Timber frame and SIPs

overview

‘Timber frame’ has several meanings:

• Platform frame where walls are assembled on site into storey height panels and errected and then floored over. This then forms the basis to build the next floor on top.
• Balloon frame are similar but the walls are double storey. This is now mainly historic because the long timber members for the studs tend not to be available.
• Structural Insulated Panels (SIPs). Large factory made panels including walls floors and roofs are preinsulated and fixed together on site. A lot of the kit houses in the UK are based on SIPs.
• Post and Beam, where timber posts and beams are fixed together on site and then the walls are filled in using studwork and sheets or infill. The floors and roof are fairly traditional, using joists (or trusses in the roof). Post and beam can cover anything from the traditional oak timber frame type of building from many hundreds of years ago right through to methods like the Walter Segal approach which utilizes standard softwood sections and standard panel sizes. Roundwood framing is also a possibility (as exemplified by the Grand Designs episode showing Ben Law building his cottage). 

The first three of these offer the advantage of fast erection on site, possibly only a few days. This is useful for getting the building watertight quickly. The post and beam method is slower but more flexible, allowing minor changes to be incorporated as the work proceeds. More of the cost is in site labour and lends itself to self build in the sense of using your own labour.

Of course, with prefabricated kit construction the pricing is more predictable for two reasons. Firstly you can get a fixed price beforehand for both the panels and for their erection. With post and beam you will be relying on estimates calculated by your architect, possibly a quantity surveyor, contractors and sub contractors, etc. Materials might vary in price as you proceed with the work.

One of the problems of designing post and beam houses is that almost all the text books are out of date when it comes to high insulation levels. Usually they show up to 100mm of insulation and the studs tend to act as thermal bridges. Until recently it was assumed that the proportion of timber in a wall was less than 10% when it can often be up around 25% when you include all the noggins, spacers, sole plates etc. When you require something more like 300mm of insulation without thermal bridging you either have to go for seperately supported inner and outer wall skins or use enginneered I beams with their thin webs. Considerable skill needs to go into the detailing of eaves, around openings and at the junction of the ground floor with the wall. Also there needs to be an approach to air tightness which utilizes membranes or sheet materials which are in simple planar configurations with uncomplicated joints.

In the BRE Green Guide to Specification timber frame rates very highly in most of its forms of construction.

One of the most comprehensive manuals on the subject is Timber Frame Construction from TRADA but this badly needs updating.

See timber’s green credentials and where to source it

The Forestry Commission do an on-line book called Designing with Timber

There is also a quarterly magazine called Timber & Sustainable Building

There is quite an interesting blog on a self build oak frame project here

walls

With the drive towards super-insulated construction the challenge becomes how to incorporate a large thickness of wall insulation without making the wall too thick (probably around 300mm in the case of a Passivhaus). With timber frame housing this can be achieved quite easily by making almost the entire thickness out of insulation.

The other challenge is to ensure a very high degree of air tightness and this can be done by providing an internal cavity for services so that the wall is never punctured by service runs. The plan detail below is one of the most successful for creating wall panels to fit into a timber frame. It is quite similar to a SIPs panel but has the advantage of having thicker insulation than the kit house suppliers usually provide. The internal lining of OSB has a far higher vapour permeability than the sarking board so a polythene vapour barrier is not necessary

The internal lining might be plaster board or a clay based lining board or possibly timber (depending on spread of flame requirements for the building regulations) fixed onto timber spacer strips to allow for service runs. The space can vary from 25mm if it’s for electrics only up to about 40mm if it needs to allow for plumbing.

The OSB boarding (usually 11mm) is a greener alternative to plywood and can be fixed directly to the I beams so that the joints are supported and there is no chance of air leakage. The insulation can be placed in situ before the OSB is fixed. This is easy with mineral fibre insulation but cellulose needs applying slightly damp and it needs to be done by an experienced contractor. The sarking boards need to be vapour permeable and are fixed directly to the I beams, similar to the OSB. Foil faced sarking cuts down on heat loss by radiation. Horizontal softwood battens (probably at 600mm centres) then provide a fixing for the final wall finish in the form of a rain screen.

This gives a total wall thickness of about 440 mm which is not excessive in terms of footprint. Nor does it cause problems with excessively deep windows. There is very little thermal bridging through the webs of the OSB beams (compared with normal timber stud work which does actually increase the heat loss quite considerably.

The I beams may not be needed if the panel is simply infill and not structural. In that case they can be substituted by OSB spacers which would be cheaper and easy for any joiner to run up.

Note that external insulation can be added to an existing wall with a similar detail using OSB, battens and a rain screen.