The Yanks are coming. And the Canadians.Not forgetting a whole host of Scandinavians. All of them are keenly promoting a new structural insulated panel system (SIPS) that, if successful, should revolutionise the ways that buildings are put together in the UK.
While SIPS are common features in the construction industry abroad, there is no SIPS manufacturer in Britain.However, this is set to change.
Angel Design and Development, representing the interests of the US National Association of Home Builders, is finalising the details of the first foray of SIPS into the British market. It is currently shipping over a factory-finished panelled dwelling from America for assembly as a precursor to setting up a manufacturing base early in the new year.This factory will enable it, as Raymond Durber, CEO of Angel Design and Development, says, to 'lead to a more aggressive programme' in the UK.
SIPS comprise an insulant captured between two skins to form a structurally stable element.The units are prefabricated in a simple process suitable for small-scale production - although large, production-line factories are taking advantage of the relatively low-cost technology.
There are a variety of types and sizes of SIPS to choose from.Typically, the external skins are made of 12mm oriented strand board (OSB) but the insulation materials vary greatly. OSB is a visually unattractive material which is rapidly becoming a more costeffective substitute for panelling than plywood.They are engineered wood panels that use fibrous strands of timber (usually by-products from other timber manufacturing processes), which are randomly overlaid and bonded. This produces a board which has strength in both directions - which assists the structural stability of the composite in terms of compressive and shear strength.
OSB is now picking up a large proportion of the sheathing market (see box) even in the UK. Due to OSB's relative cost effectiveness - even though it has to be shipped in from abroad - it is regularly used on construction hoardings in lieu of more traditional sheet materials.
Sandwich fillers The OSB sheathing panels have insulation trapped between them to form the SIPS composite. Some simple SIPS panels utilise a timber frame to fix the skin to, with polystyrene or mineral fibre insulation cut to size and inserted between the frame studs.
These panels are completed with OSB sheets mechanically fixed over the frame.This simply replicates a panel of a timber frame construction, built in factory conditions and enjoying the benefits of factory-quality systems.
This straightforward type of construction has been promulgated by self-build advocates in Britain for many years.The more advanced SIPS are as simple to use but reduce the need for internal strutting and take more advantage of the factory environment.
Plastifab, a company based in Canada, uses sheets of expanded polystyrene as the sandwich layer.
Depending on the required U-value, the expanded polystyrene layer varies between 125mm and 250mm. An industrial adhesive is uniformly applied by machine to both surfaces of the expanded polystyrene and then the OSB skins are affixed to either side. The glue expands under a prescribed pressure and adheres to the OSB and insulation.The pressure is maintained until the panels become a composite body.This cold-form manufacturing process has existed for more than 30 years. Monitoring during manufacture ensures that the bonds are acceptable and post-manufacture tests ensure that the boards maintain their integrity in use. Other types of SIPS panels have insulants with metal or plastic skins designed for different applications.
The real beauty of the system is its potential for standardisation.Pre-manufactured SIPS can be used as wall and roof units as well as floor panels.Buildings can be sized, pre-cut (and even trial assembled), to ensure maximum co-ordination prior to site assembly. The gluing and nailing patterns, the sizing, rebating and other elements can be thoroughly inspected in a safe environment to minimise the discrepancies on site.
The obvious target for SIPS is the housebuilding market although its applications extend far beyond.
In North America it is used on a range of projects - from large-span structures to balloon-framed multistorey buildings - and has also proved effective in extremes of climatic conditions.As panels come in standard dimensions at 2.5 x 8m, construction time, in comparison with more traditional methods, can be cut dramatically.
Glued and sealed Although different manufacturers have different product designs, they typically set back the insulation by about 50mm from the top and bottom edges.The assembly of walls from SIPS units is straightforward.
Floorplates laid along the perimeter act as a male connector to receive the SIPS (rebated insulation provides a female key).
Compression joints such as this have strips of adhesive and sealant liberally applied prior to finalising the connection.This caulking (combined with nail fixings) ensures rigidity, but more importantly - at least in the North American context - maintains a thorough air-seal to minimise heat loss and draughts. It is worth remembering also that most new-build housing in North America has natural or mechanical ventilation as a matter of course.With the advent of Approved Document Part L, this factor is going to have to be more clearly addressed by the UK construction sector, especially in terms of detailing and buildability.
On the sides of the panels, the insulation is flush with the edge of the OSB but has 50 x 12mm channels cut into it (see sketch).Once the first panel is in place, the next panel is introduced alongside. As the panels butt together,100 x 12mm timber splines, which have been glued into the channels, line up the channels in the follow-on SIPS.These splines not only ensure that the wall panels are flush but also that they maintain a glued and sealed joint throughout the length of the wall. In this way, complicated structures can be constructed quickly and with reduced man-power.
By infilling the head with a timber packer (raked or otherwise), the head of the wall can act directly as wall plate to accept the roof SIPS. These roof panels will span large distances as sheathing to the standard build up of the roof covering.
One note of caution about the specification of SIPS:
because it is less labour intensive and requires fewer skills of the on-site operative, there is a potential for reckless construction on site, negating the finely-tuned, factory processes of its manufacture.These panels are structural when acting together but individually, they are relatively flimsy and light.Hammering splines carelessly into channels in insulation can easily result in gaps in insulation and badly aligned walls; reliance on pre-cut units can result in on-site discrepancies; and poorly applied sealents in sealing will exacerbate any potential condensation problems at weak points.However, these difficulties simply require effective site management and control.
Angel Design and Development's initial plans include a factory, as well as a teaching centre for builders and specifiers to enable them to become fully acquainted with the materials and techniques.
Given the resistance to change within the construction sector and, more importantly, the specific existing skills-base in wet and manual trades in the UK, SIPS construction is likely to gain only a marginal share of the overall construction market in the near future.
However, once the simplicity of the methods of manufacture, design and installation have been understood, many designers and contractors are bound to realise the benefits and get on board.
Structural Insulated Panel Association www.sips.org
Angel Design & Development firstname.lastname@example.org
STRUCTURAL INSULATED PANEL SYSTEM FACTS
Laminated veneer lumber production is expected grow from 1.5 million m in 1997 to nearly 3 million m 3in 2002.
OSB capacity in the US and Canada doubled between 1993 and 1998, from about 1 billion to 2 billion m 2.
The decline in western production has bottomed out and will remain between four billion and five billion board m per year.
OSB has a 70 per cent share of the North American residential market for floor, wall and roof sheathing.
OSB outstripped plywood for the first time in North American structural panel production by a slim margin last year totalling 4.5 billion m 2.
When compared by weight, wood is used more than plastic, metal and concrete combined.
Japan is updating its building codes, which is expected to result in more timber-frame construction. The decision is expected to allow wood frame buildings to be built on 73 per cent of the available lots in Tokyo.
Analysis of earthquake damage in California and Japan revealed that panel-sheathed wood frame structures fared better than masonry and concrete buildings.
In 1980, the structural wood panelling produced in North America was enough to circle the earth 13 times.
OSB is made from a completely renewable resource - small-diameter, fast-growing trees.
Sources: Wood Technology (www.woodtechmag.com) and Structural Board Association (www.sba-osb.com)