When is a timber frame not a timber frame? When it’s a SIP.

A SIP is short for Structural Insulated Panel. Imagine a timber frame wall you have a skeleton of timber studs nailed together. On one face is a rigid sheet of ply (or similar) and the voids get filled with insulation. Now remove the timber skeleton altogether and instead glue the insulation to the rigid sheet on the outside and add another rigid sheet to the inside face you have a SIP. Its just a solid mass of insulation sandwiched between plywood or OSB (oriented strand board) skins. The result is very strong and can be used to build external walls and roof panels. Its also good for getting strong insulation into your structure.

This column has already covered the Dutch practice of using SIPs to build roofs. In North America they have been using SIPs to build whole houses. In fact, the first SIP house was built in 1953 and they have been in commercial use since the 1970s, so in some ways they barely rank as an innovation. But its only in the past three years that interest in SIPs from the USA’s huge number of self-builders seems to have ignited over here.

One US housebuilder well acquainted with the SIPs phenomenon is Steve Keller, president of two companies based in Mansfield, Pennsylvania. The first, Woodhouse, is in the oak frame business, producing around 60 homes each year for the US market. The second, Murus, makes SIPs.

“I began Woodhouse in 1980 and started using SIPs very early on”, Steve explains. “By 1987 I believe I was using more SIPs than anyone else in the US, so started Murus as a separate company, specifically to concentrate on making them. In addition to using SIPS to clad traditional timber frames, we have free standing structural panel homes in Europe, North America and Japan. We now find we are selling to a lot of new customers, some of whom have no idea what they are going to do with these panels”.

One place where Steve is aware of Murus panels being used is in Kent, where he has been working with timber frame company TJ Crump Oakwrights on their first British house for Peter and Susie Ferguson. The whole superstructure has been shipped across the Atlantic in a container. It’s an American Red Oak post and beam Woodhouse and the external walls and roof are using Muruss Polyurethane Foam Core SIPs.

The Fergusons sifted through dozens of different methods for building their home before alighting on the concept of SIPs. They travelled to the USA to meet Steve Keller and look at the product and returned impressed enough to place their business with him.

With its massive oak posts and beams, the Fergusons house is scarcely typical of a SIP house as the structural benefits of the system are not utilised. However, the couple were still drawn to SIPs because of the benefits of build speed, lack of wet trades and high level of energy efficiency. A SIPs structure fits together in a very similar fashion to a timber frame the panels are hoisted into place then clipped together. Its easier to adapt a SIPs structure if you want to add a window or a door, you can just cut out a hole on site and, provided you dont go over a prescribed width, you dont even need to add a lintel above to bridge the gap.

The insulation within the panel must be a rigid board. This means that the majority of SIPs are made with expanded polystyrene, although you can use polyurethane boards which are better insulators but much more expensive. Typically a SIPs panel is around 150mm thick, enough to give a really low U-value as well as providing a strong structural shell, but for people wanting ultra high insulation levels there is nothing to stop you going for wider panels. The Murus panels used by the Fergusons are just 118mm deep yet, because they use a polyurethane core, they have a U-value of just 0.2 substantially better than the new standards set for the UK.

So, are there any disadvantages? Well cost is one. Importing bulky building materials from the USA is never going to be cheap. However, locally produced SIPs panels are already coming onto the market and are comparable in cost to conventional timber frame walling. Another problem to be resolved is the first fixing of services. Whereas a timber frame allows you to run cabling within the skeleton, this is not an option with SIPs because of the insulation.

One way around this is to build a service cavity on the inner face and then line over this with plasterboard but this is a somewhat bulky solution.

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