‘Timber frame’ can have several meanings:
- Platform frame where walls are assembled on site into storey[for the purposes of part B (fire) of the Approved Documents to the Building Regulations] this includes - (a) any gallery[for the purposes of part B of the Approved Documents] - A raised area or platform around the sides or at the back of a room which provides extra space. if its area is more than half that of the space into which it projects; and (b) a roof, unless it is accessible only for maintenance and repair. height panels and errected and then floored over. This then forms the basis to build the next floor on top.
- Balloon frame is 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. (It was a North American practice mainly anyway)
- Structural Insulated Panels (SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on 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. More about SIPs
- Post and BeamSubstantial, usually horizontal structural member., 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 used over many hundreds of years right through to methods like the Walter Segalrefers to a simple self build timber frame form of construction pioneered by the architect Walter Segal. see details of the systemThe architect who devised a simple timber frame self build system (often simply known as 'Segal self build'). see more on the Segal method 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).
Oak frame building is a bit of an anomaly. see more on oak frame
Air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard is becoming one of the major issues for energy conservation. Sealing timber frame houses made of stable kilned softwood is difficult enough. Green oak is almost impossible due to the large number of splits which develop. An interesting example of problems experienced in achieving air tightness is in an AECBthe Sustainable Building Association article on low energy houses at the Greenoak developments.
A third problem is the cold bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a thermal bridge. see more on cold bridging introduced by the large sections involved. So oak frame is really a nostalgic distraction rather than a green building option.
The first three of these methods 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.
design guides needed
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 bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a cold bridge. see more on thermal bridging (which may well be the case for PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. standard) you have to go for engineered I beamsprefabricated timber beams with a web of OSBOriented Strand Board and softwood flanges. They are very light, efficient and regular 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 in order to maintain good insulation. 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.
One of the most comprehensive manuals on the subject is Timber Frame Construction from TRADATimber Research and Development Association A trade association with a strong reputation for research and publication on all things timber but this badly needs updating to achieve higher insulation standards.
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
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. Ground floors and roofs present less of a problem
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 3 has a far lower vapour permeability than the sarkinga waterproof layer in a roof which acts as backup protection in case the main layer fails. Usually a membrane or board and usually situated between roof laths and joists. board so a polythene vapour barrier is not necessary. See “Breathing construction”
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 regulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place)) 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 battensNarrow strips of wood. Roof battens are narrow strips used to fix slates and tiles. Also used for tile and slate hanging on walls and rain screens (probably at 600mm centres) then provide a fixing for the final wall finish in the form of a rain screenthis is a (usually thin) outer cladding on a wall which prevents rain, snow, etc getting at the structure of the wall behind. see more on 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.
Check this out if you are interested in a kit house
Passivhaus Homes with their “PH15 System, a timber framed solution, designed to simplify all the technical aspects of building a Passivhaus, enabling you to create your energy efficient home, one that will be fit for now and the future”.
The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)
Note that compartmentationa term used in the Building Regulations to denote fire resistance between two parts of a building (such as a compartment wall or compartment floor) or between one building and another (for instance a party wall), can be achieved using timber frame construction clad in plasterboard or a similar none combustible material.
With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)
Resistance to the passage of sound
The degree of sound insulation required within and between houses and flats is covered in Part E of the Approved Documents.