Fabric heat loss

Heat can escape through the building envelope by direct heat transfer through the walls, roof, floors and windows, both through the insulation itself and through direct paths of lower thermal resistance called thermal bridges.

In the UK, the U-value concept is used to quantify heat loss through plane elements of the building envelope. This U-value is defined as the overall thermal transmittance of a particular construction element (a wall or a roof for example), including the effect of surface resistance. It depends upon the thickness and thermal conductivity of its component layers and, in the case of air cavities, the emissivity of the surfaces.

U-values are measured in W/m2K, and the lower the value the better the thermal performance. U-values of simple constructions can be calculated readily but for constructions with integral thermal bridges such as light gauge steel framing, the method becomes more complex. BS EN ISO 6946 contains approved calculation methods. There are also software tools validated by the Building Research Establishment (BRE) available to perform U-value calculations using accepted approximation methods.

A thermal bridge is a localised area of lower thermal resistance in the building envelope resulting in higher heat flow and lower internal surface temperatures. Repeating thermal bridges, such as fasteners, must be included in the U-value calculation, whereas nonrepeating thermal bridges such as flashings, must be accounted for separately.

One type of thermal bridge occurs when any noninsulative material penetrates the insulated layer and becomes a heat conduction path. Examples of this include fixings, brick ties, lintels, composite cladding junctions, brackets in built-up cladding, window and door frames, cantilevers for balconies, and roof beam supporting overhangs. Thermal bridging also occurs as a result of building geometry. For example, corners can also be thermal bridges, providing a heat flow path from both adjoining walls, as are panel joints and other design features.

As well as increasing heat loss from the building envelope, thermal bridging can cause localised condensation as surface temperatures may be reduced below the dew point (condensation temperature) of the air in the space. This is a particular danger in buildings where the Relative Humidity (RH) may be high, such as canteens, laundries, swimming pools and some factories.

The relatively high thermal conductivity of steel (approximately 52 W/mK) means that careful detailing is required to ensure that thermal bridging does not occur in certain applications.

Air infiltration is a term which is widely used to describe the uncontrolled movement of air through a building in response to wind and convection-induced forces on the building envelope. Air flows through a building envelope in flow paths resulting from poor design or construction. Typical routes for air infiltration include
service penetrations, door and window seals and poor quality masonry walls. The aim of the air-tightness requirement is to minimise uncontrolled air infiltration, but with sufficient controllable ventilation to allow fresh air to maintain good internal operating conditions.

The air-tightness of a building envelope has a direct effect upon the energy performance of the building High levels of air infiltration through joints, interfaces, doors, windows and service penetrations will add to heating and air conditioning loads and consequently to CO2 emissions and energy bills.

High air leakage levels will account for a substantial proportion of energy losses for the occupier. Conversely, good air-tightness in a building reduces capital spend on heating and cooling systems, also  reducing running costs. Air leakage typically accounts for 25-50% of the heat loss from a building.

Air-tightness can vary enormously depending upon construction method, site practice and build quality. An extensive programme of tests has been carried out at The Corus Colorcoat® Centre for the Building Envelope at Oxford Brookes University. These measured the air leakage through various cladding joints at a pressure of 50 Pascals which is the pressure used for testing whole buildings.

Results confirm that for a typical shed, with conservative joint leakage figures for built-up or composite cladding, air leakage through joints accounts for less than 20% of the overall leakage.
Other factors such as poor interface detailing and leakage through masonry walls contribute significantly to air leakage. The joint leakage tests show that it is possible to build air-tight steel clad buildings with air permeability well within Building Regulation
requirements. An air permeability specification of less than 5m3/h/m2 should be achievable for most buildings with good design and site practice.

Whole building thermal modelling work has been carried out, to assess the heating load for a typical large retail unit with the building envelope designed to comply with U-values in ADL2, but with varying levels of air-tightness. This demonstrates the significant effect that air-tightness can have. 2002 Building Regulations stipulate an air-tightness of 10 m3/h/m2. The 2006
Building Regulations have set this as a reasonable design limit but best practice with pre-finished steel cladding would be less than 3 m3/h/m2.

Reducing energy usage through efficient air-tightness of the building envelope is clearly a major step towards cutting CO2 emissions. With ever-rising energy prices, this also makes economic sense, since attention to detail in construction of the building will continue to pay off for the full operational lifetime.

Under the 2006 revision of part L2, it is mandatory to consider the effects of solar gain, in order to minimise the need for air conditioning. It requires approximately 3 to 4 times as much energy to cool a building, as it does to heat it. It is therefore essential that potential causes of overheating are
minimised by:

• Reduced incidence of direct sunlight (through rooflights and windows).
• Provision of well designed solar shading.
• Use of natural or assisted ventilation to reduce reliance on air conditioning.

The effects of solar gain need to be balanced against the benefits of natural day lighting.

Modelling packages can be used to predict the natural lighting levels throughout the day/year within a building for varying areas and orientation or roof lights and windows.