The higher costs of making sure your masonry wall complies with Part L are partially offset by annual savings in energy costs. But by how much? We do the sums
The little pig that built the house out of brick had the best grasp of whole-life performance issues. But he would have been shocked to realise that to meet the recommended U-value for thermal transmittance — 0.35 W/m2K — his brick walls may have to be as thick as 1.5 m.
The driving force behind improving the thermal efficiency of buildings is the government's commitment to reduce greenhouse gases. Buildings account for just under 50% of the UK's greenhouse gas emissions. The increased energy efficiency requirements for buildings is just one of several government initiatives that are intended to help the UK meet its Kyoto commitment. It should also contribute towards the UK's self-imposed goal of reducing the UK's carbon dioxide emissions to 20% below 1990 levels by 2010.
From the building owner's point of view, reducing carbon dioxide emissions means a trade-off between capital costs incurred from designing and building a thermally efficient building and future annual cost savings made through using less energy. In this instance, part of the whole-life assessment involves analysing these trade-offs.
Generally, implementing Part L is estimated to increase the cost of dwellings by some £5-20/m2. However, these initial capital costs can be reduced through considered design and by taking advantage of other government incentives, including capital allowances for energy-saving measures such as combined heat and power plant.
The standards within the Approved Document Part L fall short of the energy-neutral building – a building that generates no more carbon dioxide emissions than it consumes. But such a building is not a pipedream. At Peabody Trust's BedZED housing scheme in south London, the strategy formulated to maximise the energy efficiency of the external envelope included:
- "Superinsulation" — 300 mm of insulation to all the external surfaces to keep heat in the building
- U-values for walls are 0.11 W/m2K
- Windows are triple-glazed and south-facing to make the most of solar energy
- Dense structure provides a thermal store for the solar energy
- Construction is draught-free with heat-recovery ventilation, so that outgoing stale air warms up the incoming fresh air.
As for cost, one of the most significant trade-offs operating at the BedZED development is that individual dwellings do not need a heating system. The money saved on this is invested in the superinsulated envelope. It is estimated that household energy savings for a three-bed maisonette could amount to as much as £240 per year.
<B>Solutions to Part L wall requirements</b>
An energy-neutral development represents a step-change in traditional design processes. The changes in Part L are a small step in the direction of energy-neutral design that is an integral part of an energy-efficient building.
The savings to the occupier achieved from increased wall insulation should be seen as part of a broader picture of overall energy cost savings. Typically, for domestic new build, annual energy saving of £5-£20 per year can be expected from improved wall insulation. The payback period is some 25-30 years.
The table below considers annual costs and savings for three house types, comparing wall insulation values at pre-2002 levels with recent construction at slightly better than the new requirement. All other values and parameters have been kept constant to enable the cost benefit of the wall insulation to be assessed.
This analysis reveals that the individual energy-saving measures may not have a huge impact on the whole-life cost of a building. For each building there will be a hierarchy of energy-saving measures, each with its own cost and future saving.
The skill in obtaining the best whole-life performance for a building is to incorporate the low cost/high saving items in preference to the high cost/low saving options.
<B>Whole-life performance of brick walls</b>
Once a brick wall is built we would really expect no, or only insignificant, costs to be associated with maintaining the wall for a generation or two. But buildings are rarely as simple as this. The reality is that walls incur repair or maintenance costs not envisaged as part of the whole-life costs. These costs are random, in the sense that nobody had planned for them in the same way as they would if redecorating a fence. For an owner of building stock it would be prudent to set aside an allowance to cover the costs associated with random failures and deterioration. Nevertheless, these costs are avoidable, as early repair and maintenance expenditures are commonly the result of poor design or workmanship.
The following tips show how to enhance the durability and performance of a brick wall, thereby minimising whole-life costs:
When masonry becomes saturated with water, the rate of deterioration increases. Water-shedding features such as roof overhangs, copings, string courses and throated sills will help keep run-off water from the wall below. Large areas of flat walls with flush sills should be avoided.
Ground water is conventionally excluded by the introduction of a damp-proof course. However, the brickwork below it is still vulnerable to water saturation. A common solution is to use engineering bricks below the damp-proof course. Providing effective drainage adjacent to the wall by laying land drains or paving to shed water from the wall reduces the potential problem even more.
<B>Detailing for exposure</b>
Where the building is in a severe or very severe location, precautions in addition to conventional detailing may be required such as rendering or other impermeable cladding, or the planting of trees to protect the building from driving rain.
Cavity walls let in water. The cavity allows this water to trickle down the inside of the outer brick leaf leaving the inner leaf dry. Openings such as windows and doors are a problem; they create a bridge between the inner and outer leaves, which can trap penetrating water as well as interrupting the thermal insulation. The former can lead to damp penetration and damage to the internal fabric of the building, the latter to cold bridging with consequent condensation and mould growth to the detriment of the occupants' health. Careful detailing of damp-proof trays, vertical damp-proof courses, weep holes to allow water to escape and insulation are needed to avoid damp penetration and cold bridging. The cost of getting it wrong can be significant.
When repointing old walls or soft brickwork use a weak or "soft", preferably lime-based, mortar rather than a strong cement-sand mortar. If the mortar is harder and less pervious than the bricks, wetting of the wall will eventually drive salts out through the brick resulting in brick spalling. If this occurs at upper levels there are health and safety risks to passers by. The repair costs of replacing bricks is greatly in excess of repointing with the correct grade of mortar.
This is not really a durability issue and the white salt deposits are not harmful to the wall. However, some consider them unsightly and there are costs in brushing off efflorescence. The risk of efflorescence can be reduced by avoiding saturation of brickwork during construction and detailing exposed parts of the wall to be protected. Given time, efflorescence usually disperses.
The cavity seems a sensible place to install insulation – out of sight out of mind. Nevertheless, there are a number of risks that may affect whole-life cost predictions:
- Care is required to ensure the risk of water penetration is not increased by providing a route for water to travel from the outer wall to the internal wall especially in the case of full-fill cavity insulations. In areas of severe or very severe exposure, full-fill cavity insulation should not be used (in fact in Scotland it is not allowed)
- Interstitial condensation occurring within the body of the insulation should be managed through vapour checks if analysis shows there is a risk
- Loose fill insulation has a tendency to settle, which causes cold spots reducing the thermal efficiency and increasing energy costs
- When moisture comes into contact with insulation by penetrating water or condensation there may be a local increase in U-values and loss of energy efficiency
- Lower external wall temperatures increase the risk of frost damage to masonry.
Weatherstruck pointing sheds water effectively. Avoid recessed joints as they retain moisture and put the wall at greater risk of frost attack.
Walls move in response to changes in temperature, moisture and applied loads. The usual practice is to incorporate movement joints, particularly where:
- There are short returns in masonry
- There are long runs of masonry at 12 m or 6 m intervals for clay or calcium silicate bricks respectively
- Dissimilar materials are used in the wall construction
- There is co-ordination with structural changes such as door and windows and intersecting walls.
The British Standard Code of Practice for Use of Masonry — BS 5268 — gives detailed guidance on these and many more durability issues.
Further informationBuilding Performance Group has developed and uses an expert software tool to calculate whole-life costs, payback appraisals, compare component options and maintenance strategies and carry out value engineering.
For further information on whole-life performance contact Peter Mayer at the Builder Performance Group at p.mayer@bpg–uk.com or telephone 020-7240 8070. For further information on energy performance contact Peter Giblin at p.giblin@bpg–uk.com or telephone 020-7240 8070
By Peter Mayer of Building Performance Group