Choosing Insulation for a Timber Frame Home

When it comes to the energy performance of timber frame and masonry building systems, there are some key design differences between the two.

Unless you are thinking about solid wall construction, the fundamental distinction between masonry and timber frame is the non-negotiable need to preserve a cavity between the inner timber structural panel and outer cladding skins, which must be kept free for ventilation purposes.

With brick-and-block, the gap can be usefully insulated to boost a masonry structure’s performance while attempting to prevent the total wall thickness from becoming too great.

That’s not the case with timber frame, where the structural panel does everything: taking load by supporting floors and roofs, providing rigidity to stop the structure from a lateral twist and containing the insulation required for acoustic and thermal performance.

The physical properties of a timber panel are important. The selection and mix of the individual components should be treated much like a formula or recipe – some vanilla, others more exotic, with a staggering range of prices from a big supplier pool all competing for your business.

Panel design components

An open-cell timber panel is made to a given height and usually to a standardised width, with special sizes to suit building dimensions, plus window and door openings. The panel is made from structural timber studs/rails, which form a perimeter frame and include intermediate vertical studs at no more than 600mm centres.

The hollow frame is formed into a panel by fixing a layer of rigid board, typically to the outside. This sheath locks the frame together and stops the rectangular or square shapes from becoming rhomboid.

This rigid (racking) board is most commonly a sheet material like OSB (oriented strand board) but could equally be plywood, MDF, Fermacell (made from gypsum and paper), Panelvent (a wood-chip derivative, without using glue) or magnesium oxide board, among others. Where window openings are located, the panel will also include a timber lintel which will be supported by additional studs.

Interfaces at floor & roof junctions There is no point in making serious purchasing decisions about a specific panel’s performance unless the same diligence is paid to the interface details of these panels to floors and roofs. Therefore, the airtightness of a building must be carefully tested at the end to ensure insulation in difficult-to-access voids is installed with care and that overlaps between VCLs are properly joined and taped. The SAP calculation software has the equivalent of robust details for these interfaces and extra scores are added if you can demonstrate compliance.

If the racking board is applied to the outside – as is most common in the UK – then this outer surface will usually be covered with a breather paper which will protect it from weather but allow the passage of moisture vapour.

Most panel manufacturers will apply this in the factory with nylon strips to show where vertical studs are located and with overlaps (temporarily folded back) that are included for sealing each panel to its neighbour.

Timber stud sizes are most commonly 38mm x 140mm sectional timber (treated softwood) which is one of two main industry standard sizes and referred to as CLS (Canadian Lumber Stock) as its origins are from North America.

The very fine wood of Northern Europe uses a different metric equivalent at 45mm x 145mm (TR24) or C24 stress graded timber, which may come out at 47mm x 147mm. For most timber frame users, the insulation options are all built on, into and around this panel chassis.

Industry products

Take a walk around any one of the exhibition halls at Build It Live and you’ll come across a vast range of panelised products, each designed to differentiate it from the competition and to help justify a variance in price.

Some add insulation in the factory, others take it one stage further and line the interior panel face with a vapour-control layer and possibly electrics and conduits. Some include a built-in service channel and others extoll the virtues of two separate stud walls with a clear insulated space between.

In picking some examples for a bit more focus, the following three generic types should cover most options.

Model 1: The first route is the conventional core panel with an additional layer of sheathing insulation in the cavity. This provides an easy opportunity for a service zone internally by adding a counter batten after the vapour control layer to give you whatever thickness of void you want for mechanical and electrical installations.

Using PIR (polyisocyanurate) rigid foam insulation as an example, impressive U-values can be achieved as calculated and published by one of the leading manufacturers (see Kingspan image, above right). Using a 140mm timber stud (selected for frame rigidity) with 60mm PIR insulation between the studs and 60mm PIR as an insulation wrap around the outside, you can attain a U-value of 0.16 W/m2K. 

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