The thermal performance of all components and systems except windows and doors is expressed in terms of R-value; for windows and doors, performance is expressed in terms of U-value.


Insulation materials are rated for their performance in restricting heat transfer. This is expressed as the R-value, also known as thermal resistance. The R-value is a guide to its performance as an insulator—the higher the R-value, the better the insulation (i.e., resistance to heat flow) it provides.
R-values are expressed using the metric unit’s m².K/W, where:

m2 refers to one metre squared of the material of a specified thickness;

  • K refers to a one degree temperature difference (Kelvin or Celsius) across the material;
  • W refers to the amount of heat flow across the material in watts.

Use the nominal R-values as listed by the manufacturer on the packaging of the insulation to determine the performance.

Products which have the same R-value will provide exactly the same insulating effect as each other, provided they are correctly installed. The higher the R-value the more effective the insulation. Products must be installed in accordance with the manufacturer’s specifications.

The information available on the product data sheet and/or label must include the R-value and whether it must be installed professionally or DIY. Ensure that it suits your particular application. Ask if performance guarantees and/or test certificates are available.

Material R-values – the thermal resistance values of bulk/mass type insulation are measured on the product alone according to international standards.

System R-values – the thermal resistance value of reflective insulation is calculated based on international standards and depend on the product being installed as specified in accordance to manufacturer’s specifications. This is known as a system R-value which incorporates air spaces.

Composite R-values – the thermal resistance values of composite insulation products are measured by testing the composite product as a unit according to international standards.


R-values can differ depending on the direction of heat flow through the product. The difference is generally marginal for bulk insulation but can be pronounced for reflective insulation.

  • Up R-values describe resistance to heat flow upwards (sometimes known as ‘winter’ R-values).
  • Down R-values describe resistance to heat flow downwards (sometimes known as ‘summer’ R- values).


Sometimes insulation or systems are rated in terms of its thermal transmittance (U-value), rather than its R-value. The U-value measures the transfer of heat through a material or a building element (thermal transmittance), whereas the R-value measures the resistance to heat transfer. U-values are often used in technical literature, especially to indicate the thermal properties of glass and to calculate heat losses and gains.

The U-value is the reciprocal of the R-value, R=1/U or U=1/R. For example, with an R-value of 2.0, the U-value is 1/2 or 0.5.

The U-value is expressed using the metric units (W/m².K) where:

  • W refers to the amount of heat transmitted across the face or through the material in watts;
  • m² refers to one metre squared of the material of a specified thickness; and
  • K or ‘degree Kelvin’ refers to each °C temperature difference across the face of the materials or through the material.

A smaller U-value results in lower heat flow, and therefore less heat loss. Higher U-values mean greater heat loss.


The overall R-value is the total resistance of a building element or system combination. It takes into account resistance provided by construction materials used in a wall or ceiling, internal air spaces, thermal bridging, insulation materials and air films adjacent to solid materials. Each of these components has its own inherent R-value, the sum of which provides the overall R-value.


The intervention added R-value or added thermal resistance is the value of the insulating material alone. This is the term most used when buying thermal insulation.

The manufacturer should provide the R-value of the insulation on the packaging. Some products will have a higher R-value for a specified thickness. For example, a 70 mm thick extruded polystyrene board and 100 mm thick glass wool blanket may have the same apparent R-value.

Reflective insulation requires that air spaces are positioned correctly within the building system to be effective, reflective membranes cannot have an R-value without the air space or air spaces.

To compare the relative performance of bulk and reflective insulation membranes, the resistance of such membrane in combination with air space(s) must be calculated. Reputable manufacturers can supply this information. Note: The effectiveness of reflective insulation installed on horizontal or sloping surfaces may eventually be reduced due to dust build-up, which reduces reflectivity, thereby increasing absorption. 


The R-value is the material thermal resistance, i.e. product only whereas the Total R-value describes the total thermal resistance of the system to heat flow provided by a roof and ceiling assembly (inclusive of all materials and air films), a wall or a floor. These values are calculated from the resistance of each component, including the insulation. Total R-values are the best indicator of performance, as they show how insulation performs within the building envelope. See example in section 7.5. 


Thermal bridging is the transfer of heat across building elements, which have less thermal resistance than the added insulation. This decreases the overall R-value. Wall frames and ceiling beams are examples of thermal bridges, having a lower R-value than the insulating material placed between them. Because of this, the overall R-value of a typical ceiling and/or wall is reduced.