U-values and Ψ-values: What’s the Difference and Why Are They Important?

Part L of the Building Regulations requires new buildings to meet minimum energy efficiency standards. Local planning authorities may also impose energy  requirements above and beyond Building Regulations.

Calculating the energy performance of a new building is done using specialist software but as with any tool, the output you get is only as accurate as the  information you put in. While it’s possible to use default values, you’ll get a more accurate picture of energy performance if you use information
specific to the design of your building and products used.

When considering building construction, two particular values are important to get right: u-values and Ψ-values.

They’re both used to measure heat loss – so what’s the difference?

U-values measure the heat loss through a square metre of a thermal element, for example, a wall, window or floor. The lower the u-value, the better insulated the element.

Ψ-values (sometimes called Psi values) measure the heat loss at junctions between thermal elements – for example where a floor joins an external wall – and around openings. Lower Ψ-values mean less heat is lost through non-repeating thermal bridges.

Superglass Blog | Builders at Construction Site

Why Does This Matter?

Thermal bridging occurs when a part of a building’s thermal envelope has a much higher heat transfer than the surrounding area. Like a hole in a bucket, it leaks heat. As buildings have become better insulated, the impact of thermal bridging on energy performance has increased. In fact, research has shown that thermal bridging can be responsible for 30% of a dwelling’s heat loss.

Calculating these heat losses accurately is the only way to get a true picture of the energy performance of the building. It will also help highlight where the fabric energy efficiency could be improved and construction details that need to be focused on during the project build.

As well as giving a more accurate picture, using u-values and Ψ-values that are specific to your products and design may also give you a lower overall predicted energy consumption. This is because the default values used in the software are often on the high side to take account of a range of specifications. This means you may not have to invest in expensive low carbon or renewable technologies to meet your energy efficiency targets.

U-values – Getting Them Right

Our in-house Technical Team can provide u-value calculations for all Superglass products as well as Building Information Modelling (BIM) objects. We can also help create bespoke products and solutions to meet the most stringent fabric energy efficiency standards.