Drop it in nail it on

While many housebuilders are still grappling with the differences between a fink truss and an attic truss, there are a growing number of options appearing which promise not only open loftspace but considerable savings in build time as well. The latest entrant into this field is Vencel Resil, better known to UK housebuilders as the company that makes Jablite expanded polystyrene (EPS) insulation. Vencel Resil is launching Jablite Roof Element in May 2000.

A structural insulated panel (the preferred acronym is SIPS) is laid from eaves to ridge and is fixed into supporting beams below, replacing the need for any form of roof truss. On a typical house roof, Jablite Roof Element will need to be supported at mid-point via a purlin as well as at the ridge and the wallplate. That’s it in a nutshell – it’s very simple. Drop it in place and nail it on.

Jablite Roof Element is a sandwich of 8 mm moisture resistant chipboard outers wrapped around a 145 mm expanded polystyrene core, enough to give a U value of under 0.25 – which is expected to be the maximum permitted by the forthcoming changes in Part L of the Building Regulations. At present sloping roofs are treated more leniently than flat ceilings. The panels are just over 1 m wide and can vary in length up to 7.5 m: being largely lightweight polystyrene means that they can be easily handled on site.

Although the product is new to the UK market, it’s been used for many years in The Netherlands where a majority of house roofs are built this way. There has been very little adaptation to meet the UK market but Vencel Resil has seen fit to obtain a BBA certificate for the product to add credence to its claims that these panels are reliable and meet all the demands of UK regulations. There is no 50 mm ventilation channel included in the panel: rather it is argued that Roof Element is a form of warm roof. This is ingenious because whereas the insulation in a warm roof normally sits above the rafter line, with Jablite Roof Element there are no rafters – the panels replace the rafters altogether.

Panels are normally craned into place and come with counter battens to take a conventional tiled finish. There is no need for any sarking or roofing felt. With a predicted cost of around £25/m2, the product looks to offer not just time savings but possible cost savings as well: attic trusses alone cost around £20/m2.

Housebuilders wanting to trial it will have to consider a number of new design issues. Switching to an SIPS system isn’t a straightforward matter of substitution. The introduction of load bearing ridge beams and purlins means that these elements have to either be extremely substantial to run from gable to gable or they have themselves to be supported at their mid-points, which calls for a design incorporating load bearing internal walls. While this is not difficult, it does mean redesigning from the ground up.

There are other issues for UK housebuilders to deal with, notably the problem of how to do the internal linings. The Dutch are happy to leave the melamine faced chipboard as a finished surface but it is felt that in the UK it will be necessary to line the face with plasterboard. And you have also to consider how to run cables when there is no void.

Jablite Roof Element does not have the SIPS field to itself. Another major Dutch producer, Opstalan, is becoming increasingly active in the UK with several projects in the pipeline, including Southern Housing Group’s Nightingale Estate in Hackney, east London. Opstalan offers both a polyurethane based panel and a fully prefabricated hinged roof. Redland is still trialling its Total Roof, another system with Dutch origins. And there are smaller producers who are moving beyond prototypes and into commercial production, like Essroof’s Esspanel system. Each system is slightly different, but housebuilders are sure to be putting them to the test soon.