Total R-value

R-value is a measure of thermal resistance used to assess the effectiveness of insulation. It illustrates how well a particular material resists the conductive flow of heat. The value is determined based on the thickness of the material and its inherent thermal conductivity.

The "Total R-value" is the overall thermal resistance of a building component comprising multiple layers of different materials, such as a wall, floor, or roof. This is calculated by adding the R-values of each layer or component together.

These layers typically include:

External Cladding: The external layer is exposed to the outdoor environment. Different materials have different R-values - for instance, brick has a lower R-value than timber. These are often ignored in building codes and best practices.

Air Gap/Ventilated Cavity: There may be a small air gap between the cladding and insulation. This space allows for ventilation, aiding in moisture management, and the air itself also has an R-value. The actual R-value of the air gap is dependent on airflow and the bounding surfaces facing into the cavity.

Insulation: The primary thermal barrier contributes significantly to the total R-value. Various types of insulation exist, including batts (like fibreglass, rock wool, etc.), rigid foam boards, and blown-in insulation (like cellulose, etc.).

Internal Lining: The inner layer of the wall facing the inside of the building. Like external cladding, different materials have different R-values, but they are minor and ordinarily insignificant.

Air Films: These are thin layers of air adhering to a wall's outside and inside surfaces. They contribute to the Total R-value but on a smaller scale compared to the primary insulation layer.

In addition to these layers, thermal bridging must be considered. Thermal bridging is a phenomenon where there's a path of least resistance for heat transfer across an insulation layer, often through a more conductive material. This occurs typically through structural elements like wood or steel studs in a wall, roof rafters, or floor joists, which can undermine the insulation due to their lower R-values. This is why the calculated total R-value can often differ from the effective R-value you achieve in practice. Strategies like continuous insulation or advanced framing techniques can be used to minimise thermal bridging.

Different climates require different total R-values for optimum energy efficiency - buildings in colder climates need a higher total R-value to prevent heat loss. In comparison, those in warmer climates need a higher total R-value to prevent heat gain. However, the effectiveness of the R-value can be reduced due to factors such as installation errors, insulation compression, moisture accumulation, and material ageing.