Prince and the revolution
Natural selection: The Natural House (above)has been described as a “simple low-tech and easy-to-build alternative for volume housebuilders
Prince Charles is once again in the vanguard of the built environment, this time with a back-to-basics house which could become a template for volume housebuilders. Martin Spring reports
If low-carbon, environmentally friendly, sustainable housebuilding has become a bandwagon hurtling across the industry, then Prince Charles can be clearly seen sitting up there on the front seat. In fact, the Prince of Wales and the Prince’s Foundation for the Built Environment are going so far as to park an entire new demonstration project at the BRE’s Innovation Park near Watford.
In the five years since its launch, the Innovation Park has emerged as a scrum of experimental buildings, all of which strike eye-catching poses with angular forms, modish materials and high-tech wind and sun traps on the roof.
But the Prince’s Foundation’s house is literally the new kid on block and has still to emerge from behind a veil of scaffolding and protective wrapping. Standing slightly apart at one end of this hyperactive village, the building has a classical symmetry of form along with white-stuccoed walls, hipped roof and a pair of trad chimneys on either side.
“It remains recognisably a house,” was how Prince Charles goaded the nation’s architects at the RIBA last summer. “It doesn’t wear its ‘green-ness’ as if it is the latest piece of haute couture. It is much more concerned with what works on the High Street in terms of good manners and courtesy.” As designed by the Prince’s Foundation, the project presents itself as a pair of sedate suburban semis. A closer inspection reveals that the project is actually a three-storey town house on one side, and a ground-floor flat topped by a duplex apartment on the other.
As part of this unassuming spirit that sets it apart from its neighbours, the building is claimed by the Foundation to offer “a simple, low-tech and easy-to-build alternative for volume housebuilders seeking to meet increasingly stringent low-carbon targets for new homes”.
As for materials, it’s back to nature with clay blocks, lime plaster and sheep’s wool insulation that are “either grown or taken from the ground” and locally sourced. These natural materials have also given the demonstration house its name: The Natural House. With the shell of the house slowly nearing completion, CM decided to put these claims to the test.
Slowly is the operative word here, as the entire house was originally due for completion last June. But after the foundations and ground-floor slab were laid, the original project and construction manager, Swan Country Homes, pulled out claiming that operating at long distance from its West Country base was becoming too much of a strain.
Chartered Building Company Kingerlee Homes of Oxford then picked up the reins, as managing director Tony Woodward explains: “When companies are donating their time and effort, you have to take a relaxed view of the programme, especially in difficult times like these.” As with its neighbours in the Innovation Park, The Natural House is being put together out of key products that are donated and installed largely free by suppliers with an eye on the publicity offered by a demonstration project.
The main supplier for The Natural House is Natural Building Technologies, which supplies both the hollow clay blocks for the external walls and the wood-fibre boards for the roof. Clay and wood are indeed both natural materials, but the blocks and boards are far from locally sourced. The clay blocks, known by the tradename ThermoPlan, are made by ZWK in Germany, while the Pavatex wood-fibre boards are imported from Switzerland.
The hollow clay blocks used for the external walls are 425mm deep and laid in single skin. But it would be stretching definitions to call them low-tech. They are the products of a sophisticated industrial manufacturing process in which the clay is mixed to a precise formula and extruded as an intricate and delicate honeycomb. After firing, the blocks are planed top and bottom to an ultra-fine tolerance of just 0.5mm. The aim of such precision manufacturing is to increase the block’s thermal insulation while decreasing cold bridging and air loss through gaps.
When it comes to laying the blocks on site, there is nothing rough and ready about the process. “You have to make sure the first course is absolutely spot on,” says Andrew Mitchell, technical manager of Natural Building Technologies. Straight after the first course is laid on a conventional 10mm bed of mortar, it must be surveyed by laser to ensure the top surface is level to a tolerance of +/-1mm.
Precision is also needed to keep all horizontal joints between the upper courses no thicker than 1mm. The mortar is a strong ready-mixed formula of lime and clay dust that acts more like adhesive. It is a far cry from traditional mortar, laid in thick buttery smears that can easily take up variations in brick thickness, but which risks creating a sizeable cold bridge through the external wall.
Laying these paper-thin mortar joints entails using a special hand-held hopper supplied by the block manufacturer. By turning the handle, the Kingerlee bricklayer unrolls a continuous mat of glass fibre that is fed through the machine’s integrated basin of wet mortar then lies on to the top surface of blocks. When he reaches the end of the course, he simply cuts off the mat and then returns to drop the next course of blocks in place before tapping them into exact alignment. As the blocks are interlocking, no mortar is required for the vertical joints.
“These blocks can be laid three or four times faster than normal bricklaying,” claims Mitchell. “You can build as high as you want in a day.”
Door and window openings are formed using specially manufactured “reveal” blocks in which the internal air cells are closed off. And shallow recesses can be quickly filed in the soft blocks on site to house joist hangers without intruding into the narrow joints.
The technique of laying these blocks can be picked up by competent bricklayers with no more than a day’s training, adds Mitchell. But the system does demand what he calls “a culture shift” because people are not used to the disciplines involved. These disciplines are different in two aspects, he explains. The first is at the design stage, when a vertical grid of 250mm should be adopted to avoid cutting whole blocks on site. The second calls for efficient management on site to ensure that palettes of blocks are left close to where the brickies are working. For this purpose, he recommends installing a static micro-crane on site.
Mitchell could have added a third discipline: get it right first time. “The adhesive is so strong that once it’s set the blocks are extremely difficult to pull down,” he warns.
The external walls will be made airtight with render externally and plaster internally, both using spray-applied Baumit lime mixes that are vapour-permeable and will allow the walls to breathe. But as Kingerlee’s Tony Woodward points out: “To make the walls really airtight, it’s important to parge [rough plaster] the area you can’t see, such as where the floor zones meet the wall’s internal face.” The overall U-value of the wall is claimed by NBT to be 0.2W/m²K.
The roof has been constructed using NBT’s Pavatex insulating wood-fibre boards, which can also breathe. They are laid in three layers totalling 254mm in thickness. And just like the blockwork walls below, all joints must be made as snug and airtight as possible. The challenge here is that, at the roof hips, the boards must be cut simultaneously at an angle and on a mitre, and this calls for the services of a qualified carpenter.
When it comes to the overall Code for Sustainable Homes, NBT’s Mitchell concedes that the Natural House should only achieve a moderately good Level 3 and will require some heating. But he adds that it should also have a unit heat loss of 40kWh/m²/year, which he says is 70% better than in the average UK home. “This isn’t NASA space science. But the real innovation is to show how exceptional high performance can be achieved by mainstream housebuilders and contractors.” The bulding cost is estimated at between £1,000 and £1,200 per m².
Hank Dittmar, chief executive of the Prince’s Foundation for the Built Environment, is keen to roll out the Natural House prototype in other speculative housing developments in which the Foundation is involved, where it could be easily adapted to a range of dwelling types. “We are already getting a positive response from developer St Modwen for the first eco-hamlet of eight houses at the Coed Darcy regeneration scheme in south Wales,” he says.
As for Prince Charles, he visited the site last autumn and laid two clay blocks imprinted with the Prince of Wales feathers on their inside face. It leaves Dittmar and Woodward with a tricky dilemma. Should they leave the royal insignia open to view or cover them with lime render? It would hardly do if Prince Charles’ only hands-on contribution to the low-carbon Natural House was a cold bridge for heat to escape.
All in all it’s just another brick in the cavity wall
The thermally efficient ThermoPlan aerated clay blocks as used in the Prince’s Foundation’s Natural House are due to come much closer to home – and cheaper to buy – once UK brickmaker Ibstock starts production here.
“We are currently developing and testing the product for the UK market,” said an Ibstock spokesman. “The aim is to manufacture them in the UK using UK materials, as the company has tremendous resources of clay around the country.”
ThermoPlan, which will continue to be marketed by licenceholder NBT, offers a simple, foolproof alternative to the traditional British cavity wall, which “has effectively
had its day”, according to Kingerlee’s Tony Woodward.
“As thermal efficiency demands rise, cavities are getting wider and insulation increases in thickness,” he says. “This causes difficulties at openings, where you have to introduce origami to get the DPCs right.”
As well as offering good U-values and air-tightness, ThermoPlan blocks have a high thermal mass that ensures even temperatures.
A unique feature is the incorporation of sawdust and polystyrene particles into the clay mix. This combustible material lowers the heat needed to fire the clay and hence its embodied energy, and it also adds to the finished product’s voids and pores and hence its thermal insulation.
The blocks have approximately 30% less embodied energy and 50% less embodied carbon than a conventional brick cavity wall meeting the same performance.
To meet UK needs, ThermoPlan is currently awaiting approval by the British Board of Agrément approval. It has already won approval from the Housing Corporation as a modern methods of construction.
Ian Beal, MD Miller Homes (Midlands & South)
Why we specified...
H+H Celcon Aircrete blocks
Miller Zero, The Pinnacle, Basingstoke
Ian Beal, managing director, Miller Homes (Midlands & South)
We wanted to challenge the industry’s preconception that timber frame is the only answer to achieving the higher levels of the Code for Sustainable Homes by building a Code level 3, level 4 and a level 6 home using high thermal performance Aircrete masonry blocks and panels.
The houses are part of a Miller R&D project to build five sustainable houses using different construction techniques and establish how much each costs. The homes are all identical in size and layout and are part of a 79-unit development of traditionally-built houses on the site.
The Code level 3 and 4 houses were built using H+H Celcon’s jumbo blocks (610mm x 270mm, 100mm thick), bonded using Thin Joint adhesive and covered with 90mm of blown-fibre insulation.
The Code level 6 house is super-insulated and its walls incorporate 200mm-thick H+H Vertical Elements panels, which were covered by a 200mm-thick layer of WeberTherm PHS insulation, capped by 6mm of WeberTherm XM render to deliver a measured U-value of 0.09W/m2K.
Using the Vertical Elements panels meant fewer joints and therefore improved air-tightness (Code level 6 requires 1.5 air changes per hour) compared to the less airtight matrix of blocks used in the Code level 3 and 4 homes. The large panels also allowed for very fast construction and the Code 6 house was completed, by contractor Minett Group, in just two weeks compared to the six-week build you would expect for a traditional masonry solution.
However, Aircrete was a very expensive solution, especially since we used it in a single thickness external wall with no cavity, which required a costly external insulation layer, plus the layer of render. Still, as part of an R&D project designed to establish the buildability of various solutions Aircrete proved very effective indeed. We’ve shown you can build a Code level 6 home using masonry.
Tips of the Trade
Machine-lay block paving
01 Consider the advantages
Machine-lay concrete block paving is supplied pre-packaged in its laying formation. The installation machine collects an entire layer from the pack and places it, in formation, on to the laying course. The process helps contractors to meet CDM Regulations and reduces workers’ exposure to manual handling, lifting and bending.
Traditionally used for large installations, such as dockside pavements, machine lay is now commonplace for installations such as roads and car parks. It is increasingly being used for the installation of permeable concrete block paving as sustainable drainage systems become more widely adopted.
03 Choose the right product
A varied range of products and laying patterns will accommodate different aesthetic and design requirements. Complementary products, such as half blocks to reduce cutting, waste and labour and one tonne bags of joint filling sand, are designed to make the process quicker and cost effective.
04 Project management
The automated processes deliver new efficiencies in speed and quality, allowing larger areas of paving to be completed far quicker than using traditional hand installation methods or alternative surfacing materials. Installations of 500m2 per day, per machine, are achievable.
05 Select the right Installer
There is a nationwide network of trained and accredited machine lay installers. Use their expertise early in the process to help you achieve the best results for your project.
Steve Attfield, commercial manager, Marshalls
THE AQUATA sustainable urban drainage system from Wienerberger is designed to control storm water before it enters into main drainage channels.
Incorporating Baggeridge Aquata permeable clay paving tiles, it filters, cleans and stores water within voids in component layers before it is allowed to drain away naturally. This cuts the volume of water entering the drainage system and reduces the risk of flooding.
The company says the system can help projects comply with Part H of the Building Regulations
THE HANSON Floodwall flood defence system incorporates prefabricated panels of either clay brickwork, concrete blockwork or natural stone.
A two-day trial of the system in Carlisle showed that 18 panels, equivalent to almost 8,000 bricks, could be installed in a day, including the placement of temporary propping, pouring concrete and striking the props.
Hanson claims this off-site technique offers consistent quality, rapid build time and the health and safety benefits.
BASF and German block manufacturer Knobel have launched a new construction system based on hollow blocks with PUR rigid foam thermal insulation already built in.
The Liapor blocks are manufactured from Lias clay, with extra air added during the production process, then filled with BASF’s Elastopor H insulation. The single-skin system does not need any additional insulation and is claimed to create substantial savings on construction time and costs.