If you have ever had a SAP assessor point out a cold bridge at a lintel or wondered why a well-insulated wall still feels chilly around the edges, you are already dealing with psi values. You just might not have called them that.
The term Psi (Ψ) comes from the Greek alphabet. In the UK it is pronounced “Si” because the P is silent. Online you will see it written in all sorts of ways, such as psi value, psi-value, psi values sap, psi value symbol, what is p s i, psi reading or what does psi stand for. They all refer to the same thing: heat loss at junctions.
Understanding psi values is becoming increasingly important as the UK moves toward stricter energy standards and Net Zero Carbon. The requirements are driven by Approved Document Part L, which covers the conservation of heat and power in buildings.
What Is Psi? What Does Psi Mean?
A psi value measures heat loss at a junction. A junction is any point where two building elements meet. These areas are always weaker than a flat, uninterrupted wall. Common examples include:
- Ground floor to external wall
- External wall to balcony or canopy
- Lintels over doors and windows
- Window sills and door jambs
- Roof or ceiling connections
Whereas U-values measure heat passing through a flat surface, psi values measure heat leaking around edges. These are known as non-repeating thermal bridges, and they are a major contributor to real-world heat loss. In a cold UK winter, even a small thermal bridge can create a cold patch that attracts condensation and mould.
Why Psi Values Matter on UK Building Sites
You do not need to be building to Passivhaus standards for psi values to be a problem. They affect everyday jobs:
- Extensions. The lintel junction on a new opening often loses more heat than the wall itself.
- Refurbishment work. Replacing windows without improving the reveals usually leaves a cold stripe around the frame.
- Timber frame builds. Gaps around cavity closers or poorly aligned insulation send psi values much higher than the drawings suggest.
- Steel balconies and canopies. Any steel that passes through insulated layers acts like a radiator fin.
Psi values also have a major impact on SAP Calculations. Many assessors no longer accept default junction values. Builders and self-builders now search for terms like psi value calculations, how to calculate psi value, psi value software, knauf psi values and psi-building because poor psi values can prevent a project from passing Part L.
How Psi Values Are Calculated
Psi values are expressed in W/mK which means watts of heat lost per metre of junction for each degree of temperature difference. They are normally calculated using 2D psi values modelling in specialist thermal bridging software.
Common tools include:
- Knauf’s PSI Value Calculator
- BRE approved thermal modelling tools
- Various junction simulation programs used by SAP assessors
For deeper technical guidance, the Building Research Establishment (BRE) publishes research notes and modelling principles that underpin how psi values feed into SAP.
Typical UK Psi-Value Benchmarks
- Poor lintel psi-value: 0.30 to 0.45 W/mK
- Improved lintel psi-value: 0.15 to 0.25 W/mK
- Thermally broken lintel psi-value: 0.03 to 0.06 W/mK
Lower numbers mean less heat loss and better SAP results.
Real-World Psi-Value Problems and Fixes
1. Lintel Junctions
A bricklayer might build a neat cavity wall, but if the lintel insulation does not align with the wall insulation you get a cold bridge. In January you may see condensation forming above windows. This is usually a psi-value issue rather than poor ventilation.
Fix: use a thermally broken lintel and ensure the insulation is tight to the lintel flange.
2. Window Reveals
If insulation stops short or the cavity closer has gaps, cold air tracks around the frame. This often produces black mould patches on the internal corners.
Fix: upgrade to quality cavity closers such as the 100 mm cavity closer and maintain insulation continuity around the opening.
3. Floor Perimeter Junctions
Cold weather pours can shrink slightly. If insulation boards are not tight to the edge or the DPM curls, cold air creeps along the slab perimeter and raises psi values.
Fix: tape board joints, secure the DPM, and avoid laying insulation on frozen or saturated ground.
4. Structural Steel Penetrations
Any steel that crosses from inside to outside forms a direct path for heat escape.
Fix: specify thermal breaks where possible and insulate exposed steel internally or externally depending on the design.
How to Reduce Psi Values on Your Build
Much of psi-value improvement comes from attention to detail rather than expensive products:
- Keep insulation continuous and well-fitted. Stagger joints and avoid gaps.
- Choose better lintels such as thermally broken options.
- Use rigid, well-shaped cavity closers for openings.
- Seal potential air paths. Airtightness and psi values are closely linked.
- Avoid steel penetrating insulation without a thermal break.
Additional support materials are available through Knauf’s technical library if you want deeper reading on junction modelling and thermal continuity.
How to Calculate Psi-Value: Practical Overview
Professionals use software to calculate psi values, but the basic steps are:
- Identify the junction, such as wall to lintel.
- Enter the wall build-up and material properties.
- Define the junction geometry in 2D.
- Run the thermal model to measure heat flow.
- Subtract the heat that would pass through the plain elements.
- The result is your psi value (W/mK).
If you want a quick check or learning tool, use the Knauf psi value calculator.
Frequently Asked Questions
What does psi stand for?
It is the Greek letter Ψ. In building terms it refers to heat loss at junctions.
What is a psi?
It is a measurement of linear thermal bridging at edges or junctions, expressed in W/mK.
Is a lower psi value better?
Yes. Lower psi-values mean less heat loss and better energy performance.
Do lintels affect psi values?
Yes. Standard steel lintels often create significant cold bridges. Thermally broken lintels provide far better psi performance.
What is a psi reading?
It is another way of referring to the psi-value result from thermal modelling.
Do SAP assessors accept default psi values?
Increasingly no. Many projects require calculated psi values sap or manufacturer junction details to meet Part L compliance.
Final Thoughts
Psi-values might sound like an academic topic, but on site they show up as cold patches, mould, condensation and poor SAP outcomes. Improving psi performance with better detailing and products like thermally broken lintels and reliable cavity closers often delivers more benefit than simply adding extra insulation.
If you want your next project to pass Part L comfortably, start by improving the psi-values at the weakest junctions. It is one of the quickest ways to tighten up overall performance.
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