Timber frame walls for a house extension

Timber frame construction is usually a series of vertical timber posts, horizontal noggins with a facing board applied to one side for stiffness and ant-racking.

CLS timber is usually used due to its dimensional stability and forms the inner skin in most cases. The stud voids are then ready for filling with high performance insulation.

A clear cavity can usually be maintained before the extension cladding is applied.

The ever improving thermal requirements of the Building Regulations have also affected timber frame design. Simply filling the studwork voids is no longer enough an internal layer of insulation board now also needs to be applied. This is quite a good feature as it stops potential wall staining of the cold bridging elements of the studwork posts.

How common this method of wall construction is within the home extension market is unclear but I would suggest that it only accounts for a very small percentage but it may now be increasing year on year due to the increasing thermal requirements.

Many banks and Building Societies are still wary of non-traditional building methods as there was a period during the 1960's where the complexities of interstitial condensation was poorly understood by architects and building designers. This is where condensation occurs within the building fabric and causes damage. Early timber framing with poor insulation without any form of vapour barrier on the warm side of the insulation allowed this to happen quite regularly with the resultant claims for damages.

Timber frame also suits off site production of the wall panels due to accuracy & speed of construction within a work shop. Almost the whole house can be built in panel format and delivered to site on a lorry for fairly quick and precise erection ready for external cladding of the weathering layer.

Singular house extension work does not therefore suit this method of construction and any timber framing is usually built bespoke on site using separate materials by the carpenter.

One of the downs sides of timber frame construction is the lack of thermal mass when heating a property.  As the inner skin is the main thermal envelope, the heating systems only heat the internal air without warming any other structures that can store the heat.  Therefore there is no latent heat built up within the structure for smoothing out the variable temperature differences that occur during the daily life cycle. 

When the heating is turned off or a door is opened the drop off in air temperature is virtually instant.  The positive side effect of timber frame construction is that the air temperature heats up very quickly as there is no absorption from the denser building fabric such as masonry.

Another drawback to timber frame relating to the lack density of the wall construction is the transfer of sound.  Again high mass structures are very good at blocking air-borne sound.  However, technology has evolved now where acoustic improvements can be made to timber frame design.

Where timber frame construction is often widely used and comes into its own is where a first floor extension is built over an existing ground floor structure and the existing bearing potential of the existing foundations or ground floor wall thickness and quality is in doubt and expensive underpinning is to be avoided.  These are usually highly engineered solutions requiring input and guidance from a structural engineer.

Another area of timber frame use within the home extension market if for roof extensions and loft conversions.  A 'hip to gable end' side roof extension for example ready for a loft conversion is an ideal opportunity to use timber frame for the infilling of the gable wall end.  Often, many older houses were only 9" solid brickwork making it impossible to form a compliant traditional masonry wall.