Building Cavendish Avenue House in Cambridge in solid wood panels created the perfect acoustic for
the “Music Box” room, where the owners play their baby grand.

In fact, bar some discord over planning and mortgage negotiations and problems with steel fastenings,
Mole Architects’ first opus in the Austrian-made cross-laminated timber (CLT) panels seems to have been a pretty harmonious affair all round.

Architect and engineer were equally enthused by the building material, the project came in on budget (albeit slightly late due to the steelwork issue) and the clients loved the result. So did the judges of the 2010 Daily Telegraph British Homes Awards who named Cavendish Avenue joint winner of the Small House of the Year category.

While CLT is used for individual houses and other smaller buildings on the Continent, in the UK to date it’s largely been restricted to bigger projects, like Norwich Open Academy and London’s Stadthaus tower block (T&SB Spring 2010 & Autumn 2007).

“But for Cavendish Avenue, we were keen to use it from a very early point, before planning,” said Meredith Bowles of Mole Architects. “At Mole we’ve worked extensively with timber, but we still find there’s a perception among some clients that standard timber frame is somehow insubstantial. We’ve tackled that by adding another layer of ply or other sheathing to provide greater substance, but CLT is a ready-made solution. It’s strong, solid and gives very good acoustic separation – and there are questions on that score too with some types of timber frame.”

An added advantage is that CLT gives you something solid to screw into. “There’s no hunting around for the stud under the plasterboard,” said Bowles. “The whole wall is wood, so you can fasten fixtures anywhere, however heavy.”

The owners of the house, which replaced a masonry building on the site, “instantly understood” Bowles’s rationale for recommending CLT, which included the aesthetics of the material and its environmental credentials (it is naturally insulating and boasts low embodied energy and high embedded CO2).

“As they hadn’t come across it before, they wanted reassurance that it wasn’t an entirely new product and going to fall apart!” he said. “But the CLT suppliers KLH were prepared for that and provided a lot of information and details of previous projects.”

Mortgage providers were also unfamiliar with the material and initially reluctant to come up with the cash. “Some still won’t contemplate any form of timber building, which is a frustration,” said Bowles. “Others told the owners they’d need NHBC approval. Eventually it did agree to underwrite the house, although, ironically, the eventual mortgage provider was happy with an architect’s certificate!”

Cambridge planners had their reservations too over the “style” and scale of the house – it’s three storeys, while neighbours’ are two. But after seeing the design details and materials samples, they were won over.

Where Mole did find positivity for the project from the outset was among the engineers at Ramboll’s Cambridge office, whose previous CLT projects included the Norwich Open Academy and the St John Fisher School in Peterborough (T&SB Spring 2010 & Spring 2009).

“We were lucky to have engineers so keen to use the material,” said Bowles. “In fact they wanted to take their involvement with it further. On previous projects, KLH took on the design engineering, but on Cavendish House – and St John Fisher School which they were also working on – they did it themselves to learn more about the material and extend their expertise.”

Bowles himself found designing in solid timber “very straightforward”. “It hasn’t exactly got a joinery-level finish, but there is a slightly magical quality to it and, without turning it into a sauna, we were keen to leave as much of the natural wood surface exposed as possible,” he said. “Through a roof light in the ground floor (which extends out further than the first) you also see the edge of the panel forming the wall above.”

The structural strength of the panels, which were pre-cut in KLH’s Austrian plant and ranged in thickness from 94-115mm, also impressed.

“Most of the CLT floors span 4.5m, but a wall panel on the first and second floor spans 9m without a down stand,” said Bowles. “It just rests on a pier either end comprising another 2ft section of CLT.”

The house, which sits on a concrete slab, also features block internal walls for heat retention and glulam beams to allow an open plan ground floor and fully-glazed south wall. Apart from that the panels comprise the whole of the structure.

“And on the first floor we’ve laid a screed on the CLT,” said Bowles. “That gives an even more solid feel underfoot and is feasible because of the panels’ stiffness.”

The timber core of the house, which was erected by KLH UK engineers, came in at around £75,000, against a total build cost of £590,000. This is an estimated 12% more than the “conventional site-built panellised” equivalent, said Bowles.

“But interestingly the price from KLH didn’t shift more than a few hundred pounds from quotation thanks to the level of detail in the design and engineering. Using other systems, I’m sure there would have been more changes and the price would have moved more.”

The build speed and efficiency using CLT usually further defrays the cost of the material, although on Cavendish Avenue House this was slightly undermined by the steelwork problem. “It’s not a good story to peddle that it was a UK element which let us down, but late delivery of the connectors from the Sheffield manufacturer pushed the project into the Christmas break and cost us a month,” said Bowles.

Glulam beams allow an open plan ground floor

Apart from this hiatus, the build went smoothly and, from start to handover, took about 50 weeks.

Internally some walls were plasterboarded and painted white to complement the exposed wood of the others and the ceilings. Externally, three walls are clad in either shingle or cement fibreboard, with 200mm of Foamglas cellular glass insulation sandwiched between this and the CLT. The front façade is different. Isonat hemp insulation is fitted on the inside between the CLT and plasterboard to leave the exterior wood surface on view.

“The wall was finished in a translucent Osmo stain and over this is a semi-reflective fritted glass rainscreen bolted onto steel plates attached to the timber,” said Bowles.

Cavendish Avenue House hasn’t been rated under the Code for Sustainable Homes, but its airtightness, wall and roof U-value of 0.13W/m2K and energy use of 15 kWh/m2/yr make it Passivhaus standard.

Heat for hot water and the underfloor heating system is provided by a ground source heat pump, which draws warmth from piping fed into two 70m bore holes. And overheating is prevented by natural ventilation, the adjustable sailcloth awnings on south-facing windows (all Protek aluminium and wood composite-framed) and a retractable awning above the stairwell rooflight.

The finished project was described by the British Homes Awards judges as a “worthy winner” both “in concept and completion” and “an example of how contemporary design can be inserted successfully into existing historic neighbourhoods”.

The owners seem equally pleased with their new home. “We’ve stayed friends,” said Bowles, “so that must say something!”

Bowles himself has also been inspired by the project to explore the possibilities of CLT further. He was recently asked to advise on planning and costs, then to be executive architect on the Living Architecture holiday home in Thorpeness in Suffolk designed by Norwegian Jarmund/Vigsnæs Architects.

“This uses CLT from Eurban, which has an impressive modelling system that helped considerably with the complex geometry of the design,” he said.

Next Mole will be working on a Passivhaus flatted development with a Swiss pre-insulated timber panel specialist that uses thin CLT soffits as part of the make-up.

“It will always be horses for courses, but I do see potential for increasing use of CLT,” said Bowles. “Besides its structural and environmental advantages, we’ve now realised it also has sculptural possibilities we’d like to make more of.”

CLT Champions

The Cambridge office of engineers Ramboll was instrumental in persuading the owners of Cavendish Avenue House to opt for CLT instead of conventional timber frame.

“Initially CLT might be perceived as more expensive but, like for like and taking into account that it doesn’t need all the same build-ups, the difference was negligible,” said Ramboll’s Tristan Wallwork.

Ramboll has been something of a CLT evangelist and has convinced other clients to make the switch to it from other building products. In fact, recognising its commitment to using the material and its pioneering work with wood-based construction generally, the company this year won the TRADA-sponsored Market Development category in the TTJ timber industry awards.

It still sees major scope for increasing use of CLT in the UK, but more for larger projects for the foreseeable future.

“It may find a following for bespoke individual homes, like Cavendish House, but the best immediate prospects are in bigger buildings, such as schools and larger social housing developments,” said Wallwork.

Ramboll is now working on the City Academy in Norwich, a sister project being built with the same team as the Norwich Academy (T&SB Spring 2010).

The build went smoothly and, from start to handover, took about 50 weeks

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