Bryan Woodley, Managing Director of Val-U-Therm Ltd, explores the history, current use and development of structural insulated panels
Extracts of an article published in Timber 2019 Industry Yearbook, reproduced courtesy of bmtrada. For the full article – click here.
Structural insulated panels (SIPs), also known as stressed-skin panels, are the basis of modern aircraft wing structures. Millions of people rely on this approach to fly safely every day, hence it is a well-used and proven technology.
Past: ‘What’s past is prologue’
The use of SIPs in construction began in the 1930s. In 1937, the US Forest Service (USFS) built a prefabricated house using SIPs in Wisconsin. The SIPs industry was good at public relations – the experimental house was dedicated by the First Lady Eleanor Roosevelt.
After 25 years of service, the house was inspected and found to be in excellent condition. It endured the severe Wisconsin climate (recorded temperature range + 46oC to – 48oC, with significant rain and snowfall) until 1998 when it was removed to make way for a new and larger building. The USFS research paper notes that the panels were designed and maintained to control moisture effects.
Despite the advantages of these early systems, they were characterised by poor insulation performance. In 1952, Alden B Dow (an architect and student of Wright) addressed this with a new panel design using a polystyrene foam insulation core manufactured by his family’s company, Dow Chemicals. He created the first modern-day SIP.
While Dow’s homes demonstrated the energy-saving benefits of SIPs, the low cost of energy limited early uptake. Following the 1973 oil crisis and the rapid increase in energy prices, many proved that the concept worked and produced durable, long-lasting homes across many climate zones – first in North America and then Europe. The trend in Europe was further accelerated by the development of the fabric-based PassivHaus standard.
Present: ‘Happy the man, and happy he alone; He who can call today his own; He who, secure within, can say; Tomorrow, do thy worst, for I have lived today.’
Currently, SIPs take the form of a rigid foam insulating core sandwiched between two structural facings. In the UK, most suppliers typically use the same facing – oriented strand board (OSB).
The bonding of the insulation to the facing material allows the whole panel to withstand structural loads. Problems have occurred with this bond in service resulting in structural concerns. The insulation core of PUR panels is auto-adhesively bonded to the OSB facings. The PUR is injected in liquid form into the panel, where it expands, and the panels are held at high pressure, making the PUR foam bond to the OSB. It is generally said that this provides better adhesion, rather than relying on glues used in a secondary process as when using PS as the insulation.
SIPs are manufactured off-site under factory-controlled conditions and hence provide relatively high levels of airtightness. However, attention to detail and quality of workmanship on site is vital and this is more important than claims about faster build speed.
The benefits of using SIPs are that they provide:
- high strength
- good thermal performance
- suitable for most building designs.
Their inherent strength allows them to be used for roof and floor panels, as well as walls. For room-in-the-roof applications they provide well-liked vaulted roof spaces. SIPs provide the capability to span from ridge to eaves with, at most, one intervening horizontal purlin. Thus, simple roof shapes can easily and effectively be covered over in hours – for this reason a SIPs roof is often combined with other build methods.
Future: ‘Your task is not to foresee the future, but to enable it.’
SIPs continue to grow market share and the industry does a good job of promoting itself – ever since Mrs Roosevelt!
As well as commercial applications, the system is increasingly well accepted in the self-build market, currently estimated at about 8% market share. In addition, SIPs are increasingly used with other building systems as a wrap-around, infill panels and roof panels.
Most recent product developments are the result of a hybrid approach. Most blank panels start their life in a 1.2m-wide format and come in various standardised lengths from 2.4m to 7.5m. Hybrid approaches have taken the width to 3.2m and lengths to 10.2m but more is theoretically possible.
Standard SIPs floors and roofs typically can reach spans of up to 4m and hybrid approaches incorporating timber I-beams have been applied up to 10m spans. Although the standard panel width is 142mm, various thicknesses are available. One of the more recent hybrids using a combination of PUR and PIR, totalling 285mm thickness, offers a wall U-value of 0.08 W/m2K, which is claimed to be the best in the world.
Another interesting hybrid SIPs-type approach is to integrate CLT with the injected insulation approach, normally PUR, to give robust panels that are particularly suited to commercial building requirements.
From a fascinating prologue and an exciting present, SIPs offer the brightest of futures for timber in construction and an opportunity for all involved.Extracts of an article published in Timber 2019 Industry Yearbook, reproduced courtesy of bmtrada. For the full article – click here.