U-Value Calculator

U-values refer to the rate of thermal transmittance through a material or structure – how good a thermal conductor or insulator it is. U-values are measured by the unit W/m²K.

A low U-value means that heat will not transfer easily through a material, making it a good thermal insulator. Different materials have different properties and may be better insulators or conductors, but a low U-value means better thermal performance when it comes to insulation.

Lower U-values indicate that heat will not transfer easily through a material or structure – so if you’re looking for thermal insulation which prevents heat loss, a lower U-value is better.

To reduce the U-value of a structure, construction materials with low U-values should be specified. Thermal performance can also be improved during renovation and refurbishment, and installing effective insulation is a proven and successful method.

There are a series of factors affecting U-value requirements – type of wall structure, type of building, and whether it is a new build, extension or refurbishment project. There are also different requirements for England, Scotland, Wales and Northern Ireland.

Part L of the Building Regulations states that standard replacements for windows should have a U-value no higher than 1.6 W/m²K. Some existing windows may be significantly higher than this, but replacements should improve performance to Part L standards.

The required U-values for floors are specified in Part L of the Building Regulations, and this can vary between new build projects and extensions or renovations. U-value targets for many floors are between 0.45 and 0.25 W/m²K.

Lower U-values indicate better thermal performance, and will vary depending on the insulation material chosen. Some materials, such as ROCKWOOL stone wool insulation, offer U-values as low as 0.038 W/mK.

R-value is a measure of heat resistance, whereas U-value is a measure of heat transfer. R-value is, therefore, the reciprocal of U-value – but both can be used to measure thermal performance.

Every part of the building fabric contributes to the thermal performance of a house, and ultimately the thermal comfort of the occupants. There are different requirements for different building elements, but low U-values indicate good thermal performance which minimises heat loss.

Different insulation materials and solutions will hold different U-values and therefore offer varying levels of thermal performance. Lower U-values indicate insulation materials with better thermal performance.

The ‘U’ in U-value does not stand for an individual word. U-value is a measure of the rate of thermal transmittance through a material or structure, described in units of W/m²K.