Do U-Values and PSI (ψ) Values Measure Heat Loss Differently?

U-values and PSI (ψ) values are both measures of heat loss from buildings, but they are calculated in different ways. They play different roles in building performance and therefore complement one another. Both values must be taken into account when assessing overall energy efficiency and carbon emissions.

Despite this, it can be argued that more people are aware of U-values and the need to meet certain targets, especially when it comes to demonstrating compliance with building regulations. Familiarity with PSI values continues to grow, however. Regulation changes are making it almost impossible to achieve compliance without having accurately calculated PSI (ψ) values in heat loss calculations.

Read on to learn more about U-values, PSI values and their role in heat loss calculations.

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What Are U-Values?

Thermal transmittance (U-value) is the rate of heat transfer throughout a structure, which could be made from conductive materials. It is measured in watts per square metre kelvin (W/(m²K)). 

Fabric choice plays a significant role in contributing to the overall thermal performance of a building, and a u-value is a heat loss measurement between thermal elements. When a building element has poor insulation, thermal transmittance will be higher than normal due to thermal conductivity.

A U-value measurement indicates how effective a structure is in preventing unnecessary heat transfer.

While U-values relate to thermal conductivity, they can be confused with Y-values. The heat transfer coefficient (Y-value) measures how materials can absorb heat losses and transfer them. This is also known as thermal admittance, which is similar to transmittance. 

Y-values are not the same as U-values, either. Instead, thermal bridging refers to weak points, or cold bridging, where heat passes through more easily because it has the least resistance.

U-Value Calculations

To calculate a U-value, the thermal resistance of each material making up a building will be added together. The final value will be the reciprocal of this sum of thermal resistance. 

You can also do this heat conduction calculation layer by layer of the thermal elements, but this won’t account for cold bridges, air gaps and other thermal elements and properties. It’s much easier and more efficient to find an accurate U-value with the use of U-value calculators.

What Are PSI Values?

Think of the average room temperature in winter; it’s the combination of the internal temperature and heating system. In summer, there is a big temperature difference. There are no active heating systems in place, so the temperature is a combination of the internal temperature and cooling system.

While insulation focuses on thermal resistance, PSI values measure specific heat capacity in linear thermal heat transmittance. These values are measured in watts per metre kelvin (W/mK) and indicate where heat losses occur and how much energy is lost between building elements, such as the point between the ground floor and a cavity wall. 

This total heat loss calculation is beneficial for the construction design industry, as it enables building designers to create buildings with optimal energy performance. Accurate PSI value calculations help determine whether there is more heat required. It is also extremely helpful for understanding ventilation heat loss through window calculations or elsewhere. 

In the past, many people in the construction design industry used default assumed PSI values and Accredited Construction Details. These are no longer used to meet building standards and pass the Standard Assessment Procedure (SAP). 

With the introduction of new standards, it is no longer acceptable to use the old recognised construction details with assumed values to meet regulations. With AutoPSI, compliance with Part L of Building Regulations is made more efficient to ensure the use of accurate values.  BR497 also mandates the use of 3D PSI values for high-rise construction sites.

PSI Value Calculations

To calculate PSI values, measurements are taken through the junctions of a building. These junctions can be between a ground floor and an external floor, between windows or elsewhere. 

To calculate a PSI value, you need to take into consideration the heat transfer simulation online and the impact of the relevant U-values. To get the final value of PSI, the output value is multiplied by the length of a junction. You can do these calculations manually, but this can be incredibly time-consuming and complicated.

What is the Difference Between U-Values and PSI Values?

PSI values are quite similar to U-values as they’re both important heat loss values — which is why they are often confused. However, there is quite a big difference between a heat loss U-value and a PSI value.

While U-values measure the loss of heat through thermal elements, these values cannot account for the total heat loss in a junction. 

At places of junctions in a building with two thermal elements, such as the ground floor and an external wall, there can be additional heat loss and air changes. This is known as non-repeating thermal bridges. These thermal bridges can result in an inside temperature change.

PSI values can measure the total heat loss of buildings, including non-repeating thermal bridging, where there are meeting junctions. These values can help designers address air changes and lower temperatures where necessary.

How Do U-Values and PSI Values Measure Building Fabric Heat Loss?

Temperature difference and heat loss through the main building fabric elements are established by calculating U-values and thermal resistances. These fabric elements include floors, internal and external walls, and roofs (both pitched and flat, including a cavity wall). The elements are assessed as a series of layers, taking into account the thermal conductivity and performance of each material and their surface areas.

There are simplified and detailed methods for calculating a U-value, but the end result is the same: you measure heat loss through one square metre of the element in question. The value can then be multiplied by the total area of that element to establish its total heat loss.

Where two different elements meet, however, the construction becomes more complicated. Junction details are not a simple build-up of consistent material layers. Structural considerations often take precedence, usually making it difficult to achieve continuity of insulation from one element to another.

Assessing the performance of these junction details in a home or large construction site is where PSI values come in. They take into account the heat loss that occurs between elements, modelling it along one linear metre of the junction detail. As a U-value does with surface area, a PSI value can be multiplied by the actual junction length to establish its overall heat loss.

Why are PSI (ψ) Values Required Even for Simple Junctions?

A U-value measures heat loss as it flows directly through the building fabric from the warm interior to the cold outside temperature. One square metre of internal surface area corresponds to one square metre of external building fabric.

Imagine the corner of a building and an external wall changing direction by 90 degrees. Even if the wall insulation maintains perfect continuity around that corner, the external surface has a greater area than the internal surface of the corner. The rate of heat flow is, therefore, higher and cannot be measured by a U-value.

Here as well, PSI (ψ) values are needed. Without them, it’s not possible to have a complete picture of heat loss across the whole building envelope. 

Because PSI values cannot be calculated using the simplified method usually used for U-values, people often prefer to assume default or worst-case values for junction details instead of using accurate heat loss calculations. This penalises the whole building’s performance in compliance calculations, leading to over-specification elsewhere to make up for it.

The Benefits of Using Accurate U-Values and PSI Values

There are many advantages to using accurate PSI values and U-values for heat loss calculations in the construction design industry, from meeting regulations to preventing heat from escaping building materials. Here are some of the stand-out benefits of using these values.


  • Saving money on energy bills
  • Preventing surface mould growth
  • Improving indoor temperature
  • More environmentally-friendly

PSI Values

  • Meet regulations and compliance requirements
  • Lower carbon emissions without retrofitting
  • Save money on immediate engineering costs and material waste
  • Save time with efficient, editable templates and customisable junctions 

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Heat Loss Calculations – FAQs

Still have a few questions about calculating heat loss with a U-value or PSI value? Take a look at our frequently asked questions below.

What is a normal U-value?

Every U-value is different, depending on the type of material you’re using, but they are all measured in Watts per square metre per Kelvin (W/(m²K)). So, it will show the number of watts transmitted per degree difference in temperature. Common materials like brick walls are 2 W/(m²K), insulated walls are 0.18 W/(m²K), and a solid timber door would be 3 W/(m²K). 

Is a higher or lower U-value better?

As U-values measure insulation, the lower the value is, the better the insulation is in a material. Therefore, a lower U-value is always better and indicates that the thermal envelope will maintain consistent temperatures during seasonal temperature changes and extreme weather. For example, a low amount of heat flow would be closer to a U-value of 0.1 W/(m²K).

Who calculates PSI values?

Generally speaking, PSI value calculation tools are most often used by architects, companies providing a service to housebuilders, SAP assessors, house builders and technical teams. AutoPSI provides an online tool to make PSI measurements simple and efficient. Our straightforward software allows people in the construction design industry to streamline calculations.

How to measure heat loss in a house?

Heat loss in a home can be measured by understanding a house’s temperature design and surface area. You’ll also need to calculate the R-value and U-value and the amount of air infiltration heat loss, followed by the total heat loss calculated through a PSI value calculation.

Is a lower PSI value better?

PSI values are quite similar to U-values in that the lower the PSi value, the better. A lower PSI value will indicate better thermal performance. Most PSI values range between 0.04 and 0.48.

What is the formula for current heat loss?

You can calculate current heat loss with the following formula: q = (U × A) × Δt. In this formula, ‘q’ is the total heat loss of materials in thermal units per hour, ‘U’ is the total coefficient of heat transmission, ‘A’ is the surface area and ‘Δt’ is the temperature difference (both inside and outside).

What are the types of heat loss in buildings?

There are four general types of heat loss in buildings. 

  • Thermal radiation usually happens in windows when heat escapes outside from glazing. 
  • Convection is the transfer of energy from a door depending on wind changes. 
  • Conduction is the loss of heat due to the movement of air around spaces like windows and door frames. 
  • Air infiltration is the escape of heat through other spaces, like cracks under doors or windows.

Have a few more questions about U-value or PSI value calculations? Reach out to the friendly AutoPSI team for help, or sign up for a demo to test our fully online PSI measurement tool.

AutoPSI: Simplified Heat Loss Calculations 

At AutoPSI, we recognise the importance of getting exact U-values and PSI values to successfully complete SAP heat loss calculations. AutoPSI is an online modelling tool that allows you to calculate PSI values quickly and easily and therefore achieve more balanced building fabric specifications. We designed our heat transfer simulation software as an efficient thermal modelling calculator to streamline how you calculate heat loss in a building. 

Our fully online, user-friendly software performs heat loss calculations by modelling bespoke, customisable junction details, with products and components added and removed using a drag-and-drop interface, to help work out the best solution for heat loss in homes or other buildings.

Our heat loss calculator lets you add or remove whichever data components are necessary. With accurate calculations tailored to your buildings, you will use less energy from planning to project completion and pass regulations easily. 

AutoPSI has worked with many clients who have streamlined their projects with our heat energy software. We aim to help the construction design industry save time and money with our PSI value calculator. We can assist by accommodating PSI values, SAP calculations and thermal modelling into your project and set you up for success.

Now that you understand the role of U-values and PSI values in building heat loss calculations, find out how AutoPSI can help you with industrial or domestic heat loss calculations.

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