What is R-value (insulation)?
R-value (insulation) is a measure of thermal resistance, specifically how well a two-dimensional barrier, such as a layer of insulation, a window, or a complete wall or ceiling, resists the conductive flow of heat, in the context of construction.
Why does R-value (insulation) matter?
The R-value is an important factor in building design and construction, as it directly affects the energy efficiency of a building. The higher the R-value, the more insulating the material is, which can reduce heating bills in cold weather and cooling bills in hot weather.
Key Facts about R-value (insulation)
- R-value can be expressed in both metric and United States customary units.
- When expressed in metric, the term RSI-value is often used.
- R-values expressed in United States customary units are approximately 5.68 times as large as R-values expressed in metric.
- An R-value can be given for a material (e.g., for polyethylene foam), or for an assembly of materials (e.g., a wall or a window).
- In the case of materials, it is often expressed in terms of R-value per metre, or per inch in the US.
- R-values are additive for multiple layers of materials.
- R-value is defined as the temperature difference needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions.
History of R-value (insulation)
The R-value has been a standard measurement in the construction industry for many years, but its specific definition and units have evolved over time. The current definition of R-value is based on the International System of Units (SI) and the United States customary units.
Examples of R-value (insulation)
R-value can be applied to various materials and assemblies, such as:
- Polyethylene foam with an R-value of 2.8 per inch
- A wall with an R-value of 15.6 per square metre
- A window with an R-value of 1.8 per square metre
R-value (insulation) and Building Envelope
The R-value is an important factor in designing a building envelope that is energy-efficient. A building envelope is the outermost layer of a building that separates the interior from the exterior. The R-value is used to calculate the overall heat transfer coefficient, known as the U-factor or U-value, which describes how well building elements conduct heat per unit area across a temperature gradient.
FAQ
What is the unit of measurement for R-value? R-value can be expressed in both metric (R-value) and United States customary units (R-value per inch).
How do multiple layers of insulation affect R-value? R-values are additive for multiple layers of materials, meaning that the R-value of each layer is added together to calculate the total R-value of the assembly.
What is the difference between R-value and U-factor? The R-value is a measure of thermal resistance, while the U-factor is the overall heat transfer coefficient, which is the inverse of the R-value.
How is R-value expressed for materials and assemblies? An R-value can be given for a material (e.g., for polyethylene foam), or for an assembly of materials (e.g., a wall or a window).
What is the relationship between R-value and energy efficiency? A higher R-value indicates better insulation, which can reduce heating bills in cold weather and cooling bills in hot weather.