What are Thermal Bridges?

Thermal bridges are localised areas in the thermal envelope of a building where there is increased heat loss. A building envelope will have 2 types of thermal bridge.

1. Repeating thermal bridges
2. Non-repeating thermal bridges

Repeating thermal bridges are accounted for in the calculation of a buildings U-values (U-values measure how effective a material is an insulator).  Any material that penetrates the insulation repeatedly and predictably in a heat loss element would be classed as a repeating thermal bridge. Examples of this type of repeating thermal bridge are mortar joints in a masonry wall or timber studs in a timber framed construction. The increased heat loss from these materials is incorporated into the U-value calculation through bridging factors.

Non-repeating thermal bridges occur where building elements meet i.e. walls / floors or walls / roof and around windows. Psi (Ψ)-values (pronounced ‘si’) are the numerical value used to measure the amount of heat loss from these junctions.

The Effect of Thermal Bridging

Over recent years the effect of heat loss through non-repeating thermal junctions on the performance of a building in complying with Part L has become more and more pronounced. It is quite a rare occurrence now to find a Part L1A 2013 SAP Calculation that has not included an assessment of the buildings thermal bridging lengths. Recent research suggests now that as buildings are better insulated that up to 30% of a buildings heat loss can come through its non-repeating thermal junctions.

The Growing Availability of Calculated PSI Values

Many building material and insulation providers have now calculated their own set of psi values for customers to use. These calculated psi values are becoming a differentiating factor within the construction industry. Buildings that are able to qualify for the use of these calculated psi values will find enhanced thermal performance in the completion of the SAP Calculation which will ease compliance with Part L. 

Non-Traditional Construction Methods

Although there has been a growing awareness of the importance and impact of thermal bridges on the performance of a SAP Calculation, there are still some parts of the construction industry that this information has yet to reach. Any building project where calculated psi values are not available for use will see an increase in heat loss through non-repeating junctions of their building and there will be a noticeable decrease in the thermal performance of the SAP Calculation, leading to additional spend and specification elsewhere to compensate for this loss.

Well Well Well

There has been a lot of talk over the past decade about the energy efficiency of our buildings. More recently the conversation has shifted to start to include the impact that buildings are having on our health. One of the other values that is calculated with every psi value is the surface temperature factor. This value is used to assess the risk of mould growth through these parts of the building. Moulds produce allergens, irritants and even toxic substances and can cause asthma attacks and other respiratory illnesses.

Small Gains

The calculation and use of psi values is now something that every construction project should incorporate. A calculated psi value is not project specific but related to the construction method and so can be used repeatedly of subsequent projects whether the same construction specification is used. This makes the calculation of psi values a very cost effective solution to energy conservation and energy efficiency. Through using calculated psi values the heat loss of a building can be more accurately illustrated which will help SAP Calculations comply more readily with Part L. This will save money from being invested in other aspects of the building which would be needed to compensate for the poor thermal performance at the buildings thermal junctions.

As experts in sustainability and energy efficiency we can help you to achieve enhanced thermal performance and carry out your SAP Calculations which will ease compliance with Part L Regulations.  Wellbeing is a huge consideration for us and we will help you ensure your building is not only efficient but also a healthier place to be.

Written by Michael Brogden, Technical Director, Darren Evans Assessments


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