CHANGES in legislation are forcing the construction industry to change the way it works to introduce energy efficiencies to buildings and reduce the UK’s CO2 output.

Energy efficiency is measured not only by the amount of energy consumed by the enduser but in the embodied energy within each building product – assessed by the environmental impact of origin and extraction of raw materials, energy use and pollution from manufacture and transport of finished product, life expectancy, and recyclability or disposal.

Approved Document L1A, relating to the conservation of fuel and power, sets minimum energy performance requirements, which include annual CO2 emission rate calculations, solar shading to limit use of coolants in the summertime, and pressure testing to evaluate airtightness.

SAP2005 (Standard Assessment Procedure) software can be used to aid complicity with these new requirements, and new ‘competent persons’ schemes have been approved, to undertake pressure testing and energy performance calculations.

Evaluating old and new

Traditional brick and block build is being challenged by claims by the offsite construction sector that modern methods of construction have higher insulating properties and quicker construction times. Offsite manufacture includes volumetric or modular construction, panellised systems and prefabricated units, structural insulated panels (SIPs), and timber frame, and other methods such as Tunnelform or Insulated Concrete Formwork (ICF), which consists of twin-walled expanded polystyrene (EPS) panels filled with ready-mixed concrete.

SIPs generally consist of an inner insulating core of EPS or polyurethane foam (PUR), sandwiched between two outer skins of oriented strand board. The board is made from purposegrown trees, which, felled young and replanted, supply more oxygen and remove more carbon dioxide from the atmosphere than do mature trees.
EPS and PUR are free from ozone-depleting gasses, but are non-biodegradable in landfill, emit toxic fumes if burned, and are derived from petrochemicals. Having said that, the energy saved by the use of EPS and PUR insulated products is greater over a period of 50 years or more than the energy used in their manufacture.
SIPs can be used as external, load-bearing, and internal walls. A SIP of 125mm will give a Uvalue as low as 0.21, with a 175mm panel achieving 0.14.

Used in walls, SIPs do not require additional supporting frames. Being pre-engineered and made to measure with interlocking joints, the SIP contributes to the airtightness of a building, its speed of construction and reduces on-site waste.

Laid as a floor, the SIP requires supporting joists. Types include the Ecojoist, which consists of two parallel timber battens connected by galvanised V-shaped steel webs. These reduce the amount of timber used and limit waste and installation time. The Trusjoist works on ‘wholelog’ technology as a laminated veneer which reduces timber waste. It can be made in long lengths to span an entire floor, which limits floor noise. The external finish on a SIP can be brick slips, render, or timber cladding.

Getting back to brick and block

Builders, however, have not gone over entirely to timber frame and SIPs in order to comply with energy efficiency requirements. Hanson, with its house built for the Building Research Establishment’s Offsite 2007 exhibition, addresses issues such as overheating and solar gain, and resolves them with a concrete and masonry structure, having a ‘smart’ ventilation system and increased thermal mass.

The manufacture of brick comes under heavy criticism from environmentalists, who point to the quarrying needed to obtain clay and high temperature kiln firing of between 1,000OC and 1,200OC, But the industry is fighting back, pointing out that it has already met the first targets in its aim to reduce manufacturing energy consumption by over 10% by 2010. Reclamation of quarried land, fluoride and sulphur dioxide-removing exhaust purification equipment, and research in to the use of recycled materials such as glass and pulverised fuel ash (PFA) add to the brick industry’s sustainability.

PFA is a waste product of coal-fired power stations. In Australia, researchers have made bricks entirely from PFA which are 28% lighter and 24% stronger than comparable clay bricks. In the US, products made from PFA are considered ‘green’ as they reuse an otherwise waste product.

Brick’s heavy thermal mass, which is greater than 1,500kg/m3, is well able to absorb solar gain, and less likely to require air conditioning in the warmer months, or in the projected heating of the country in future decades, which helps to reduce the occupier’s energy consumption.

Although heavyweight construction has, in manufacture, a higher embodied energy, it should be taken in to consideration that bricks can last hundreds, if not thousands, of years, and can then be reclaimed and reused after that time.

Cement manufacture also gets a bad name through its quarrying of limestone and high firing process of up to 1,870OC. The UK cement industry is responsible for 1.7% of UK CO2 emissions.

In 2004 the UK cement industry launched the Sustainable Development Task Force to improve the industry’s environmental objectives. Modern plants are using heat recovery systems, as well as burning alternative fuels, such as scrap tyres, to fire kilns.

Aerated concrete products can have enhanced thermal and acoustic performance, and are fire resistant and recyclable as aggregate.

Aircrete has recently developed the use of thin layers of adhesive in place of thick mortar joints which results in a lower cross section of mortar acting as a thermal bridge. Although considered a modern method of construction, this is an ideal illustration of the use of traditional products in an innovative way to promote energy efficiency.

Non-structural insulation materials include mineral wool, popular for use in cavity walled construction, timber frame and floor and partition wall applications. Mineral wool is manufactured from basalt or volcanic rock, and spun into a fibrelike structure. Slag wool is a similar material, but recycled from iron ore blast furnace slag. Glass wool is made from sand or recycled glass, and can contain up to 80% of recycled material. When installed correctly, for each tonne of CO2 produced in the manufacturing process of mineral wool, about 200 tonnes of CO2 are saved over a 50-year period, lowering their overall embodied energy figure.

Build tight, ventilate right

New standards of construction may make a building airtight, but this very achievement results in a lowering of natural ventilation or air movement. Relative humidity should be kept below 70% for a comfortable living environment, and airtightness requires mechanical means of air exchange. To achieve ‘carbon zero’ the power used in the air cooling and extraction exchange, it must not exceed that saved by the insulation itself.

The Energy Saving Trust recommends that kitchens and bathrooms have extraction ventilation so that moist air is removed at source, and ‘whole-house’ ventilation elsewhere. Its document GPG268 compares passive stack, extract fans, background ventilators, heat recovery and positive input ventilation.

Uncontrolled ventilation is calculated by pressure testing. All external windows and doors are taped over, as are features such as bathroom vents. The blower fan is set in to a frame placed over the open front doorway and the  internal air pressure is increased to a pre-determined level.

Additional air input to maintain this pressure is measured, being equal to the amount of air escaping, or the rate of air permeability.

Thermal imaging can be used to determine paths of heat loss and smoke pencils can help to identify local leakage.
Each individual building’s pressure test value will have been defined by the designer in the SAP2005 evaluation, although the maximum allowable air leakage at 50Pa is 10m3/h/m2. The Dwelling Emission Rate should be less than the target carbon dioxide emission rate.

This summer the BRE will host Offsite 2007 to showcase full-scale buildings and demonstrate innovative technologies and a range of modern methods of construction. Jaya Skandamoorthy, project manager of Offsite 2007, is enthusiastic about this year’s exhibits. “Aspirations to carbon zero mean that we have to raise the bar in terms of what can be achieved,” he says. “In conventional housing, 70% of consumed energy is space heating.”

Osborne will show off its Jabhouse, which is constructed from SIPs and took only one and a half days to erect. Skandamoorthy says the superinsulated house has achieved 40% better than Part L requirements.

Construction companies and government housing committees, though, should always keep in mind that the primary function of housing is to be a home that people want to buy, and want to continue living in, for decades to come.

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