looking up
wood fibre insulation
November 15, 2022
The recent wet weather and some technical conversations we've been having with external consultants and suppliers and building control inspectors has led us to think more about cladding.
For a long time now, our preferred method of construction has been timber frame, using wood-fibre insulation between studs and with a compressed wood-fibre installation sheathing board externally. This is a fairly common build up used across northern Europe (particularly Scandinavia and Germany), and is great for the environment due to its very low, or even negative, embodied carbon cost and biodegradability. It’s also great for the people that live within the building as these natural materials do not off-gas like many petrochemical insulation products.
This build-up is very efficient and cost effective; particularly with a thin-coat render system applied directly to the external sheathing board. However many building warranty providers (inc NHBC and LABC) do not accept this design approach. Despite the suppliers and manufacturers being very happy with this direct-render approach, these UK warranty providers, rightly or wrongly, take a more cautious approach and side with the long-standing British Standards' suggestion of providing a drained cavity in the construction to stop the transmission of moisture. Building Control inspectors we've worked with previously have accepted the newer approach, but recently a couple have taken this more cautious view too.
To create a rendered, timber frame building we therefore find ourselves stuck in the unfortunate position of being forced to add 'unnecessary' layers into the wall in the form of battens and a render carrying board. While we would agree this is a more fail-safe detail from a weathering perspective it does add cost (and a little thickness). Given that this low capital cost was the key advantage to a rendered finish, we are now reconsidering our design thinking and questioning builders and Quantity Surveyors to establish the next best thing. The answer is often project specific, so if you have any experience or direct cost comparisons do get in touch! We're also raising this issue with clients and recommending other finishes (like slate or timber) which require much less maintenance and last longer. This means the small uplift in capital cost is outweighed by the benefit of low running costs.
These thoughts have also aligned with our recent opening of an office on Totnes High Street, where slate and timber weatherboarding is very common; at least half of the buildings are clad in one or other of these materials. Interestingly the use of slate is much more prevalent on one side of the street (facing south) than the other. While this may be a coincidence, we presume it is a very conscious decision by owners to favour a more robust finish where their buildings are more exposed to the weather - we look forward to finding out more of the town's history as we settle in! While north-facing rendered facades can become discoloured easily (especially if there is not much air movement), they are less prone to wind and rain damage. One imagines that render was used historically as it was a bit cheaper (much like it generally is today).
As the industry becomes more aware of embodied carbon and whole-life cost/carbon analysis, we suspect (and hope) that the 'cheap rendered box' might die out in favour of robust natural materials. Time will tell!
