Environmental concerns and the rising cost of fuel mean that there is an increased focus on the minimisation of energy use during the natural occupational life of a building. The thermal performance of the building envelope can make a significant contribution to reducing the overall building energy usage.
Reducing operational carbon emissions from buildings is imperative in the drive to combat global warming. The European Union Energy Performance of Buildings Directive (EPBD, 2002/91/EC), published in 2002, aims to promote building energy efficiency across the whole EU, and requires energy performance to be calculated to a national standard.
In response, the 2006 revisions to Part L of the Building Regulations (Conservation of Fuel and Power) in England and Wales is projected to save over 1 million tonnes of carbon emissions by 2010 and incorporates a new National Calculation Methodology for non-domestic buildings.
Enhanced thermal performance of the building envelope, both in terms of improved insulation and air-tight construction, plays a key role in minimising energy use for heating and cooling and hence in reducing carbon emissions.
CO2 emissions targets can be met by a combination of means, such as:
- Efficient insulation and better detailing of the building envelope.
- Air-tight construction of the building envelope.
- Energy efficient appliances and fittings (e.g. boilers and lighting).
- Automatic controls and building management systems.
- Use of zero-emission technologies such as solar water heating and photovoltaics.
Over the years, well-proven cladding systems have been developed using pre-finished steel for the outer and/or inner skin of the building envelope. Highly insulated, air-tight cladding systems, with well designed junctions and interfaces can make a significant contribution to reducing the overall carbon emissions of a building over its lifetime.
“Reasonable improvement” for the conservation of fuel and power shall be made whenever building work is being carried out, where it is “cost effective” according to criteria contained in ADL2B. Any extension or significant refurbishment to a building, must meet defined criteria, documented within ADL2B, including improvements to the existing building.
Established pre-finished steel over cladding and other refurbishment solutions are available to meet these requirements.
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.
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.
For further details on airtighness click here
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.
- Large areas of rooflights can lead to excessive solar gain causing the building to overheat.
- Rooflight areas greater than 15% will almost certainly lead to a certain amount of overheating.
- For low energy design, the lowest sensible lighting level should be specified.
- Most rooflights will need to be triple skin to achieve the limiting U value standard of 2.2 W/m2K as specified in the latest building regulations.
- For large single storey buildings, 10% rooflight area can be considered as a good practical starting point when considering a daylight requirement.
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 non-repeating thermal bridges such as flashings, must be accounted for separately.
One type of thermal bridge occurs when any non-insulative 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.
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.
Cladding system suppliers will generally perform these calculations for their systems and provide a standard U-value.
- Defining sustainable development
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- The sustainable specification