Thermal U-Value Calculator
Calculate transmittance rates across multiple material layers.
Layer 1: External Finish
Layer 2: Insulation
Layer 3: Inner Leaf
Thermal Analysis
Thermal Engineering: U-Values & Efficiency
Heat Retention
With energy prices rising and UK climate targets tightening, understanding U-values has never been more important. A U-value tells you exactly how much energy is escaping through your walls every second for every degree of temperature difference between inside and outside.
By lowering your U-value, you reduce your heating bills and carbon footprint while creating a more comfortable living environment.
UK Building Regs Targets
The Science: R-Values vs U-Values
In many countries, insulation is sold by its **R-value** (thermal resistance). In the UK construction industry, we prefer the **U-value**.
The R-value measures a single material's ability to resist heat flow. The U-value measures the performance of the *entire* assembly (including air gaps and surface effects). Mathematically, the U-value is the reciprocal of the total R-value.
PIR Board (Celotex/Kingspan)
With a lambda value as low as 0.022 W/mK, PIR boards are the most efficient common insulation in the UK. They allow you to achieve modern U-value targets with thinner wall profiles.
Mineral Wool (Rockwool)
While mineral wool has a higher lambda (approx 0.044 W/mK), it provides superior acoustic insulation and fire resistance compared to plastic-based boards.
Pro Tip: Cold Bridging
A U-value calculation assumes a perfect layer of insulation. However, "thermal bridges" (like timber studs or metal wall ties) can bypass your insulation and leak heat. Building control officers look closely at these junctions during inspections.
Thermal Performance FAQ
Technical Disclaimer
U-value calculation is a regulated process. For official SAP or EPC ratings, calculations must be performed using specialized software (like BRE's BR443) by a certified energy assessor. This tool provides an educational estimate only.