The need for heating is a reaction to heat losses – no heat losses will result in a zero heating requirement. Therefore, it seems logical to try to reduce the amount of energy passing through the building fabric in order to decrease space heating requirements. An uninsulated house will lose heat in a similar way to a poorly equipped person exposed to cold temperatures. Large amounts of heat are able to move in and out of the property via infiltration and exfiltration, as quantified in Figure 2-4, for a house built to meet the requirements relevant to Part L of 2010 Building Regulations. (Woodford, 2014).
Figure 2-4 - Contribution of Building Elements to Whole House Heat Losses (%) Source Data: (Woodford, 2014, Web)
There are three key ways in which building fabric can be used as a vehicle to improve thermal efficiency, namely by reducing the thermal transmittance (u-value) of materials, by reducing the effects of thermal bridging, and through employing a holistic airtightness and ventilation strategy (Energy Saving Trust (EST), 2010).
The choice of materials used to construct the building envelope will have a major impact on thermal performance. As insulation levels are increased, u-values (transmittance levels) are decreased and the thermal resistance (R value)
increases. Low thermal conductivity (K value) usually indicates greater insulating properties in proportion to thickness (Ibstock, 2011). Insulating materials generally work by reducing levels of conductive and convective heat transfer, and may also act as thermal stores, or reflectors of radiation to reduce solar gains. The resistance of the insulating layer enables a temperature difference to be maintained between the internal and external environments (Pohl, 2011).
It is not uncommon to insulate the roof and walls (filled cavity, solid cavity, internal wall or external wall solutions) of a house. Whilst it is relatively simple to incorporate insulation into upper floors, the ground floor can be problematic to treat as it requires isolation from the earth that supports it. It should also be noted that the calculation of heat losses to the ground is a complex issue due to the unique behaviour of earth as a thermal store, and is governed by standard procedures detailed in EN ISO 13370 (International Organisation for Standardisation, 2007).
In more modern properties, the foundations of the house will generally incorporate insulation to mitigate against heat loss. In older buildings, suspended timber boards can be thermally enhanced by incorporating insulation. Where there is a cellar, insulation can be added between joists from below, and in the case of solid floors a damp proof membrane with an insulative overlay may be a possible solution (English Heritage, 2012a, 2012b).
It has been suggested that heat losses through the floor of a building may account for up to 15% of the total energy balance (Claesson et al., 1991, p. 195), so a strategy to prevent this pathway should be incorporated at the design stage. There has also been counter evidence presented that suggests that savings attributable solely to improvements to ground floor insulation could be as negligible as 3%, raising the question of whether the cost and invasive work required to install floor insulation in older properties is actually worthwhile (George et al., 2006, p. 28).
Whilst insulation is important, other elements of building design may increase heat losses or lower the overall u-value of a building. Thermal bridges are areas or points within a structure where materials with a different thermal conductivity either fully or partially penetrate the building envelope, where there is a change in fabric thickness, or where there is a difference between internal and external areas (for example at junctions between walls, ceilings and floors) (European Committee for Standardization, 2007a). It is estimated that anywhere between 15% and 30% of total fabric heat losses can be attributed to poor detailing at thermal bridge junctions (Energy Saving Trust (EST), 2008b; 2010, p. 4 & 41). However, it is also possible to almost eliminate thermal bridge effects through careful design and attention to detail during construction (Chartered Institution of Building Services Engineers (CIBSE), 2006;
Kalousek et al., 2013).
The effect of solar radiation may also influence the heat energy balance. Direct energy can enter a building via transmittance through glazing (predominantly on the south side), and then be absorbed by thermal mass provided by construction materials, or by other objects within the property. When the temperature of the materials falls below that of the internal environment, the stored heat is released due to conduction or convection processes. Whilst this process can be beneficial in terms of reducing space heating demand, it can also lead to overheating in buildings and uncomfortable living conditions. Solar shading can be used to prevent excessive solar gains in the summer months, and when installed at the correct angle will also allow sunlight to reach the building in winter months when it may be useful as a heating aid (Figure 2-5 (Reardon, 2008)).
Figure 2-5 - Designing to Mitigate Solar Effects Source: (Reardon, 2008, Web)
Ventilation strategy is central to the design of a thermally efficient home, as this directly influences the nature of air flow in terms of temperature, velocity and circulation flow rate (Roulet, 2008). When internal gains and solar gains are accounted for, it is possible that, in an airtight house, background infiltration rates alone will not be sufficient to maintain an adequate air change rate to provide a healthy environment for residents (National House Building Council (NHBC), 2009).
Banfill (2011a) suggests that several key measures are required to reduce dwelling space heating demand, namely increased airtightness, high levels of insulation, and installation of a mechanically ventilated heat recovery system (MVHR). The savings are achieved due to a decrease in infiltration levels, in conjunction with an elevated base air temperature obtained via the preheating of supply air using recovered heat from the extracted air. However, this is balanced against the energy costs associated with the running of the MVHR system. The effectiveness of an MVHR system is directly dependent upon the
correct balance between the efficiency of system fans, efficiency of the heat recovery unit, air flow rate, and building airtightness (Banfill et al., 2011b).
There is considerable debate concerning the level of air tightness at which it becomes necessary and cost effective to install such a system. Part L of the current UK Building Regulations (Department for Communities and Local Government (DCLG), 2010b, p. 15) specifies a minimum air tightness level of 10 m3/(h.m2) @ 50Pa for new build domestic dwellings. However, best practice standards seek to achieve a value as low as 3 m3/(h.m2) @ 50Pa (Energy Saving Trust (EST), 2007a, p. 3). Research suggests that the latter value should be adopted as a minimum in order to observe sufficient MVHR operational efficiency levels to realise overall energy savings from such a system (Banfill et al., 2011b).
When considering the UK housing stock, it is characterised by a wide range of air tightness levels, as shown in Figure 2-6 (Stephen, 2000).
Figure 2-6 - Air Leakage Rates for Survey of 471 UK Dwellings Source Data: (Stephen, 2000)(Web)
Stephen (1998, 2000) undertook a detailed survey of 471 UK dwellings, and found that those constructed between 1900 and 1930 had mean air permeability values of approximately 10 m3/m2/h @ 50pa, those constructed between 1930 to 1960 exceeded 15 m3/m2/h @ 50pa, and in properties after that date the value had returned to 10 m3/(h.m2) @ 50Pa. This is far in excess of the 3 m3/(h.m2) @ 50Pa that may be required in order to achieve efficient function of an MVHR system. The emergence of more energy efficient designs, more stringent minimum requirements for building airtightness, and increased rates of retrofit projects to improve existing housing, could gradually lead to a more widespread need for installation of such systems in order to provide adequate ventilation in dwellings (Zero Carbon Hub, 2012c).