Thermal Bridging – How can I Design to Pass?
To succeed in the latest versions of SAP, it is necessary to address thermal bridging. Thermal bridges are junctions between parts of your build through which heat can escape.
Psi values (pronounced ‘sigh’ values) show the rate of heat loss between at least some of these junctions. Addressing thermal bridging usually means choosing from construction types for which Psi values have already been calculated. These are known as approved details.
What is Thermal Bridging?
Thermal bridging is one of the most significant sources of heat loss within a building. A conductive material or a gap in the insulation can form a ‘bridge’ between the inside of a home and the outside world, where heat can escape and be wasted.
When we talk about thermal bridging in SAP calculations, we are looking to improve construction to stop this escape of heat, and save energy. Reducing air gaps through which warm air can escape will also help with Air Permeability Testing.
Designing to address thermal bridging is now a must when constructing to pass the building regulations.
The SAP methodology caters for the fact that in many junctions thermal bridging is likely to be significant, commonly where two pieces of insulation meet, or between one surface and another. By constructing these areas with thermal bridging in mind we can lower the Psi values and improve efficiency. The most common method is by following the construction types shown in approved details schemes.
What Thermal Bridging Schemes Exist?
Historically there have been two sources of accredited details which contractors can follow. One was provided by the Planning Portal (a partially government scheme) and the Energy Saving Trust. They provide the ACD and ECD schemes as below. However these are often deemed outdated and and may not be sufficient to meet stringent current regulations. Therefore more schemes are now available including those from insulation manufacturers, who have produced their own approved details using proprietary products.
Thermal Bridging Schemes from Independent Bodies:
Many independent industry bodies have created schemes of approved details to allow constructors to address thermal bridging. This is not an exhaustive list but includes:
Accredited Construction Details – Most of the junctions covered in SAP are represented in this scheme. This is the most common set of approved details that we use in SAP, and we have written more about these junctions on our Accredited Construction Details page. The junctions can be viewed on the Planning Portal website.
Enhanced Construction Details – These details are specifically made to be used in conjunction with ACDs. Enhanced Construction Details cover fewer junctions but are designed to improve upon and replace some of the less efficient ACDs.These can be seen on the Energy Saving Trusts website.
BRE Certified Thermal Details and Products Scheme – In an effort to create a standard directory of approved details, BRE has launched a database that ‘allows users to search a wide range of accurate and independently assessed thermal junction details’. There are already some listing from Xtratherm and Marmox among others, but take up remains slower than hoped. You can view the listings on BRE’s website.
LABC – Local Authority Building Control (LABC) is a not-for-profit membership organisation that represents all local authority building control teams in England and Wales. Their full list of approved details can be viewed on their website.
Advanced Details – Advanced Details is an online Thermal Bridging database aimed at simplifying the process of calculating junctions, whilst offering options for improved performance.
Accredited Construction Details (Scotland) 2015 – The Building Standards Division of the
Directorate for Local Government and Communities in Scotland has produced an improved set of details, which can be viewed here.
Thermal Bridging Schemes from Manufacturers:
Many insulation manufacturers have produced their own approved details which feature construction methods with their own products to limit PSI values. Xtratherm and Celotex both provide calculations for applications of their PIR boards, as does Kingspan with its TEK SIP systems.
Xtratherm – Xtratherm Advanced Detailing was created by the insulation manufacturer Xtratherm, and has been available to constructors since SAP 2009.
Kingspan – Kingspan have produced a set of approved details for their TEK SIP system, beating us hands down for the best thermal bridging pun with their ‘PSI of relief’ document. You can find out more about this scheme on the Kingspan website.
Celotex – Celotex now provide a comprehensive toolset for Celotex Members, with both the Celotex U-value calculator and thermal bridging models. You can view these items on the Celotex website.
Calculate Your Own
Under certain circumstances it is possible to create your own Psi values from bespoke construction types. This does take additional time and cost so it is worth weighing up the benefits. This route makes sense for housebuilders and those who are building frequently with a standard set of junctions. There is also a case for developers and contractors who may be struggling with other design restrictions (perhaps being off mains gas for instance) and so may use improved junctions to compensate for other elements.
If thermal bridging is addressed at design stage, then the chances are that at least one of the approved junction types above is suitable. That said, if this is a route you want to explore, we may be able to help you calculate your own Psi values. Like all things SAP, thermal bridging is best addressed at design whilst changes can still be made.