Glossary
What is a U-Value?
A measure of how well a building element prevents heat loss
U-value measures the rate of heat transfer through a building element in watts per square metre per degree Kelvin (W/m2K). A lower U-value means better insulation. UK building regulations require new windows to achieve a U-value of 1.4 W/m2K or below.
Where it wins
- Standard metric for comparing insulation performance
- Used in UK Building Regulations for compliance
- Helps identify where a home is losing most heat
- Applies to walls windows roofs and floors
Where it struggles
- Does not capture air leakage or thermal bridging
- Calculated values may differ from real-world performance
- Difficult for homeowners to measure without specialist equipment
- A single U-value does not show whole-house efficiency
How it works
01
Measure the temperature difference across the building element
02
Measure heat flow in watts per square metre
03
Divide heat flow by temperature difference
04
Lower result = better thermal performance
U-Value: common questions
What is a U-Value?
A measure of how well a building element prevents heat loss
How does a U-Value work?
U-value measures the rate of heat transfer through a building element in watts per square metre per degree Kelvin (W/m2K). A lower U-value means better insulation. UK building regulations require new windows to achieve a U-value of 1.4 W/m2K or below.
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