The key difference between R value and U value is that higher the R value better the insulation, whereas lower the U value better the insulation for a particular material.
R value and U value are important factors regarding the insulation ability of different materials. These values indicate the thermal performance of a barrier structure.
CONTENTS
1. Overview and Key Difference
2. What is R Value
3. What is U Value
4. Side by Side Comparison – R Value vs U Value in Tabular Form
5. Summary
What is R Value?
R value the temperature difference per unit of heat flux required to sustain one unit of heat flux between the warmer surface and colder surface of a particular barrier under steady-state condition. In the field of building and construction, this value measures how well a 2D structure (e.g. a layer of insulation), a window, or a complete wall or ceiling resists the conductive flow of heat.
In the building industry, the R value is the term for thermal resistance per unit area. Sometimes, this term is denoted by RSI value when we are using the SI unit system. We can give the R value for a particular material (e.g. polyethylene foam) or a collection of some materials (e.g. a wall). Usually, we can express the R value in terms of R value per unit length when considering a particular material. Moreover, we can add R values for layers of materials. Higher the R value better the performance of insulation. The mathematic expression for this value is as follows:
What is U Value?
U value is the rate of transfer of heat through one square meter of a barrier divided by the difference in temperature across that barrier structure. In the field of building industry, this term refers to the overall heat transfer coefficient that can describe how well a building element conducts heat. In this context, the term element refers to the assembles of many layers of components, such as the components that make up walls, roofs, floors, etc. The mathematical expression for this value is as follows:
The U value measures the rate of heat transfer through a building element across a given area under standard conditions. Usually, the standards for this measurement include a temperature difference of 24 Celsius degrees, at 50% humidity where there is no wind. The unit of measurement is typically Watts per meter squared Kelvin or W/m2.K. Therefore, higher the U value, worse the thermal performance of the building envelope. In other words, a lower U value refers to a high level of insulation.
What is the Difference Between R Value and U Value?
R value is the temperature difference per unit of heat flux required to sustain one unit of heat flux between the warmer surface and colder surface of a particular barrier under steady-state condition. U value, on the other hand, is the rate of transfer of heat through one square meter of a barrier divided by the difference in temperature across that barrier structure.
The key difference between R value and U value is that higher the R value better the insulation whereas lower the U value better the insulation for a particular material. These values indicate the thermal performance of a barrier structure.
Below is a summary of the difference between R value and U value in tabular form.
Summary – R Value vs U Value
R value and U value are important factors regarding the insulation ability of different materials. The key difference between R value and U value is that higher the R value better the insulation whereas lower the U value better the insulation for a particular material. These values indicate the thermal performance of a barrier structure.
Reference:
1. “Insulation.” Energy.gov, Available here.