Nov 21, 2019 Last Updated 10:52 AM, Aug 14, 2019

How an all-inclusive method to thermal performance will benefit public sector buildings

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Andy Mitchell from NBT – a UK distributor of Pavatex wood fibre insulation – discusses the reasons why so many buildings fail to meet their designed thermal performance.


Pressure on public sector budgets has prompted a more creative and strategic approach to planning and designing public sector property assets. Local authorities have not only reviewed their property portfolio and how it can deliver best value; many have also embarked on rationalisation programmes, often involving the construction of new buildings fit for the 21st century, where services can be combined to reduce operational costs.

Critical to that cost management goal is the sustainability of public sector buildings, in terms of thermal performance, maintenance requirements and service life. The insulation used and the way in which this is specified and installed as part of the building fabric is critical to all three sustainability parameters because it will determine heat loss, solar gain and the risk of issues associated with moisture build-up, such as damp, mould and condensation.

The same principles apply to social housing properties, where longevity of the asset, minimising maintenance, reducing heating bills and ensuring tenant comfort are all important specification and design considerations.

All of this may seem fairly fundamental and integral to Part L compliance. However, there’s a hitch. Many buildings fail to meet their designed thermal performance due to issues with airtightness and thermal bridging, and there is no legal requirement to check if the energy performance of the building, once built, meets its designed energy performance. Meeting Part L is essentially a paper exercise, and this is a missed opportunity to work with the supply chain to simplify design details and ensure thermal coherence of the finished build, which would often result in a faster build programme and less waste too.

The U-value issue

The most commonly quoted thermal performance criterion is the U-value, which is calculated based on the heat loss of a building’s principal areas – such as walls, roof, windows etc. – and considers every component of each. However, to achieve genuine thermal performance this should not be considered in isolation but in combination with psi values (thermal bridging and airtightness.

To achieve the required U-values, the specifier selects an insulation material with low thermal conductivity – the Lambda value. The lower the conductivity of the material, the higher performance it gives as an insulator. In theory, therefore, materials with very low conductivity, such as PIR insulation, can be specified in smaller quantities to achieve high levels of thermal performance in the finished structure. But there are a couple of problems with this assumption.

Firstly, we don’t construct buildings using a single material in isolation, so the Lambda value of one element of the wall or roof build-up, i.e. the insulation, can be seriously compromised by the additional materials that surround it, often required for structural or weatherproofing purposes. Instead, the thermal conductivity of the insulation material needs to be considered in the context of the entire building envelope and floor structure.

A good illustration of this is wood fibre insulation, like Pavatex. Wood fibre has a higher Lambda value than PIR, but it is high density and can be applied to the external envelope of a building as a complete thermal wrap requiring much smaller section fixings for external weathering surfaces such as cladding. Its thermal performance in practice is better, therefore, despite the fact that its Lambda value would suggest otherwise on paper. Consequently, it is vital that the specifier considers the verified Lambda value within the context of all the repeating components of the principal areas of the building envelope when designing a project.

Secondly, the effectiveness of the building envelope also depends on the psi values (thermal bridging detailing), collectively known as the Y-value. Proportionately, thermal bridging has little impact on poorly insulated older buildings as there is so much heat loss through the principal areas. For a Part L-compliant building, however, as much as one-third of a building’s heat can be lost through thermal bridging.

Once again, the solution to this is delivering a thermal wrap around the principal building fabric, which can perform even better when solutions are incorporated to address high heat loss thermal bridge details, such as overlapping insulation onto window frames, for example.

Achieving improved airtightness

The other major cause of heat loss is air escaping from the building and we measure this as airtightness at m2/m3@50pa. The Part L requirement of 5m2/m3@50pa is the equivalent of a hole the size of a 20p piece in every square metre of the building envelope: not quite the hermetically-sealed boxes that people fear airtightness will deliver! Insulation can only trap the heat within the building if the envelope provides good levels of airtightness because warm air will naturally escape through any gaps, increasing heat loss.

While designers and specifiers cannot always control construction integrity, good airtightness can be aided at the design stage with simple details that are more easily executed on site. In most cases, airtightness is better delivered from the inside of the building envelope, enabling a pressure test at first fix to show up any issues so that the contractor can make good. Additionally, careful consideration should be given to the sequencing of the construction.

We must be mindful also that incorporating elements to improve the thermal performance of our building changes the physics of the structure. We must, therefore, avoid creating new problems, such as trapped moisture or summer overheating as a byproduct of the focus on preventing heat loss.

Ultimately, we must stop focusing on U-values alone; the most costly of the three ways of reducing heat loss. By delivering thermal performance through a combination of U-values, Y-values and airtightness, we are more likely to deliver the low-energy public sector built environment we need to drive performance, whilst reducing cost.

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