A new wave of timber frame innovators have created significant improvements to both the process of building in timber and, more importantly, the performance of timber homes. Here, we explore how.
Timber Floors and Roofs
Traditionally, floors and roofs were made of solid timber, cut and installed on site. Around 25 years ago, trussed roofs started to become the norm to the point where a cut roof is now a rarity.
Floor joists went through a period of change with the introduction of:
- the I-beam (two lengths of solid 90mm x 30mm timber, separated vertically by a 9mm-thick sheet of orientated strand board (OSB) or plywood)
- the metal web joist (two lengths of 90mm x 30mm timber, spaced apart by pressed steel webs) also called Easi-joists, Ecojoists or Posi-joists
Metal web joists (above, from WB Timber) and I-beams (below) have revolutionised construction and performance
What these innovations did was allow floors and roofs to be manufactured off-site with greater precision and reduced time on site. This then led to the introduction of computer technology to the design and manufacture of a house.
This approach leads to flexibility in the design and accuracy in the manufacturing — attributes which the housebuilding industry is not generally noted for. It also means that the design specification for thermal, acoustic and structural performance is actually achieved on site.
One of the best-recognised advantages of timber frame over masonry is its thermal performance.
The accuracy allowed by computer-aided design (CAD) and manufacturing (CAM) means that the thinnest of timber frames – 90mm – will comfortably meet the Building Regulations’ required U value of 0.3W/m²K. A more typical stud width of 140mm with 120mm polyurethane rigid foam insulation can achieve a U value of just 0.18W/m²K.
But the majority of heat is lost from a building by uncontrolled ventilation. Building Regulations require an airtightness of 10m³/hr. The use of impermeable materials – OSB, as well as airtight vapour and moisture control layers – means that timber frames can reduce heat loss by half without any trouble.
Frame Wise recently won a prestigious award for its Wise Wall concept, which is essentially a variation of the twin-wall system (similar to the Larsen frame developed in Canada in the 1970s). It is essentially two small timber frames, spaced apart to form a single solid wall.
The effect of using two walls in this way is to provide greater strength and flexibility. Either skin of the wall (or both) can be used for load bearing. Insulation can be installed between the studs to eliminate cold bridges, and soft insulation can be used to provide better acoustic isolation. The glulam finish can be left bare internally. The whole 270mm panel system achieves U values as low as 0.09W/m²K.
Timber frame house built by Fforest Timber Engineering
By 1980, over 27% of all new-build homes in the UK were constructed from timber frame. A combination of adverse publicity and a slowing of the housing market, saw market shares fall to around 6%. This led to mortgage lenders getting cold feet.
Interestingly, the market in Scotland remained around 45% for new-built housing throughout that period. The last few years have seen the figure for the UK grow to around 8%. However, in the USA, Sweden, Norway and Australia, over 90% of all new homes use timber frame (figures from TRADA — Timber Research and Development Association).
In fairness, there were issues with timber frame houses in the UK in the 1970s and ’80s, as Robin Aldridge of Fforest Timber Engineering explains:
- Timber grown in the UK grows more quickly than in Scandinavia due to our warmer wetter climate. This can lead to more knots and weaker timber.
- Timber is kiln dried, and UK drying techniques in the 1970s were not what they might have been.
- There was a lack of understanding in the UK around the shrinkage that timber frame houses undergo.
Taken together those issues led to poor-quality houses, but the industry recognised and dealt with these issues so that longevity is now not a concern. That is supported by NHBC figures showing fewer warranty or insurance claims for timber frame houses than for masonry.
Timber Frame Systems Stick build:
Timber is cut to size off-site and the frame assembled on site — most common in the USA, Canada and Japan.
Open panel: The most common system in the UK, where panels are part-manufactured off-site with one side left open to accept insulation. The panel is then closed on site.
Closed panel: The frame is fully manufactured off-site, insulation installed, and the panel closed before delivery to site. Panels are heavier than open panel and have to be installed with a crane.
Drivers for Change
Mass builders are now looking at the technology – albeit reluctantly – as the advantages of timber are tangible:
- It is one of the few truly renewable resources in the construction industry
- It contains less embodied energy than comparable materials
- It can sequester CO?
- As Building Regulations move towards zero-carbon homes (from 2016) it is difficult to see how this will be achievable with current methods employed by mass builders
- It has clear price and performance advantages
- It reduces time on site (and time is money)
So, with increased competition and greater use of CAD/CAM driving prices down and improving performance even further, perhaps we can look forward to the UK’s timber frame industry finally coming of age. >>>>>>>>