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Published on 19 April 2017 09:15

Cavity Closers and Psi values: Taking the Devil out of the Detail

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Key Learning outcomes
  • The importance of thermal bridging and how it relates to energy performance
  • How linear thermal bridging, HTB and SAP are related
  • How psi-values are calculated and the parameters that can influence them
  • How premium performance cavity closers can help achieve significant reductions in thermal bridging

The “performance gap” between the design and actual energy performance of buildings has never been more relevant. Junction detailing was raised as a significant issue by the Zero Carbon Hub in 2014 and this was reiterated recently in the ‘Each Home Counts’ review, which identified approaches to ensure consumers receive value for money when making energy saving investments. One area identified for immediate attention was “thermal insulation detailing around corners, junctions, edges”.

Poor detailing around openings can lead to condensation on cold reveals, mould growth and eventual deterioration of plaster, wallpaper and paintwork. Cavity closers provide a simple and effective method for closing cavities around openings in masonry and steel framed cavity walls. Premium performance phenolic insulation cores within cavity closers are thermally efficient and inhibit heat transfer, and so reduce linear thermal bridging in these locations.

This CPD covers a thermal modelling study by BRE Scotland of jamb and sill details for differing cavity wall constructions using premium performance cavity closers. This CPD aims to show those who are serious about designing low energy buildings that attention to detail is critical, but there are ways to make the process simpler and help take the devil out of the detail.

1.0 Thermal Bridging & SAP 2012

Linear thermal bridging describes the heat loss or gain at junctions between elements and around openings where there is an interruption in the insulation layer. These bridges can be detrimental to the overall effectiveness of highly insulated constructions, as heat naturally flows through these paths of least resistance. Linear and point thermal bridges are considered when calculating the energy performance of a building. There are two types of linear and point thermal bridging: repeating and non-repeating.

Repeating thermal bridges occur where there are regular interruptions in the building fabric caused by materials with poorer insulating properties, e.g. timber studwork. The differing (typically additional) heat flow incurred by this is accounted for in the U-value calculation for that building element.

Linear (non-repeating) thermal bridges occur at intermittent points in the building fabric where:
• the thermal insulation layer is discontinuous, e.g. sills, jambs or lintels around windows or doors; or
• at the junction of two or more planes, such as at the corner of an external wall or at the eaves.

The differing heat flow is typically greater than through the adjoining elements. This heat flow, caused by the thermal bridge, is known as linear thermal transmittance. This is measured in W/m.K, referred to as ‘psi-value’ and expressed as ‘ψ-value’. The lower the psi-value, the better the performance.

SAP & Linear Thermal Bridging
The Standard Assessment Procedure (SAP) is the methodology used by the government to assess and compare the energy performance of dwellings through accurate and reliable assessments.

The SAP calculates typical annual energy costs for space, water heating and lighting, plus the CO2 emissions of a dwelling. It is used in the production of Energy Performance Certificates (EPCs).

A building’s overall heat loss due to linear (non-repeating) thermal bridging is accounted for in the SAP by the transmission heat transfer coefficient (HTB), expressed in units of W/K.

To calculate the HTB, the length of each individual thermal bridge is multiplied by its psi-value. The overall heat loss, due to linear (non-repeating) thermal bridging, is determined by adding the figures together.

In England and Wales, the overall heat loss due to linear (non-repeating) thermal bridging may be estimated rather than calculated. In this case, individual thermal bridges are not considered but instead a default ‘y-value’ of 0.15 W/m2.K can be taken to cover an overall lack of thermal bridging detailing.
Thermal bridges can occur wherever poor conductivity material bridges the insulation layer of a construction, resulting in additional heat lost through that bridge.
Thermal bridges can occur wherever poor conductivity material bridges the insulation layer of a construction, resulting in additional heat lost through that bridge.
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Anna Watkins
Tel +44 (0)1544 387471
The information contained in the CPD article web pages is not intended and accordingly shall not be relied upon either as a substitute for professional advice or judgement or to provide legal or other advice with respect to any particular circumstance. RIBA Enterprises accepts no responsibility for loss occasioned to any person acting or refraining from action as a result of the information contained.
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