The greatest wood he’s ever machined is Wayne Davies’s ringing endorsement of heat-treated US hardwoods.

"It’s amazing," said the proprietor of Carna Joinery. "And you should see these timbers, they are astonishingly beautiful."

Carna Joinery’s heat-treated ash and tulipwood window was on show on the Wood Awards stand at Ecobuild last month – and attracting the attention of some major window manufacturers, signalling, perhaps, a more widespread adoption of the product in the future.

That would certainly fit in with the American Hardwood Export Council’s (AHEC) plans for heat-treated US hardwoods, which have been on the scene – admittedly in small volumes – since the mid-2000s.

"North American hardwood species respond very well to heat treatment," said AHEC’s technical consultant and treatment expert, Neil Summers, of Timber Dimension. "The two most beneficial effects of the process are to dramatically improve the stability and also the durability of species that have an inherent low durability, such as ash and tulipwood," he continued.

"Thermal modification significantly reduces the ability of the timber to absorb moisture, greatly improving its stability properties. Heattreated wood also contains a substantially reduced amount of hemicelluloses – fungisusceptible material – and, as a result, the process enhances the durability to Class 1. This makes it suitable for Use Class 3 exterior applications such as cladding and high quality garden furniture."

Its use for window manufacture is being investigated and is at the development stage, he added. That’s where Carna Joinery comes in.

Wayne Davies was looking for a solution to the problem of moisture ingress and subsequent movement in timber windows. "I was fed up with going on site and planing off factory-finished windows because the plasterers render the house out, nobody opens the doors and windows and water is introduced into the building and absorbed by the joinery," he said.

Having "dabbled" with other forms of modified timber and deciding it wasn’t for him, he sought advice from his supplier, International Timber, a distributor of Northland Forest Products’ Cambia brand of heat-treated timber. International put him in touch with AHEC.

"Wayne agreed to make a sample window for us to assess the suitability of heat-treated US hardwoods and we sent him some boards that were left over from the Martino Gamperdesigned Infinity Bench that was exhibited at last year’s London Design Festival," said Mr Summers.

The bench used five species – ash, soft maple, tulipwood, yellow birch and quartersawn red oak [it has been found that quartersawn red oak performs better in the heat treatment process than flat sawn, although this is not a factor in the other species]. The species were heated to 210°C to achieve the durability and stability properties to enable them to be used for exterior applications.

The results from Carna Joinery were outstanding, said Mr Summers – and the timber was a joy to work with.

"It has no resistance or tension and there’s no wear and tear on the blades," said Mr Davies. "I machined the profile in one hit."

Not only does it machine well and look good, its performance is excellent too.

Heat-treated timber has a thermal conductivity that is 20-25% lower than timber in its untreated state. Mr Davies cites the example of a German window manufacturer which is achieving a sub 0.8W/m2K U-value on a triple-glazed frame comprising modified timber, heat-treated tulipwood and redwood. As Carna’s window goes a stage further and is made entirely from heat-treated timber, Mr Davies is confident he can get below the 0.8 benchmark that would meet Passivhaus standards.

Further testing and investigation are needed, said Mr Summers and Carna Joinery is enthusiastic about conducting its own experiments. When it came to the glue for bonding the lamella, for example, conventional D4 PVA didn’t work.

"The wood doesn’t absorb the moisture, so the glue doesn’t go off," said Mr Davies. "I spoke to Abrabond and they sent me a special melamine urea formaldehyde glue. We did some tests and it worked very well."

Mr Davies now intends to experiment with heat-treated soft maple for window manufacture and is thinking of extending the use of heattreated species into other joinery products. "I’m going to experiment with bonding heat-treated timber to door blanks," he said.

Mr Davies has already supplied the first batch of Cambia-framed windows for a restoration project and reports that the architect was delighted that he was "using his brains" to deliver better U-values and windows that wouldn’t warp – a fact that would improve the longevity of the paint, too. And there was another bonus.

"The windows are double opener casements and if you used regular timber you’d have to have a mullion," said Mr Davies. "But because the heat-treated timber doesn’t move, a mullion isn’t necessary."

This architect engagement is likely to be key to further take-up of heat-treated US species in the UK. "It just needs a ‘light bulb’ moment when someone specifies it for a major project," said Mr Summers. "I’m convinced that will happen."

At present there are two major US producers exporting heat-treated timber to the UK, Bingamen & Sons Lumber and Northland Forest Products and UK distributors include Morgan Timber, which is supplied by both producers, and International and Thorogood Timber which stock Northland’s Cambia.

Volumes at present are relatively low but much larger quantities of American hardwoods are being exported to mainland Europe, heattreated there and then redistributed.

"At Carrefour in France last year heat-treated timber was everywhere," said Mr Summers. "And at a recent wood modification conference in Slovenia it was stated that there are now 93 commercially operating heat treatment kilns in operation across Europe."

"They can’t all be wrong in believing that this type of technology works and is here to stay. We may well see more US sawmills installing heat-treating kilns in the future so that they can gain the added value that this opportunity presents."

Northland Forest Products turns on the heat
Northland Forest Products started heat treatment in the summer of 2008 and processes predominantly white ash and tulipwood, with some soft maple and red oak and a little hard maple and yellow birch.

"Wood structure, cost and demand are the primary drivers," said president and chief executive officer Jameson French. "For example, white oak doesn’t work very well, it cracks and checks, while ash is very tolerant of the treatment. And tulipwood is a lower value material that we can add value to easily."

The colour of the heat-treated wood, which can vary from light brown to a deep "roasted" brown, depends on the treatment time and the temperature at which it is heated and can be very much down to customer choice.

"We turn light coloured woods into walnut coloured material and many buyers choose Cambia for the colour – flooring buyers use red oak, for example," said Mr French. "We also do a lighter colour, lower temperature version but other buyers might want the full 210°C thermal modification process because they are making cladding or outdoor furniture and the treatment is more important than the colour."

The Cambia brand is well known in the US in specialist sectors, he said, and it’s already a viable export product, including to markets as far-flung as Australia.