Zero Carbon Hub - Energy Efficiency Standard and impact on building elements
The first step on this path to ‘zero carbon’ homes is the proposed Fabric Energy Efficiency Standard (FEES).
This month we look at how this is defined and focus on how the common external cavity wall construction can be adapted to meet the fabric standard.
The Energy Efficiency Task Group set up to advise the Government on the standard, comprised representatives from across the construction industry. This was to ensure that the proposed standards could be achieved using existing technology and in time for the 2016 zero carbon deadline.
The detailed report can be found at: www.zerocarbonhub.org/resources.aspx
Building Regulations Approved Document Part L1 (ADL1) specifies the reductions in CO2 emissions that new housing must achieve in order to demonstrate compliance. This takes into account the overall performance of the building fabric, services and any renewables. By contrast, the FEES uses a different measurement – kilowatthours per square metre per year (kWh/m2/ yr) and sets the maximum amount of energy houses need for space heating and cooling. The methodology for evaluation will only take into account the performance of the building fabric.
Two different performance levels have been set in the FEES; one for apartment blocks and mid-terrace houses and another for end-terrace, semi-detached and detached houses (fig. a). This is in recognition of the fact that some types of dwelling have inherently higher space heating and cooling demand than others, due to a larger exposed surface area. If the standard were the same across all house types, an end-terrace house would require different external wall construction from its mid-terrace neighbour. To avoid this, end-terrace, semi-detached and detached houses will be set targets that are more practical to deliver. fig b. gives a table of indicative specifications that would meet the Energy Efficiency Standards. While detached houses will have to have slightly better levels of insulation to achieve compliance, the additional space that this may require is more easily accommodated than it is in more compact terraced and flatted housing.
IMPROVED THERMAL PERFORMANCE OF FABRIC
Improving the energy efficiency of houses not only requires an improvement in the U-values of the basic building elements (wall, floor, roofs, windows, doors), but also in the quality of construction, in particular heat losses caused by air leakage and by thermal bridging need to be tackled. Current Building Regulations already require for the building fabric to achieve an air-permeability rate of 10 (m3/m2/hr@50Pa) or less and this is likely to remain the same when the new version of ADL1 is introduced later this year.
However, to meet the Energy Efficiency Standard, a new house is likely to require an airpermeability rate of around 3 (fig. b).
Thermal bridging, or cold bridging, occurs where the rate of heat loss is higher than adjacent surfaces due to a change in material or geometry, for example where a solid window cill returns across the cavity. This is measured as a ‘y-value’ (W/m2/K), which you may be familiar with from SAP calculations. As overall U-values are improved, heat loss caused by thermal bridging becomes a more significant proportion of the overall heat loss. ‘Accredited Construction Details’ and ‘Enhanced Construction Details’ provided by the CLG and the Energy Saving Trust respectively, may be used to demonstrate that the ‘y-values’ used in the SAP calculation are correct.
Ensuring these details are adhered to on site will require particular attention from contractors.
EXTERNAL WALLS: MASONRY
Wall thickness
The first area clients, designers, quantity surveyors and contractors are likely to look at, will be the design of the external wall. To increase the thermal performance of walls, the insulation thickness will increase. This can impact on the foundations, lintels, cills as well as the wall’s own components. Illustrated in fig. c & d are build ups for traditional cavity walls, demonstrating how lower U-values affect the overall width of the external wall. In both figures, specification B relates to the recommended FEES U-value for external walls.
As illustrated, fully filled cavities can help limit the overall wall width for some U-values, but this build up may not be suitable in exposed locations.
As an alternative to dense blockwork, aerated blockwork has a significantly lower thermal conductivity and can be used to limit the insulation required, but it has limited use in load bearing situations. Insulation may also be added on the inside of the inner leaf, behind or integrated with the plasterboard. This is not ideal because the intermediate floors breach the line of insulation, which can be a potential thermal bridge.
The larger the cavity, the more consideration must be given to the detailing of junctions between different elements of the building. One example is the window lintel illustrated in fig. e. This shows that a single lintel may be appropriate for a wall with a higher U-value, but as the cavity width increases, separate lintels for the inner and outer leaves are likely to be preferred. Although potentially more complicated for the builder, separate lintels do at least help to reduce thermal bridging heat loss.
Wall Ties
With an increase in the width of cavities, non-standard wall ties may be required and these may have a thicker cross section. Wall ties bridge the inner and outer leaves of cavity walls and contribute to heat loss. The thickness, number and material of the ties is taken into account in the U-value calculations, but their relative impact on the overall heat loss is greater the more the insulation is increased.
Ties in materials other than steel like plastic/glass polymers and basalt fibre, are becoming more easily available and have a significantly lower thermal conductivity. Using low conductivity wall ties could reduce the amount of insulation required within the build-up, amounting to savings in cost and space.
Condensation
Condensation will occur in external walls at the point where warm internal air meets cold air or a cold surface. Partial filled cavity walls effectively address this issue by having a ventilated void to the outside of the insulation layer, allowing for any accumulated moisture to be drained away. Fully filled cavity construction is acceptable in regions of low to moderate exposure to rain and also incorporates weep holes in the external leaf to allow for any interstitial condensation to be drained.
However, if insulation is applied on the inner surface of a wall build-up, there is the possibility of condensation occurring between the internal layer of insulation and inner leaf, where warm internal air would come in contact with a cooler surface. This region is not likely to be ventilated adequately and this type of construction should be assessed for a risk of condensation before being implemented.
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