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What Does Thermal Performance Mean?

When we discuss the energy efficiency and thermal comfort of buildings, the topic of thermal performance takes centre stage. Especially when retrofitting existing buildings or designing new ones, understanding how to optimise the thermal behaviour is crucial. This is where external wall insulation comes into play. However, to truly comprehend the significance of this, one must be familiar with terms such as lambda values, U-values, and R-values. Let’s dive in!

Thermal Performance

Thermal performance is a multifaceted concept in the world of building science, encompassing how buildings respond to external temperatures and how they manage internal heat loads. Let’s break it down further:

1.1 Heat Transfer Mechanisms

Heat moves in and out of buildings through three main mechanisms:

  • Conduction: This is the transfer of heat through materials, like when the heat from a hot cup of coffee moves through the mug to your hand. Different materials have different levels of heat conduction, which is why understanding lambda values, as discussed later, is crucial.
  • Convection: This involves the movement of heat due to air currents. For instance, warm air rises because it is less dense, and cold air falls. Proper insulation can prevent cold drafts and uneven temperature zones in a building.
  • Radiation: This is the transfer of heat in the form of electromagnetic waves. An example is the sun heating a room through a window. Reflective barriers and specific types of glazing can help manage radiant heat.

1.2 Impact on Energy Consumption

The effectiveness of a building’s thermal performance directly affects energy consumption. If a building cannot retain heat in cold months, heating systems work harder, consuming more energy. Similarly, in hotter months, if a building absorbs too much heat or cannot expel it efficiently, air conditioning systems are burdened, leading to higher energy costs.

1.3 Comfort and Health Implications

Optimal thermal performance is not just about energy savings; it’s also about human comfort and health. Cold spots, drafts, and over-heated spaces can cause discomfort for occupants. Moreover, poor thermal performance can lead to issues like mould growth due to condensation, which can have health implications.

1.4 Environmental Considerations

Better thermal performance means reduced energy consumption. This not only reduces costs but also, when relying on non-renewable energy sources, reduces the carbon footprint of the building. As we move towards a more sustainable future, the importance of the thermal efficiency of buildings becomes even more pronounced.

Lambda, R, and U-values

Lambda value, commonly denoted by the symbol λ, refers to the thermal conductivity of a material. It measures how well a specific material conducts heat. The lower the lambda value, the better the material is as an insulator.

Given in W/mK (watts per meter Kelvin), common insulating materials often have low lambda values, meaning they are not good conductors of heat. This is what we want in insulation – materials that resist the flow of heat.

The R-value measures how well a material resists the conductive flow of heat. It’s the thickness of the material divided by its lambda value:

R = λ / d


R = Thermal resistance (m²K/W)

d = Thickness of the material (m)

λ = Lambda value (W/mK)

The higher the R-value, the better the material’s insulating properties. So, if you have two insulating materials with the same thickness but different lambda values, the one with the lower lambda value will have a higher R-value and, therefore, will be a better insulator.

While R-values measure the resistance of a single material, U-values measure how effective a construction element (like a wall or roof) is as an insulator. It considers all layers and materials.

The U-value is the reciprocal of the sum of the R-values of each layer:

U= ΣR / 1

Expressed in W/m²K, a lower U-value indicates better insulating properties. When discussing external wall insulation, the goal is often to achieve a wall with as low a U-value as possible to minimise heat transfer.

Thermal performance of different insulation materials

Insulation Material Thermal Conductivity (λ) [W/mK] Key Characteristics Common Applications
Expanded Polystyrene (EPS) 0.030 – 0.038 Lightweight, rigid, moisture-resistant. Widely used external wall insulation due to cost-effectiveness.
Extruded Polystyrene (XPS) 0.029 – 0.038 Denser than EPS, excellent moisture resistance. Basements, ground-contact scenarios.
Mineral Wool (Rock/Slag Wool) 0.035 – 0.040 Non-combustible, good sound insulation, vapour-permeable. Cavity wall insulation, fire-resistant applications.
Polyisocyanurate (PIR) 0.022 – 0.028 Closed-cell foam, moisture-resistant, excellent thermal performance. Spaces with minimal thickness where high insulation value is needed.
Polyurethane (PUR) Slightly higher than PIR Similar to PIR but can vary in certain properties. Spaces with minimal thickness, where high insulation value is needed.
Wood Fiber 0.038 – 0.050 Natural, breathable, good acoustic insulation properties. Ecological constructions, sustainable projects.
Cellular Glass 0.040 – 0.050 Inorganic, water-impervious, non-combustible, pest-resistant. Underground applications, flat roofs.
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