timber & joinery 71
A solid case for timber
Nick Milestone of the Structural Timber Association and B & K Structures sheds some light on the whys, wherefores and appropriate residential applications for structural timber technology.
W
e have an outstanding inheritance of timber architecture. Until now, the greatest period of timber building
was between 1200 AD and 1700 AD, a period which saw the development of sophisticated prefabricated systems which provided many buildings throughout the UK. Fast-forward over 300 years and we are
now truly realising the full capabilities of this strong, sustainable and technically advanced structural solution. The rapidly developing timber sector has
resulted in many innovative structural timber products coming to market, and now that residential developers have a wealth of choice, it is important to specify the right structural timber solution for the job. This means a system that can not only meet the budget, but also the loadbear- ing and performance requirements. Developers, architects and engineers are
beginning to recognise the impact of building details and specification on in use energy performance, and are keen to promote best practice. The construction of a house maximising offsite timber technology typically takes four to six weeks, which is nearly a quarter of the time taken by traditional methods, with an average timescale of around 20 weeks, providing weather conditions permit. Offsite construction methods reduce the potential impact of bad weather on build-times and swift weather-proofing of the structures diminishes delays for follow on trades. This makes offsite timber construction the most viable option when working to meet the shortfall in housing stock. From a cash flow perspective shortening the cycle from outlay to sale means that ramping up volume can be achieved without the same capital employed as traditional means. Many purchasers have no concept that their
traditionally built new home may be five years out of date as far as energy performance is concerned. The sooner the house buying
public begins to look at home energy running costs in the same way as car buyers look at MPG, all the better from the structural timber sectors perspective. Energy efficient buildings are achieved
through good building fabric design, optimising the performance of the building envelope. There is a vast array of options within the timber technology portfolio that can deliver high performance buildings envelopes including:
Structural Insulated Panel Systems (SIPs)
SIPs are an advanced method of construction, exploiting composite panel techniques – delivering excellent structural and thermal characteristics in one system. SIPs have two parallel faces – usually Oriented Strand Board (OSB) – sandwiching a rigid core of Polyurethane (PUR) foam. The result is a lightweight system which is quick to erect and free from the complications surrounding compression shrinkage and thermal bridging that is often associated with other forms of construction. Delivering rapid and robust energy efficient
buildings, with superior insulation, structural strength and airtightness – SIPs are used for walls, roofs and floors. They offer extremely high thermal performance throughout the lifecycle of the building, while minimising wall thickness. The Polyurethane (PUR) core of rigid insulation and OSB3 facing panels achieve U-values as low as 0.10 W/m²K, making significant savings on operating costs. Using SIPs technology for residential
applications will reduce build programmes enabling houses to be completed much faster than conventional building methods. If the house design facilitates habitable living space in the roof zone, SIPs are a fantastic solution as they do not
require roof trusses, ensuring maximum space availability, while providing superb thermal performance and limited air leakage. There are two fundamental applications for
SIPs – a full structural wrap or infill walling. In all cases the product will be engineered for load bearing capability, racking resistance and wind loading requirements.
Infill walling
SIPs are often specified as infill to steel, concrete or engineered timber structural frames and can sit inside or outside the frame itself. Infill walling is incredibly quick to install, making it an innovative solution for high-rise residential appli- cations to deliver a rapid dry building envelope.
Full structural wrap
SIPs can be used to form a loadbearing full structural wrap. Currently five storeys are the maximum practical height for loadbearing SIP buildings. Above this, an additional structure needs to be incorporated into the building to carry the imposed load.
Timber frame
Modern timber frame structures are precision- engineered, strong and durable, although the same basic rules apply. The build method relies on timber frame as a means of structural support – carrying the loads imposed by the floors and roofs, before transmitting them to the foundations. The construction of timber frame based
structures utilises factory manufactured wall panels, alongside floor and roof panels, which are often referred to as cassettes. The systems used are classified as either open panel or closed panel.
continued overleaf...
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