Poundbury. The very name of this 21st-century housing model strikes fear into the hearts of specifiers everywhere, as it demands strict compliance with tough design rules – and under the watchful eye of a rather important man. We meet a valiant developer who wouldn't be deterred
You have to be brave to build in Poundbury. Arch traditionalist Prince Charles owns the land, and if he doesn't like your buildings he can insist they're torn down and rebuilt at your expense. A costly and time-consuming business – not to mention the embarrassment of being slated by the future King of England.

Something like this happened to developer Kim Slowe when his recently completed office fell foul of the Duchy of Cornwall's strict design guidelines. Two columns at the front of the building were too small. "I had to knock them down and rebuild them," he says ruefully.

However, the experience hasn't stopped Slowe from developing in Poundbury, which lies on the outskirts of Dorchester in Dorset. Far from it – his company Cornhill Estates has just completed a terrace of five three-storey homes. Slowe says he appreciates the Duchy's traditional approach to design and its fastidious attention to detail. "There's an element of complexity at Poundbury that makes building there challenging and exciting. If it wasn't complex, Poundbury wouldn't be the place it is," says Slowe.

Although Slowe's homes look as though they were built in the late 18th century, they are made with modern building systems. Slowe wanted to prove that he could build financially viable, energy-efficient homes within the architectural constraints of Poundbury. This meant that he had to find low-cost and innovative building systems that did not compromise the Georgian appearance of the building. And central to Slowe's sustainable goal was the specification of the masonry. As well as providing a high level of thermal insulation the blockwork system had to interface with the other building components cheaply and efficiently.

The three-storey terrace of five homes is being built as part of the second phase of Poundbury. This extension to Dorchester is an attempt by the Prince of Wales to engineer a traditional urban community where the pedestrian takes precedence over the car. Buildings have to be built in the local Dorset vernacular to a high density and private and social housing has to be built alongside factories and shops to create an integrated urban mix.

Architect Stephen Mattick has based the four-bedroom homes' appearance on the Georgian town houses typically found in London and Bath. He was careful to adhere to the Duchy of Cornwall estate building code, which offers extensive guidance on what building materials should be used for walls, lintels and non-masonry items such as roofs, windows and doors (see "By royal decree: Poundbury design guidance", right). The detailed designs were carried out by architect John Matthews, who ensured that all the homes complied with Building Regulations.

For the homes' structural walls, Slowe specified Thermalite's thin joint masonry system. He considered using a timber-frame system, but decided that the greater thermal mass of two skins of aircrete blocks would mean that the homes would be less prone to overheating. (See "Grand designs: The wall specification", overleaf).

Thin joint mortar was selected because it offered a better thermal performance than conventional mortar. "With thin joint there's far more insulated block and less of the mortar, which has poorer insulation properties," says Chris Kirby, general technical manager at Thermalite. The amount of mortar in the wall has been reduced from 7% mortar to less than 1% according to Kirby, which means a U-values of 0.24 W/m2K and 0.20 W/m2K can be achieved for the ground floor walls and other walls respectively.

The use of thin joint mortar did throw up design challenges for contractor KDJ Slade. The reduced depth of mortar meant that Celotex insulation sheets had to be cut before being inserted into the cavity. Each 450 mm sheet was meant to be the same depth as two courses of blockwork, but because of the thin mortar, the actual depth of the aircrete was only 432 mm. This meant that 18 mm of Celotex had to be shaved off each sheet. KDJ site manager Paul Studley says that 432 mm depth Celotex sheets would be specified next time.

The thin amount of mortar also had repercussions for the installation of the wall-hung floor joists. As the hangers are thicker than the thin joint mortar, KDJ had to rasp a groove 2-3 mm deep in the aircrete blocks to allow the hangers to fit flush with the top edge.

One of the biggest specification challenges for Kim Slowe was choosing an appropriate render for the facades. Walls constructed from aircrete blocks are softer and are more likely to move than if they were built with dense blockwork. This means the render may crack because it will move at a slower rate than the aircrete. Slowe shopped around, looking at a number of solutions before opting for a system from Alsecco that offered a 10-year anti-crack guarantee.

The Alsecco Flexewall render system has a similar compressive strength to aircrete, which means it is flexible enough to absorb the thermal movements of the blockwork. A glass-fibre mesh is applied on top of a layer of render, which is sprayed onto the outer leaf of blockwork. A second layer of render is sprayed over the mesh and then rubbed down when it has gone partly off to achieve a dimpled finish.

As well as constructing thermally efficient walls, Slowe was keen to specify high levels of acoustic insulation between homes. To achieve this, he insisted that the blockwork was wet-rendered before it was drylined with plasterboard on dabs. Slowe says that the cost of the extra wet-rendering was negligible: only £700 for all five homes. This separating wall construction method features in the House Builders Federation Robust Standard Detail programme – a set of design details to comply with the latest edition of Part E, the acoustic regulations. If the government accepts the principle of these RSDs, housebuilders will be able to use them from 1 January 2004.

Slowe also insisted on filling the homes with as many sustainable features as possible. They include greywater recycling, heat recovery and solar water heating systems. The energy-saving devices, in conjunction with the thermally-efficient aircrete walls, meant that Slowe only needed to specify a cheap, simple boiler that requires minimal maintenance.

One area where the developer was able to specify a sustainable product without affecting the homes' appearance was in the loft. Rather than selecting mineral wool for the loft insulation, Slowe specified sheep's wool from Ochre. The UK distributor Joulesave says that although natural wool is more expensive than mineral wool, it is easier to fit and has a lifespan twice as long as synthetic alternatives (see "Raising the baa: The sheep's wool insulation", page 6).

The selection of sheep's wool is typical of Kim Slowe's unconventional approach to housebuilding. As a latecomer to the industry – he was a Captain of a Navy Frigate until five years ago – Slowe operates with an outsider's perspective. His direct involvement in the specification process shows how keen he was to prove that sustainable and innovative systems can be used in masonry-built homes. Mainstream housebuilders, he says, are too conservative to try something similar.

His approach makes commercial sense too. He knows many housebuyers are put off by wacky-looking homes that wear their sustainable hearts on their sleeves. As he proudly remarks, his energy-efficient homes "don't look as though they've come from outer space". A sentiment that will no doubt please the firmly grounded Prince of Wales, Duke of Cornwall and King Charles III-to-be.

Suppliers

Thin joint system Thermalite www.thermalite.co.uk
Concrete blocks forming boundaries and garage walls Aggregrate Industries www.aggregate.com
Cavity insulation Celotex www.celotex.co.uk
Sheep’s wool loft insulation Joulesave www.joulesave.com
Insulated render Alsecco www.alsecco.co.uk
Reconstituted stone Rebastone Masonry www.rebastone.co.uk
Chimney brick Terca www.terca.co.uk
Floor joists Trus Joist www.trusjoist.com
Greywater recycling system Gramm Environmental www.waterdynamics.co.uk
Heat recovery system Villavent www.villavent.co.uk
Solar water heating system Thermomax www.thermomax.co.uk

Raising the baa: The sheep’s wool insulation

The loft insulation used in the terrace is made from sheep’s wool produced by Irish manufacturer Ochre. The wool used at Poundbury originated from New Zealand but by the end of the summer, sheep’s wool will be sourced from the UK through a new factory in Ireland.

UK distributor Joulesave EMES says that the Ochre wool has a similar acoustic performance to Rockwool and will pass Part L of the Building Regulations if laid to a thickness of 250 mm (it is manufactured to a thickness of 125 mm).

A specialised form of polyester is used as a bonding medium, which Joulesave says does not increase the risk of fire or vermin. In the event of a fire, the product will not burn or emit toxic fumes, but melt instead.

The wool has a low level of embodied energy, according to Joulesave, and contains no fibrous dust. It can also retain 40% of its own weight as moisture, which it expels when the temperature rises without any loss of insulation.

Ochre wool is roughly twice the price of synthetic wool. In a loft of 80 m2, the average cost of providing insulation is £760 with Ochre sheep’s wool and £320 with synthetic insulation. Despite the higher cost of Ochre, Joulesave says that it is a cheaper product to use because fitting costs are considerably lower than its competitors. Joulesave also claims that sheep’s wool has twice the lifespan of synthetic equivalents.

By royal decree: Poundbury design guidance

Any architect working at Poundbury has to adhere strictly to a building code imposed by the Duchy of Cornwall, the Prince’s estate that owns the land. The purpose of the code is to ensure that traditional building materials are used and that the building’s designs reflect the local Dorset vernacular. The rules on using masonry are particularly restrictive and include the following dictates:

Walling materials

  • Stone should be sourced from local quarries at Hamstone, Portland, Purbeck or Marnhull. It should be split-face rather than sawn and laid as random or coursed rubble.
  • Reconstituted stone can only be used where agreed with the Duchy’s development manager.
  • Bricks should be laid in English or Flemish bond. Stretcher bond is not permitted.
  • Expansion joints in brickwork and render are only permitted when they can be disguised behind other details (on Slowe’s terrace, the expansion joints are hidden behind the rainwater down pipes).
  • Airbricks must be of terracotta, built tile, painted cast iron or unpainted drilled stone.
  • Mortar joints should not be weather struck, raked, concave or ribbon.
  • Party walls rising above the roof and raised or parapeted gables should be no less than 215 mm thick.

Lintels

  • Lintels should be true load-bearing constructions in stone, brick, flat tiles or timber. Where brick or stone is used, lintels should be formed as true arches.
  • Flat arches with bricks upright (soldier arches) are banned.

Chimneys

  • The building code states that chimneys should be of brick and rise “generously” above roofs.
  • Chimneys should not appear “stout” or “dumpy” and the tops of stacks should be finished with corbels and oversailing courses.

There is a degree of flexibility in the design guide. The Duchy will sometimes allow compromises in the design if the changes can be justified. Kim Slowe, for instance, persuaded the Duchy to let him build a bathroom in the front of the house: normally this would not be permitted. The bathroom was allowed because Slowe promised that clear rather than frosted windows would be specified, which meant that the traditional appearance of the facade was preserved.

Grand designs: The wall specifications

First and second floors
  • All the walls have large format (double coursing height 430 mm) Thermalite blocks with thin-joint mortar and stainless steel ties.
  • The separating walls have two leaves of 100 mm 660 kg/m3 Thermalite party wall blocks with a 75 mm cavity in between. The leaves facing the room were parged with sand cement render to a minimum of 6 mm thickness before a single layer of plasterboard on dabs was applied.

  • The specification of the external wall is 140 mm 2.8 N, 480 kg/m3 Turbo block for the internal leaf, followed by 48 mm Celotex Tuff R insulation, with 50 mm cavity, and 100 mm of 4 N, 550 kg/m3 blocks finished with Alsecco render. The internal finish was a single layer of Lafarge plasterboard on dabs.

The ground floor

  • The party walls, inner leaf of external walls, and partitions on the ground floor were constructed using standard height (215 mm) blocks of 7 N strength, and 730 kg/m3 density. The outer leaf is made from 4 N, 550 kg/m3 Thermalite Shield blocks and has an Alsecco render.
  • All walls are of thin joint construction and the block thickness, insulation and finishes are the same as the first and second floors.

Sound test results on each of the separating walls at first and second floor levels achieved levels close to 60 dB Dntw + Ctr, well in excess of the Part E requirements.

Downloads

Credits

Client Cornhill Estates Concept architect Stephen Mattick Executive architect John D. Matthews Main contractor KDJ Slade

Masonry