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Open wide for warmth

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SUSTAINABLE BUILDING: Michael Hammett looks at the wide cavities at the innovative BedZED

New development conceived with serious 'green' intent demands a rigorous, holistic approach to design. During the past ten or so years several notable houses have been built that explore the implications and practicalities of constructing, equipping and living in homes that minimise their environmental impact by ensuring low energy consumption and eschewing total dependence on public utilities.

The virtue of the philosophy is gaining support and a ground-breaking project has been developed in Surrey.

The Beddington Zero Energy Development (BedZED) will make 82 homes and 1,600m 2of workspace available for sale or rent. Also provided are leisure facilities, a health centre, a nursery, energy generation and water recycling facilities. BedZED is funded by the Peabody Trust in collaboration with Bioregional, an independent environmental organisation, and Bill Dunster Architects, a practice that specialises in sustainable design.

The development has attracted attention from both prospective customers and the press. Detailed coverage of the whole development is to be expected on completion, scheduled for late 2001, but construction of the highly insulated cavity walls is complete and this is worth attention.

Triple-glazed facades on southerly elevations take advantage of solar energy, but opaque walling is of traditional materials and conventional construction. Brickwork is predominant with timber boarding to provide colour and textural contrast.

New bricks more cost-effective

Brickwork is an economic choice as an initial installation and in use too it is probably the least costly of all materials as it is practically maintenance-free for the life of the building. Reuse of materials was a principal consideration in the specification and large quantities of reclaimed timber joists, floorboards, doors and structural steel sections were used. Reclaimed bricks were considered, but their popularity is reflected in high prices and new bricks were justifiable on grounds of sustainability and cost-effectiveness.

The stock bricks used come from a Surrey brickworks about 20 miles from the site - one of several within the 35mile limit considered reasonable.

The bricks form the 102mm outer leaf of a cavity wall. The inner leaf is of medium-density concrete block and the inner finish is 12mm plaster. The thermal capacity of the masonry inner leaf and internal compartment walls helps to stabilise the temperature within the houses by absorbing heat as the temperature rises and slowly releasing it into the rooms as it falls.

U-value of 0.1 achieved

The cavity between the brickwork and blockwork leaves is 300mm. In this two layers of 150mm mineral wool insulation bats are placed as the walling is raised. The wall has a very low Uvalue of 0.1 (1).

Two-piece stainless steel wall ties, 400mm long, join the two leaves at 450mm horizontal and vertical centres (2). At BedZED the inner leaf is raised first with the shorter piece of the tie:

the insulation and outer leaf follow later. The two-part ties are more practical, the longer, second-placed piece being hooked in to the mating piece as required. Long, one-piece ties, unless both leaves are raised together, get in the way of following work and can be dislodged or cause injury.

The recent revisions to the Building Regulations Part L: Thermal Performance require a maximum Uvalue of 0.35 for external walls when using the Elemental Method of compliance. To meet this requirement with cavity insulation, cavity widths greater than 150mm may be needed. It is not widely appreciated that the Code of Practice for Masonry, BS 5628: Part 1, now permits cavity widths up to 300mm. It gives guidance for the specification and spacing of wall ties (see table).

Avoiding condensation

Enhanced thermal insulation in any element creates greater risk of condensation where insulation is interrupted locally or reduced by some detail of construction, for example where cavities are closed at window heads, jambs and sills. At BedZED the cavities are not closed by masonry returns in these positions (3). The insulation continues to the reveal where it abuts a heavy bituminous/polymer DPC sheet that is fixed by adhesive to the reveal surfaces of the brickwork and blockwork. Separate lintels for the inner and outer leaves permit a similar arrangement at the head. Later, when the window has been secured to the brickwork (by galvanised steel straps spanning back to the blockwork) lining board is fixed to the interior reveals (4).

The BedZED walls will undoubtedly result in reducing heat loss to very low amounts, and such a wide cavity with full-fill insulation runs little risk of rain penetration. Recently published BRE guidance, Good Building Guide 44 'Insulating masonry cavity walls' (Parts 1 and 2), gives guidance on various constructions relative to exposure rating and the risk of rain penetration.

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