Psi-Value Calculations

PSI Value Calculation

What Is A PSI Value Calculation?

The rate of heat movement per degree temperature difference per unit length of a thermal bridge is known as ‘linear thermal transmittance.’ It’s measured in W/mK and is used to figure out how much heat is lost or gained when a thermal bridge is present.

Psi-Values are used to calculate the overall heat loss of a building at a junction (also referred to as non-repeating thermal bridges). This is in contrast to U-Values, which exclusively determine heat loss through thermal elements (such as walls, floors, and windows). A thermal bridge is a region of a building where two thermal elements connect and can have much higher heat transmission than the surrounding materials.

Why would I need Psi-Value Calculations?

If you request a SAP or SBEM assessment of your building but do not specify unique Psi-Values, a default Psi-Value will be used. This is usually lower than a building’s actual Psi-Value result, and it can effect whether or not your building meets standards.

Completing a specific Psi-Value calculation as part of your assessment will provide more precision than using an assumed Psi-Value, increasing the chances of your property meeting compliance the first time around.

The Building Regulations/Standards require that all SAP and SBEM assessments include a thermal bridging component, regardless of whether you include a separate Psi-Value or not. This is especially crucial if you are striving to meet higher levels of sustainability.

The following are examples of typical ‘junctions’ that frequently show greater heat loss:

  • where an exposed wall meets a ground floor
  • where an external wall meets a balcony
  • Sills and lintels
  • Jambs on windows or doors

Psi-Value Calculations are performed using specialised thermal modelling software, such as Trisco.


Why are Psi values important?

The heat loss through thermal elements is accounted for by U-values, but not the total heat loss through the fabric. There is additional heat loss at the junctions, which is referred to as non-repeating thermal bridges.

This is due to the geometry of the junction as well as, in many cases, the junction’s construction:

The geometry is important because, in SAP, we measure internal dimensions (see dotted red line below) and assign U-values to thermal elements measured in this manner (e.g., external wall and ground floor). As a result, we overestimate the total heat loss. We would overestimate total heat loss if we measured external dimensions (assuming no construction psi-value)
Because the materials used at the junction have a higher thermal conductivity than those used in the thermal elements, the construction (e.g. a block of concrete at the junction rather than continuous insulation around the junction).

What Should You Do About Psi Values?

In SAP Assessments, we consider PSI values for all junctions as well as U-values for all exposed elements to best model a home’s energy performance. Psi values must be kept to a minimum in order to meet the most recent building codes.

In fact, in order to have a realistic chance of achieving compliance, designers have had to adhere to established schemes such as Accredited Construction Details and Enhanced Construction Details.

These offer predetermined junction designs based on the type of construction, such as a set for timber, a set for masonry, and a set for steel. These schemes, however, are now obsolete, and their use will be restricted in the next release of SAP and Part L regulations.

This means that designers are more likely to use certified make-ups from manufacturers (with associated, predetermined Psi values) or have their own calculated.

Psi values are an important consideration in the design of low-energy buildings.
Historically, design teams have given them a cursory glance and have frequently abandoned them in the land of default values and ‘assumed performance.’ However, times are changing, and understanding Psi values is critical when creating an efficient building design.

Briary Energy offers bespoke Psi-Value calculations, which can increase the likelihood of your SAP/SBEM assessments meeting Building Regulation/Standards compliance and resulting in greater energy savings for your property.

To organise a Psi-Value calculation, please use the contact form below.

We will not be beaten on our price or delivery times for PSI Value Calculations.