Thermal Performance of Assemblies: Effective R-Value vs. U-Value
In a building, each material has its own R-value, which is the thermal resistance in imperial units (or RSI value in metric units).
A material’s thermal resistance is its ability to slow the heat flowing through it. Therefore, the higher the R or RSI value, the lesser the heat transfer, thus lesser heat loss. Some materials, like insulation, have a positive impact on thermal performance, while others, like jambs, have a negative impact because they are conductive.
Whether for an exterior wall, roofing or other structure, an assembly is composed of a variety of materials. All these materials (and other elements) contribute to the thermal performance of an assembly, which is the effective R-value or effective RSI value.
Effective R-value is the overall thermal resistance of an assembly. It is a weighted average of all the components in this assembly. It is therefore necessary to know the thermal resistance of all the materials in the assembly, including structural elements, exterior siding, exterior and interior air barriers, and air spaces, in order to calculate the effective R-value.
Remember that the R-value is linked to a material, whereas the effective R-value is linked to an assembly.
When talking about effective R-value, we must link it to the National Energy Code for Buildings (NECB) requirements. For each region, a minimum thermal resistance value is prescribed and must be respected. This is based on the amount of heating degree days required for each building area, which is established by the National Energy Code for Buildings (NECB) or other applicable regulations in that zone.
Rather than using the effective R-value for its requirements, the NECB uses the overall thermal transmittance, also known as U-value. It represents the rate of heat transmission through the elements of an assembly when exposed to temperature variations. It is the exact opposite of thermal resistance in metric units. In simple math, U equals to 1 divided by effective RSI value. The fact that U-value is reciprocal of effective RSI value means that a lower U-value represents a more efficient assembly. The lower the U-value, the better the thermal performance of the assembly. The NECB tables present maximum U-values in metric units. The requirements are categorized by climatic zones. An assembly complies with the Code only if its U-value is INFERIOR to those listed in the tables.