U-Value Calculator
Free online U-Value Calculator that runs directly in your browser.
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Heat transfer coefficient U calculator
| Material | Thickness (cm) | λ (W/mK) | |
|---|---|---|---|
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Other tools you may find usefulHeat transfer coefficient U - a key parameter of the building's energy efficiency
The heat transfer coefficient U (formerly marked as k) is one of the most important technical parameters in construction. It determines how much thermal energy (expressed in watts) penetrates through one square meter of a given building envelope (wall, roof, ceiling, window or door), with a temperature difference of 1 kelvin (or 1 degree Celsius) on both sides. The unit of the U-value is W/(m²·K). The lower the value of this coefficient, the better the thermal insulation properties of a given partition, which directly translates into lower heat losses and lower building heating bills.
Our professional U-value calculator for a multi-layer partition allows you to quickly calculate thermal transmittance based on the thickness of individual layers (structural wall, insulation, plaster) and their thermal conductivity lambda (λ), facilitating the design of partitions in accordance with the latest WT 2021 regulations.
How to calculate the U-value of a multi-layer partition? Formula and resistances Rsiand Rse
Calculation of the heat transfer coefficient U involves determining the total thermal resistance of the partition RT, which is the sum of the resistances of individual layers and the heat transfer resistances on the external and internal surfaces. The formula for the U coefficient is:
U = 1 / RT
Where the total thermal resistance RTis calculated from the formula:
RT= Rsi+ R1+ R2+ ... + Rn+ Rse
In individual components:
- Rsi:Heat transfer resistance on the inner surface of the partition (depends on the direction of the heat flow - vertical, horizontal or upwards). According to the PN-EN ISO 6946 standard, it is e.g. 0.13 m²·K/W for external walls.
- Rse:Heat transfer resistance on the outer surface of the partition (for the external environment it is standard 0.04 m²·K/W).
- R1, R2, ..., Rn:Thermal resistance of individual material layers. We calculate the resistance of a given layer by dividing its thickness d (expressed in meters!) by the lambda thermal conductivity coefficient (λ) of a given material:
R = d / λ
Table of maximum U-factor values according to WT 2021 standards in Poland
From January 1, 2021, stricter technical conditions (WT 2021) regarding maximum values apply in Poland heat transfer coefficient U for newly constructed buildings. The table below shows the requirements for individual partitions:
| Type of building partition | Maximum U valuemax(WT 2021) | Average required insulation thickness (styrofoam λ = 0.035 W/(m·K)) |
|---|---|---|
| External walls(heated rooms) | 0.20 W/(m² K) | Approx. 15-20 cm (depending on the construction material) |
| Roofs, flat roofs and ceilings under unheated attics | 0.15 W/(m²·K) | Approx. 25-30 cm of mineral wool |
| Floors on the ground(in heated rooms) | 0.30 W/(m²·K) | Approx. 10-12 cm of floor polystyrene |
| Standard vertical windows | 0.90 W/(m²·K) | Requires triple-glazed profiles |
| Roof windows (roof windows) | 1.10 W/(m²·K) | Requires energy-saving profiles |
| External doors | 1.30 W/(m²·K) | Requires doors with thermal insulation |
Frequently asked questions (FAQ)
What is the difference between the U coefficient and the thermal resistance R?
Thermal resistance R (m²·K/W) determines the ability of a specific layer of material to stop the flow of heat - the thicker and better the insulating material, the greater R. The heat transfer coefficient U (W/(m²·K)) determines the total permeability of the entire partition (including the air near the wall) - the lower the U value, the better. These are inverse values: U = 1/R.
How to calculate the thermal resistance of a single wall layer?
The thermal resistance R is calculated by dividing the layer thickness d (given in meters, not centimeters!) by the thermal conductivity lambda (λ) of the material. For example, for a styrofoam layer with a thickness of 15 cm (0.15 m) and a lambda of 0.035 W/(m·K), the resistance is: R = 0.15 / 0.035 = 4.28 m²·K/W.
What is lambda (λ) and how does it affect the U-factor?
Lambda (λ) is the thermal conductivity coefficient of the building material expressed in W/(m·K). It determines how well a given material conducts heat through its structure. The lower the lambda value, the better the insulator the material is. For example, graphite polystyrene (λ = 0.031) insulates much better than classic white polystyrene (λ = 0.040) at the same thickness.
What heat transfer resistances Rsi and Rse should be assumed for the calculations of the external wall?
According to the PN-EN ISO 6946 standard, for a vertical partition (external wall) with a horizontal heat flow the following values are assumed: internal resistance Rsi= 0.13 m²·K/W and external resistance Rse= 0.04 m²·K/W. The total sum of air resistance is therefore 0.17 m²·K/W.
What requirements of WT 2021 must the external wall of a residential building meet?
From 2021, the heat transfer coefficient U for an external wall in a heated room cannot be higher than 0.20 W/(m²·K). To meet these requirements, a traditional wall made of aerated concrete or porous ceramics with a thickness of 24-25 cm must be insulated with a layer of polystyrene or wool with a minimum thickness of 15-20 cm.