Key design differences of timber frame versus a masonry build
Unless you are thinking about solid wall construction, the fundamental difference between masonry and timber frame systems is the non-negotiable preservation of a cavity between the inner timber frame structural panel and outer alternative cladding skins which must be kept free for ventilation purposes. Last month’s article on insulating a masonry home, identified how the cavity can be usefully insulated to boost a masonry wall’s performance in an attempt to keep the overall wall thickness from becoming too thick.
Not so with timber frame where the structural panel does everything from taking load by supporting floors and roofs, provides racking to stop the structure from a lateral twist as well as insulation for acoustic and thermal performance and overall air tightness. So, the physical properties of a timber panel are important and the selection and mix of the individual components used should be treated much like a formula or recipe; some pretty vanilla, others more exotic and a staggering range of prices from an industrial pool all competing for your business.
Core design components in a timber frame panel
At its heart is an open cell timber panel made to a given height and usually to a standardised width with special panels to suit building dimensions and 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. This hollow frame is formed into a panel by fixing a layer of rigid board, typically to the outside in the form of a sheathing layer, which 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 (orientated strand board) but could equally be plywood, MDF (medium density fibreboard), Fermacell (made from gypsum and paper), Panelvent (a wood-chip derivative without using glue) or magnesium oxide board, amongst others. Where window openings are located, the panel will also include a timber lintel which will be supported by additional timber studs.
If the racking board is applied to the outside, as is most common in the UK, then this outer surface is usually 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 but included for sealing each panel to its neighbour.
Timber stud sizes are most commonly 38 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. Northern Europe, which also has very fine timber, uses a different metric equivalent at 45 x 145mm (TR24) or C24 stress graded timber which may come out at 47 x 147mm. For most timber frame users, the insulation options are all built on, into and around this panel chassis.
Industry examples
Take a walk around any one of the Build it exhibition halls and you’ll come across a vast range of manufacturer panel combinations; each designed to help differentiate their product 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 perhaps cover most options.
- Insulation between the studs and also an insulated cavity wrap.
First, 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 [Kingspan – brochure attached]. Using a 140mm timber stud (selected for frame rigidity) and with 60mm PIR insulation between the studs and 60mm PIR as an insulation wrap around the outside, you can achieve a U-value of 0.16 W/m²K, which could be improved to 0.15 with a rendered external block or reduced slightly to 0.17 with a light weight cladding hung on the panel as opposed to brick. If you were to take the full 120mm thickness of insulation and put it all as a wrap around the outside of the panel (with no insulation between the studs, the U-value could be improved to 0.14 with the rendered blockwork.
This confirms the effect of thickening the external insulation wrap rather than using the space between the studs which eliminates the effects of cold bridging from the timber studs themselves. This technique is much more significant when it comes to steel frame as clearly a steel stud is much more conductive than timber. However, the negative effects of doing this are an overall thickening of the external wall with implications for a reduction of useable internal accommodation and potentially an increase in foundation thickness. This could be mitigated by using a more slender stud size, e.g. 89mm as opposed to 140mm, but the rigidity of your frame will then be reduced. So, like most design decisions, it’s a balance. My preference when insulating in the cavity is to use up the thickness of the timber studs with insulation and then add a thinner insulated wrap around the outside of 25mm or so.
- Insulation between the studs and an additional layer on the inside
However, another consideration with the cavity wrap would also be the lack of permeability of PIR insulation as, in the design examples above, we are adding this to the outside of our panel’s breather paper which is to some extent counter intuitive and could cause interstitial condensation between the breather paper and the insulation wrap. So, our second generic design option is the conventional core panel but this time with an additional layer of insulation on the inside rather than the cavity. Using an industry manufacturer who specialises in mineral wool insulation [Knauf – brochure attached] with a limited range of PIR laminated plasterboard options, we can get some equally impressive performance standards although not quite as good as the PIR. Published data suggests that 140mm thickness of Earthwool Frametherm 32 between the studs and an additional 40mm of PIR internally with qualifying plasterboard and vapour control layers (VCL) will achieve a U-value of 0.17 W/m²K when clad externally with brick. However, this does not provide a service zone and so one would have to be created on the inside if needed with counterbattening on the warm side of the VCL.
In terms of cost, a recent quote from one of the national merchants for a genuine large order supply marked 100mm mineral wool cavity batts at just over £13/m² versus 100mm top of the range PIR at £25/m². Unfortunately, there is no logic to insulation pricing as the manufacturers won’t supply direct and the national merchants get, and offer, competitive volume rates on certain brands and punitively high rates on products that they don’t normally stock. So, you’ll have to work the system and get multiple quotes from multiple suppliers and you’ll work out the best combination of supply deals in your area.
- Twin wall system with minimal structural ties
Third, is the twin wall system using effectively two thinner core panels, mechanically joined but with limited thermal bridging. If you’re interested in passive standards then this is probably the route to go. These systems can have racking on both the inner and outside faces which provides good structural rigidity and the space between the twin panels is completely insulated save for the minimum requisite ties to join the two panels together. Published U-values can get as low as 0.12 W/m²K but for the ultimate enthusiast this can probably be improved upon!
Cross thermal bridging and solid timber panels
One final thought is the conductivity of wood and the principles therefore of cross bridging with timber studs in panels. Wood has a conduction of heat (0.13-0.15 w/mk) which is about 5/6 times that of the best insulations (0.02-0.03 w/mk). But by comparison wood is about 5 times less conductive than a clay brick or a dense concrete block. Notwithstanding this, there is much interest in the use of cross laminated timber panels in construction which are essentially solid timber panels. These are usually selected for their strength and they are remarkably strong. But, from an insulation point of view they do need an insulation wrap, using any one of the industry options, on either the inside or out, but more likely the out.
Permeability and the cavity
Although there is much posturing from technical teams about the merits of their systems, one simple principle with timber frame is good to hang on to; and that is the principle of increasing permeability through the panel components. Effectively we are trying to design our panels so that they are air tight and then to control internal moisture within the building from a mechanical ventilation point of view. So, we start on the inside of the panel face with the lowest level of vapour permeability which is usually our polythene vapour control layer (VCL). This used to be called a vapour barrier but, by definition, it is not perfect and so it became a VCL. Then we have our chosen insulation followed by our external sheathing layer and our breather paper wrap. So, with this principle, if moisture does find its way through our imperfect VCL then it will find it easier and easier to get through the subsequent layers until it migrates out into our vented cavity where is gets naturally dissipated away. What we don’t want is vapour getting stuck in our panels which then condensing; QED interstitial condensation.
Interface at intermediate floor and roof junctions
And finally, there is no point is making serious purchase decisions about a specific panel’s performance unless the same diligence is then paid to the interface details of these panels to floors and roofs. Therefore, the air tightness of a building must be so carefully tested at the end to ensure insulation in difficult-to-access-voids is installed with care and that overlaps between VCL’s 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 demonstrate compliance.
Written and published in July 2017
