Last edited 02 Sep 2019

Thermal performance


[edit] Introduction

Thermal performance is a measure of the thermal conductivity of materials or assemblies of materials. Thus, materials which are regarded as having a good thermal performance are those which also tend to be good insulators, ie they do not readily transmit heat. In contrast, materials with a poor thermal performance tend to be better conductors of heat and therefore will allow more heat to pass through, say from a warm building to a cooler external environment.

In summer when external temperatures can be much hotter outside than inside, the reverse is true – a building with poor thermal performance overall will allow more heat to pass through – and so will be hotter inside – than one with a good thermal performance.

The thermal performance of the building fabric – which is simply an arrangement of materials adjacent to or near each other – is directly affected by criteria such as seasonal and temperature changes; daily diurnals (ie, the difference between highest and lowest temperatures in 24 hours), the amount of solar gain and shading, incoming and outgoing heat radiation, water and moisture absorption, air movements and pressure differences.

Thermal performance has become a critical consideration in the design of buildings. This is because current building regulations aim to conserve fuel and minimise carbon emissions from space heating by limiting the heat lost from a building to the external environment. However, it should be noted that embodied carbon is not regarded as an aspect of thermal performance.

[edit] Measuring thermal performance

[edit] Conductivity

Conductivity is the ability of a material to conduct heat. In considering conductivity in relation to the building fabric, the following equation tends to hold:

High conductivity materials = low thermal resistance = low thermal performance = poor insulator

Low conductivity materials = high thermal resistance = high thermal performance = good insulator.

The units of thermal conductivity are W/(m.K)] (SI units) and [Btu/(hr ft °F)] (imperial units).

Thermal conductivity (λ = lambda value) is measured by the amount of heat flow (Watts) through a metre squared of surface area over a temperature difference of 1K per metre of thickness. However, it is more convenient to measure and compare the thermal performance (or insulation properties) of materials by using the thermal resistance value ‘R’ – a measure of thermal resistance rather than thermal transmission. Thermal resistance is the reciprocal of thermal conductivity.


The transmission rate of all the layers of a construction from the inside to the outside is called a U-value. U-values are used to gauge the thermal performance of constructions ie assemblies of materials such as cavity wall constructions.

U-values (sometimes referred to as heat transfer coefficients or thermal transmittances) measure how effective elements of a building's fabric are as insulators. That is, how effective they are at preventing heat from transmitting between the inside and the outside of a building.

The lower the U-value of an element of a building's fabric, the more slowly heat is able to transmit through it, and so the better it performs as an insulator. Very broadly, the better (ie, lower) the U-value of a building's fabric, the less energy is required to maintain comfortable conditions inside the building.

[edit] Typical U-values

U-values are measured in watts per square metre per degree Kelvin (W/m²K). For example, with a double-glazed window with a U-value of 2.8, this means that for every degree difference in temperature between the inside and outside of the window, 2.8 Watts will be transmitted every square metre.

Typical U-values are shown below for the purposes of comparison only:

For further information, see U-values

[edit] Air tightness

Air tightness is another measure of the overall thermal performance of a building. Even if it is constructed with materials of high thermal performance, a building will have an overall compromised thermal performance if it fails on air tightness tests and has a high rate of air leakage (defined by the ATTMA as the '...uncontrolled flow of air through gaps and cracks in the fabric of a building).

Approved document F, Ventilation, defines airtightness as ‘…a general descriptive term for the resistance of the building envelope to infiltration with ventilators closed. The greater the airtightness at a given pressure difference across the envelope, the lower the infiltration.’

For further details see Air tightness in buildings.

[edit] Related articles on Designing Buildings Wiki