Free U-value Calculator from Knauf Insulation

A U-value measures the amount of heat energy that is lost through of a building element and is expressed as Watts per square metre, per degree Kelvin, W/m2sK. The U-value is calculated through the full construction, taking into account all materials, layers, and thicknesses. This includes the thermal conductivity and thermal resistance values for each material and the effect of thermal bridges, air gaps and fixings.

A U-value can be calculated using our free, online U-value calculator here. Alternatively, please contact our Technical Services Team for assistance with your build-up.

U-values are prescribed in Building Regulations set out by each individual nation.

Each element of a building has a different target U-value to meet. As the UK’s leading manufacturer of high performance, non-combustible insulation used in new build housing, we can help you achieve cost-effective compliance, without compromising on quality, combustibility, whole-life carbon or your customers’ expectations. The new Approved Document L for England and Wales contains ‘limiting U-values’ for insulating fabric elements in new dwellings (excerpt below). These are sometimes mistakenly referred to as ‘minimum U-values’ however this term is not quite accurate. Whilst they indicate a minimum thermal performance requirement for each element, they are actually maximum U-values (i.e. heat transfer through the element should not exceed this value). Limiting U-values for new fabric elements in new dwellings All roof types 0.16 W/(M2K) Wall 0.26 W/(M2K) Floor 0.18 W/(M2K) Party wall 0.20 W/(M2K)

A lower U-value is better. The U-value measures how much heat is transferred through a material or building element (e.g., walls, roofs, floors, or windows). It indicates the material's thermal performance, with lower U-values meaning better insulation and less heat loss. This helps improve energy efficiency, reduce heating and cooling costs, and maintain a comfortable indoor temperature. In summary, the lower the U-value, the better the thermal performance of the application.

A good U-value for floors depends on the energy efficiency standards and the specific requirements of the building. Generally, lower U-values indicate better insulation and energy performance. Here are typical recommendations:

Floors with lower U-values help reduce heat loss to the ground, improve indoor comfort, and lower energy costs. The exact target U-value may vary depending on the climate, type of building, and local regulations.

A good U-value for roofs depends on the energy efficiency standards and climate of the region. Generally, roofs require better insulation than walls or floors because heat rises and can escape more easily through the roof. Here are typical recommendations: Energy-efficient buildings: A U-value of 0.11–0.20 W/m²K is considered very good for roofs. Passive House standards: Roof U-values are often required to be 0.10 W/m²K or lower. Building regulations: In many regions, the maximum allowable U-value for roofs is around 0.20–0.25 W/m²K, but lower values are preferred for better energy efficiency. The exact target depends on local building codes and the type of roof (e.g., flat or pitched). Lower U-values indicate better insulation and reduced heat loss, which is ideal for energy savings and comfort.

A U-value measures the rate of heat transfer through a specific build-up, with lower values indicating better performance. Here are some general guidelines: Walls: U-value for walls in energy-efficient buildings is typically 0.18–0.30 W/m²K. Roofs: Roofs generally require better performance, with U-values around 0.11–0.20 W/m²K. Floors: For floors, a good U-value is usually 0.15–0.25 W/m²K depending on the type of floor and build-up. Windows: For double-glazed windows, U-values of 1.0–1.6 W/m²K are common, while triple glazing can achieve values as low as 0.8 W/m²K or better. These values are often aligned with building regulations and energy efficiency standards, such as those in the EU (e.g., Passive House standards) or other regions. Lower U-values are generally better for reducing heat loss and improving energy efficiency.

Considering U-values is important as they measure the thermal performance of building applications, indicating how well materials prevent heat transfer. Lower U-values mean better insulation performance and energy efficiency, which is crucial for several reasons: