to heat the dwelling may have the biggest bearing on whether you will pass or fail Part L1A compliance.

5. Thermal bridging

Often overlooked, linear thermal bridging can offer significant gains when aiming to achieve compliance. In some circumstances the heat loss through junctions between building elements has been shown to account for up to 25% of total heat loss from a dwelling.
The amount of heat lost through a junction between elements is known as a Psi-value. The lower the Psi-value, the less heat loss through the junction.

Elmhurst Energy and Part L

Elmhurst’s knowledge of building regulation compliance is built on over 25 years of industry experience.
Elmhurst is currently providing external Part L1a and Part L2a awareness training at the offices of local authorities and other organisations. If you would like further information about this training opportunity please call 01455 883 250 or email:

Elmhurst will be exhibiting at CABE’s annual conference and exhibition on 3-4 October; Managing Director, and fellow CABE member, Martyn Reed will be in attendance and is looking forward to meeting fellow CABE professionals:

CABE's Annual Conference provides a fantastic opportunity for Elmhurst to speak with other like-minded industry professionals. We always enjoy the discussion and wide range of industry topics which are delivered by expert speakers throughout the day.

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