Free Online U Value Calculator
Use our free online U Value Calculator to calculate the U value for your project.
Simply choose your units of choice, add in the layers you need, and calculate the u values.
U-Value Calculator
Heat Loss Estimation
A U-value (thermal transmittance) measures how effectively a material or building element (such as a wall, roof, window, or floor) allows heat to pass through it.
It is expressed in W/m²·K (watts per square meter per degree Kelvin), where lower values indicate better insulation and reduced heat loss. The U-value is calculated as the reciprocal of the total thermal resistance (R-value) of all layers in a construction assembly, including surface resistances. It is a crucial metric in building energy efficiency, as regulations and energy codes often set maximum allowable U-values to improve insulation and reduce heating and cooling costs.
In some regions, U-values may also be expressed in BTU/(hr·ft²·°F), the imperial equivalent.
How are U Values Calculated?
A U-value tells us how much heat passes through a material or a combination of materials. The lower the U-value, the better the material is at keeping heat inside (or outside) a building.
To calculate the U-value, we first find the thermal resistance (R-value) of each layer in the structure. Thermal resistance depends on two things:
1. Thickness of the material (how thick it is, measured in meters).
2. Thermal conductivity (how easily heat moves through it, measured in W/m·K).
We calculate the R-value of each layer like this:
R = Thickness ÷ Thermal Conductivity
Once we have the R-values for all layers, we add them together to get the total resistance (R-total) of the structure.
Finally, the U-value is simply:
U = 1 ÷ R-total
The lower the U-value, the better the insulation!
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Why do U Values matter?
Lower U-values mean better insulation, reducing heat loss and improving energy efficiency in buildings.
What are the regulatory standards for U values?
Different countries have building regulations that set maximum allowable U-values for different components:
• Walls: ~ 0.18 - 0.35 W/m²·K
• Roofs: ~ 0.12 - 0.20 W/m²·K
• Windows: ~ 1.0 - 3.0 W/m²·K
Impact on Energy Efficiency
A low U-value reduces energy demand, making buildings warmer in winter and cooler in summer.
U Values for common materials
| Material | U-Value (W/m²·K) |
|---|---|
| Single Brick Wall (no insulation) | 2.00 |
| Cavity Wall with Insulation | 0.30 |
| Solid Concrete Wall (no insulation) | 2.20 |
| Timber Frame Wall with Insulation | 0.25 |
| Double Glazed Window | 2.70 |
| Triple Glazed Window | 0.80 |
| Flat Roof (uninsulated) | 4.50 |
| Flat Roof with Insulation | 0.18 |
| Pitched Roof with Insulation | 0.12 |
| Floor (concrete, no insulation) | 1.50 |
| Floor with Insulation | 0.20 |
U Value comparison for common building materials
The above chart shows a comparison of the U values for common building materials.
As you can see, an uninsulated flat roof has a much higher U value than an insulated Flat Roof. This highlights the higher the U value, the worse the heat insulation value of the material.
Hopefully, you've been able to use our free online u value calculator to create a picture of how well insulated your project may be.