Which Junction Details Require PSI Values?
Where two building elements meet and form a junction, a linear thermal bridge is created. Its heat loss should be calculated in the form of PSI values.
A change of geometry in a building element – for example, where a wall forms an internal or external corner – also results in a linear thermal bridge. This is because there is a difference in the building fabric’s corresponding internal and external surface areas, resulting in a heat loss differential that cannot be accounted for by U-values.
Continue reading to discover the importance of using PSI values and which junction details require PSI value calculations.
Why Are Junction Details Important?
Junction specifications are vital to ensuring buildings perform as intended after construction and don’t experience a building performance gap.
A junction needs to be thoroughly designed and its components well-constructed in order to minimise the risk of poor energy performance. The materials required for a well-constructed junction detail need to have suitable characteristics and account for various building elements.
The individual building elements need to meet specific building requirements. These elements should be designed to achieve acoustic performance, boost thermal performance, provide stability to the structure, and allow for weatherproofing.
Through the use of accredited construction details (ACDs) or detailed junction drawings, the construction industry aimed to develop buildings that are easy to maintain and meet energy performance requirements.
How are PSI Values Relevant to Junction Details?
Historically, commercial buildings and residential dwellings could pass SAP calculations by using a default PSI value. However, default or assumed PSI values are no longer implemented as they are considered outdated and insufficient for ensuring optimal thermal performance.
Precise PSI calculations enable building designers to achieve a much more accurate insight into the potential for heat loss in a new-build site. These metrics help designers implement methods to combat or mitigate thermal bridges and improve insulation efficiency. These methods include having a thermal break installed, using more suitable materials, and employing sealant at critical joints.
PSI values ensure accurate SAP calculation, enabling better regulation compliance before and after construction and ensuring optimal insulation.
What Junctions and PSI Values are Listed in SAP Appendix K?
Linear thermal bridging is taking on increasing prominence within building regulations, as evidenced by the Part L 2021 uplift. It is worth remembering, though, that building regulations are only a minimum standard.
From the perspective of complying with national building regulations, we are mainly interested in how the Standard Assessment Procedure (SAP) deals with PSI values. Appendix K of SAP lists all of the different junctions and linear thermal bridges, along with their default PSI values.
The default values are used in the compliance calculation if no project-specific value is entered, and they penalise the calculation heavily. Achieving better performance than the default values makes compliance easier and is essential if going down the route of a voluntary standard such as Passivhaus (which has its own requirements for thermal bridging and surface temperature factors.)
Appendix K groups details into the following three types of junctions.
- Junctions with an external wall.
- Junctions with a party wall.
- Junctions within a roof, or within a room-in-a-roof.
Understanding Different Junction Types in More Detail
Appendix K of the SAP guidelines provides guidance on which components of each junction type need to be considered for construction.
E1–E25 External Wall Junction Details
‘Junctions with an external wall’ is the broadest category in Appendix K. It includes:
- Different floor types – ground floor, basement floor, exposed floor, intermediate floor and so on
- Sills, jambs and lintels – where doors and windows interact with the surrounding construction
- Other lintel types – including steel lintels
The remainder of the junctions covered in this category are balconies, corners and party walls.
P1–P8 Party Wall Roof Junction Detail
The ‘junctions with a party wall’ category is much smaller and covers different ground floors, exposed floors and intermediate floors. It also features roof junctions where insulation is either at the ceiling or rafter level. For roof junctions, it includes different eaves, gable and flat roof options.
With regard to party wall junctions, sound insulation and fire resistance, along with thermal considerations, need to be taken into account.
R1–R11 Junctions Within a Roof or Room-in-Roof Junction Details
Finally, this category covers a variety of specialist junctions, such as those relating to roof windows and lights, and different interactions between ceilings, ridges and external walls.
When calculating the thermal performance of a roof, it’s vital to factor in roof window thermal performance as well.
AutoPSI: Streamlining PSI Value Calculations for Different Junction Details
With many sources for both pre-calculated and project-specific PSI values, it can be difficult to know where to get the right calculations to meet the needs of individual projects. To encourage wider adoption of accurate PSI values, and to make bespoke junction detailing less onerous, BRS Technology has developed AutoPSI.
AutoPSI is an innovative online-only thermal modelling software that offers quick and easy calculation of accredited PSI values in accordance with relevant standards and conventions like BS EN, ISO 10211 and BR497. This tool offers a smart way of modelling fully customisable junction details by using a user-friendly drag-and-drop interface.
Products and components can be added and removed quickly. This makes it easy to experiment with different configurations during the construction design stage and instantly see the impact on the calculated PSI value.
Find out how AutoPSI can help you simplify your thermal modelling calculations and pass SAP assessments with ease. Sign up for a seven-day free trial today.
To find out more, visit the AutoPSI website or visit our LinkedIn page.
