Timber frame is the dominant form of housebuilding in northerly latitudes throughout the world. Wherever there are lots of conifer trees, the locals use them to construct their homes. The further north you go, the more prevalent timber building becomes and, not surprisingly, the acknowledged experts are the Canadians and the Scandinavians. We see this northerly bias even within the British Isles where there is a marked preference for timber frame in Scotland.
The Scottish experience is instructive. Almost all timber used for housing in the UK is imported, so the reason the Scots have taken to it isn’t because it’s conveniently growing in their backyard. Instead they appreciate that timber framed buildings don’t need time to dry out and are very quick to build. These qualities are invaluable when the weather is wild and the hours of winter daylight few. Timber frames also have a well-earned reputation for delivering warm, comfortable homes.
Traditionally, a large number of UK-wide self builders have seen things in much the same way as the Scots. Whereas most professional developers south of the border take little interest in moving away from their tried and tested methods, self builders are more open to new ideas and have long embraced timber frame as an excellent way to build.
Statistics are hard to come by, but anecdotally we reckon that around a third of UK self builders use timber frame construction, or one of its related methods. For timber frame is no longer a homogeneous product — it has evolved through the years so that its customers are now faced with a range of choices.
The Quick Read
- Timber frame systems enjoy a popular niche among self builders looking for a fast build time on site and factory standards
- There are several different types of timber frame system, from open ‘stick-built’ systems through to panels delivered to site with insulation, wiring and plumbing pre-fitted
- Most innovation is based around using engineered timber to achieve bigger spans and greater factory finishing for better airtightness: in many ways the race to lower U values has moved on
At its most basic, timber frame walls consist of timber studwork fixed in place with sheets of plywood or, more usually, orientated strand board (OSB). The studwork doesn’t have to be that thick — 38x90mm is usually structurally adequate for a two storey house.
The strength and rigidity is supplied by the board, which, when nailed to the studwork, makes a very rigid box known as an open-panel, into which you add insulation on site. The frame is wrapped on the outside with a waterproof barrier and then wrapped around this are the external wall elements, which can be either built up out of the ground (brick or stone), or hung off the timber frame walls — as would happen with timber cladding or tiles.
The timber frame factory produces these panels, leaving holes where the doors and windows will be placed later, and then ships them off to site where, together with flooring elements and roof trusses, the superstructure of the house is erected in a few days. The roof is left with a waterproof covering and, once the frame erectors have finished, work can commence both inside and outside of the house. It’s very speedy and you can work in the wet: you can see why the Scots love it.
Open-panel timber frame became the standard way of doing things in the UK in the 1970s and it’s still widely used, though the timber sections tend to be rather wider than 90mm these days because the insulation standards have risen. Today, most timber framers now use either a 140mm or 180mm frame depth.
While open-panel framing is quick and flexible compared with on-site construction, it still leaves a lot to be done on site. In countries such as Sweden, Germany and Switzerland, where self build is the prevalent way of delivering family homes, there are many housebuilders who have taken the process to another level so the walls are virtually finished in the factory — hence the term closed-panel.
The floors and the roof are also delivered in semi-finished panels and even the electrical wiring and the plumbing is pre-installed so that there is very little to do on site after the house is delivered. The more work that is carried out in the factory, the less time is spent on site.
As with many things in timber frame, there is no clear-cut distinction between open-panel and closed-panel. Many people interested in closed-panel homes gravitate towards Scandinavian and German businesses working in the UK market, but home-grown options are appearing.
One such is The Timber Frame Company (TTFC), whose sales manager Leigh Porter commented that the distinction is “usually drawn by whether pre-glazed joinery is pre-fitted into the panels in the factory”. If it is, then the erection service has to switch to cranes and, once cranes are used, the panel sizes can get much bigger. Again, the more work that is done in the factory, the less time is spent on site.
Structural Insulated Panels (SIPs)
SIPs construction is sometimes referred to as timber frame without the timber. The strength of the panels derives from bonding insulation within an inner and outer skin of board to make a very rigid, highly insulated shell. It’s a technique originally developed in the USA in the 1950s and has occupied a small but thriving niche within the wider timber frame industry — principally selling to people wanting higher levels of insulation and airtightness. One area where SIPs have proved especially adept is in providing open or vaulted roof spaces, where the panels are laid over beams.
More and more, we are seeing established timber frame businesses like Kingspan Potton offer their customers not only a choice of house types, but also building methods. Customers can elect for a SIPs build, which is usually 5–10 per cent more expensive but offers lower heating costs. SIPs homes are always fitted with mechanical ventilation systems, whereas with traditional timber frame it remains an additional option.
Other timber frame businesses, such as Scotframe and Flight Timber, are now producing pre-insulated walls with several different elements and it’s no longer easy to tell where timber frame ends and SIPs begin. Technically, if the load is borne by the vertical timbers, then it’s timber frame, but most SIPs systems have timber elements encased within them.
Where is the Innovation?
The demand for lower U values is driving the industry forward and is the reason behind innovations in wall techniques. Currently, the UK regulations seem to be happy with a U value of around 0.19, but this is expected to fall to 0.15 in a couple of years, which is very close to PassivHaus level.
There is really very little point in building to even lower U values than this because the added energy savings are so small that it doesn’t justify the extra cost, the extra material use or the added wall width. Instead, the new emphasis is on achieving very high airtightness levels — something that SIPs and closed-panel systems are inherently good at.
Cross-Laminated Timber (CLT)
If SIPs are niche, CLT is positively cliquey. Here, the walls are made up of solid timber, albeit cross-laminated or engineered timber panels. It’s a style of timber building that has grown in popularity, mainly in Austria, and is mostly used for larger buildings like schools and blocks of flats, where structural strength is more important.
There are a few architects in the UK, like Mole Architects in Cambridge, who have specified it on family homes and it makes for a stunning structure where the timber can be shown off to its best advantage. But unlike the frame systems, the insulation isn’t embedded within the walls and so it has to all be built up externally on site. CLT has to date been used on very contemporary designs and it has a growing fan base — so much so that moves are afoot to start manufacture in Scotland using native timber.
How to Choose
In terms of cost, it’s the original open-panel systems that remain the cheapest to produce, but as the requirements for lower U values and better airtightness levels increase, the cost differentials are being narrowed. The SIPs and closed-panel options tend to cost something like 10–15 percent more for the frame, but offer better performance and less site-time in compensation.
Bear in mind that the timber frame element of a house is rarely more than 30 percent of the overall budget, so the added costs may fall to as little as 3–5 percent on the overall build total — which is little different to what you might spend on a more luxurious kitchen or a whole-house lighting scheme.
Perhaps of more interest to self builders is how timber frame stacks up against traditional masonry builds. In terms of cost, there isn’t a clear winner. The cost of masonry builds tends to go up and down with market conditions — sometimes bricks are hard to come by and bricklayers hit the headlines for charging £1,000 a week. In contrast, timber frame prices tend to vary much less.
In terms of buildability, timber frame tends to work best with simpler-shaped structures and it really comes into its own when combined with lightweight wall claddings like timber, tiles or render on boarding. This can reduce wall widths by up to 150mm, which adds a surprising amount to internal floor areas.
The Different Systems
The variables between the different systems are more shades of grey than black and white. Here are some examples of the key systems:
Structural Insulated Panels: Pre-cut structural OSB sandwich panels with either polystyrene or polyurethane insulation factory-fitted.
Open-Panel: After being erected and panellised, the frame is fitted with insulation and then ‘closed’.
Cross-Laminated Timber: Super-strong engineered timber can be used in panels or for posts and beams.
Oak Frame: A traditional type of post and beam framing, the oak is now commonly encapsulated in timber panels.
Closed-Panel Innovations: Frame Wise’s Wise Wall system is effectively a twin frame full-filled with insulation (similar in principle to a full-filled cavity wall). At a depth of 270mm it can achieve U values of 0.09 kW/m².
Engineered Timber: Manufactured to improve strength allowing longer spans.