R-value (insulation)

The R-value 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. The higher the R-value, the more insulating the material is. Higher R-values can reduce heating bills in cold weather and cooling bills in hot weather. R-value can be expressed with 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. The U-factor or U-value is the overall heat transfer coefficient and can be found by taking the inverse of the R-value. It is a property that describes how well building elements conduct heat per unit area across a temperature gradient. The elements are commonly assemblies of many layers of materials, such as those that make up the building envelope. It is expressed in watts per square metre kelvin. The higher the U-value, the lower the ability of the building envelope to resist heat transfer. A low U-value, or conversely a high R-value, usually indicates high levels of insulation. They are useful as it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials.

R-value definition R-value is defined as

R val = Δ T ϕ q , {\displaystyle R_{\text{val}}={\frac {\Delta T}{\phi _{q}}},}

where (using SI units):

R val {\displaystyle R_{\text{val}}} (K⋅m2/W) is the R-value,

Δ T {\displaystyle \Delta T} (K) is the temperature difference between the warmer surface and colder surface of a barrier,

ϕ q {\displaystyle \phi _{q}} (W/m2) is the heat flux through the barrier. The R-value per unit of a barrier's exposed surface area measures the absolute thermal resistance of the barrier.

R val A = R , {\displaystyle {\frac {R_{\text{val}}}{A}}=R,}

where (using SI units):

R val {\displaystyle R_{\text{val}}} is the R-value (m2⋅K⋅W−1)

A {\displaystyle A} is the barrier's exposed surface area (m2)

R {\displaystyle R} is the absolute thermal resistance (K⋅W−1) Absolute thermal resistance, R {\displaystyle R} , quantifies the temperature difference per unit of heat flow rate needed to sustain one unit of heat flow rate. Confusion sometimes arises because some publications use the term thermal resistance for the temperature difference per unit of heat flux, but other publications use the term thermal resistance for the temperature difference per unit of heat flow rate. Further confusion arises because some publications use the character R to denote the temperature difference per unit of heat flux, but other publications use the character R to denote the temperature difference per unit of heat flow rate. This article uses the term absolute thermal resistance for the temperature difference per unit of heat flow rate and uses the term R-value for the temperature difference per unit of heat flux. The greater the R-value, the greater the resistance, and so the better the thermal insulating properties of the barrier. R-values are used in describing the effectiveness of insulating material and in analysis of heat flow across assemblies (such as walls, roofs, and windows) under steady-state conditions. Heat flow through a barrier is driven by temperature difference between two sides of the barrier, and the R-value quantifies how effectively the object resists this drive: The temperature difference divided by the R-value and then multiplied by the exposed surface area of the barrier gives the total rate of heat flow through the barrier, as measured in watts or in BTUs per hour.

ϕ = Δ T ⋅ A R val , {\displaystyle \phi ={\frac {\Delta T\cdot A}{R_{\text{val}}}},}

where (using SI units):

R val {\displaystyle R_{\text{val}}} is the R-value (K⋅m2/W),

Δ T {\displaystyle \Delta T} is the temperature difference (K) between the warmer surface and colder surface of the barrier,

A {\displaystyle A} is the exposed surface area (m2) of the barrier,

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