The UK government clearly recognises that timber-based construction must play an increasing role in the country’s built environment.
It’s seen as key to meeting the dual need for more housing and lower building sector emissions. In 2021 it set up the public-private sector ‘Timber in Construction Working Group’, tasked with producing a roadmap to plot how this is to be achieved.
Besides an increased supply of UK-grown timber, conclusions included that there was a need for greater timber building capacity and efficiency. The route forward is seen as an adoption of new technologies and more offsite manufacturing. With its superior strength to weight, timber is identified as a prime material for the offsite approach.
Timber frame is the energy efficient answer
The current government’s aim to see 1.5 million new dwellings built in its five year-term has stepped up the urgency to increase building output. At the same time the new Future Homes Standard requires the construction sector to meet ambitious new levels in terms of build quality, energy efficiency and emissions more broadly.
Timber frame offers all round carbon performance
Timber-based building stands out as a construction method to achieve these new goals. Timber is lower carbon to process and transport than energy intensive alternatives, notably steel and concrete. Moreover, it stores the carbon sequestered by trees as they grow for the life of the timber building product and it’s an inherent insulant.
Timber frame is also a strong performer when it comes to airtightness due to use of precise, engineered components and capacity for incorporating insulation.
Weather resistant barrier holds key to durability and quality
A weather resistant barrier (WRB) protects a timber frame structure from the elements during the build, preventing moisture penetration in use and playing a role in air tightness.
When specifying the right solution for a particular application, it’s important to look at the product’s ease of use, performance and installation methods: significant factors in terms of timber frame quality, long-term performance, build speed and efficiency of delivery.
Pros and cons of more “traditional” house wraps
The weather barrier type perhaps most commonly recognised is polyethylene or polypropylene site-fixed breathable house wrap membranes. Also referred to as flexible wraps, they are waterproof and allow moisture in the shell to escape. They are relatively cheap and fixed in place with plastic capped nails.
Requiring skill and experience to fit, they must overlap accurately to prevent moisture ingress and effectiveness of taping joints can be compromised if running over nails. They can also snag and tear, especially if exposed to adverse weather, and any offcuts need to be disposed of. Additionally, they add a step to both the site construction process and offsite manufacture, partially offsetting timber frame’s quick build benefits.
It is becoming more common for this type of product to be fixed to timber panels offsite, however similar issues to the above still stand. There is also the newly added concern of transportation. If not handled correctly the panels could tear, needing to be rectified onsite.
Old school building paper deficiencies
Building paper is an older approach, generally comprising asphalt impregnated kraft paper. These Building papers are billed as more water resistant than plastic but can be less durable. Being non breathable, these traditional products can potentially trap vapour in the envelope leading to mould and rot, a particular drawback for timber frame.
Skill and time to fit is required to mechanically fix these building papers, which can rip during installation. Being heavier and less flexible than plastic equivalents with lower surface traction it can be harder to install on complex surfaces. In addition, it can become dry and brittle, leading to performance deterioration.
Weather weakness of self-adhered WRB
Self-adhered or peel and stick water resistant and vapour permeable weather barriers are bonded to the substrate, making them more resistant to water and air leakage. An adhesive primer is added to the sheathing, the backing of the WRB is then peeled off and the product applied. In general they are quicker and easier to fit more accurately than plastic wraps and also, while more expensive, the cost is still a small percentage of overall build budget.
Vapour permeance, however, can vary between products so it’s vital to choose the right one for build type. They tend to stick poorly to wet surfaces and if the adhesive primer step is omitted, they can lose adhesion in cooler weather. The primer should be used with OSB sheathing as the surface is not smooth, and fluid applied flashing products should be compatible with WRB type. Peel and stick weather barriers also add a stage to both the site built or offsite manufacture.
Fluid-applied WRBs promise quality but need expertise
Vapour permeable fluid-applied WRBs are promoted as better than plastic house wraps and self-adhered products for achieving a quality water resistant and airtight result. Although more expensive, the price is a small fraction of build cost. They can be used in damp conditions and brushed, rolled or sprayed onto a surface to achieve a monolithic coating, with the latter application method said to be best to ensure the right wet thickness, but all require training and experience.
The joint fabric embedded in the coating has to be lapped correctly to shed water, with a reinforcing joint fabric then embedded followed by a second coat of WRB. It is recommended to wait for warmer temperatures, as cold weather application can require heating of coating and hoses. An average house takes around one and half to two days to coat.
Thermally-fused weather-resistant barriers – all round performance
Thermally-fused weather-resistant barriers, such as Arctek® Dryshell™, are a whole different technology and method for achieving a high-performance weather resistant barrier for timber frame buildings. It’s a proprietary resin-based protective overlay, which is applied at source in factories by wood panel producer.. This ensures a quality-controlled application process, whereby the overlay is thermally fused to the panel. The result is an all-round homogenous coverage and consistently uniform, high level weather protection, with zero waste onsite. Thermally-fused weather-resistant barriers are both water resistant and vapour permeable. They provide damage resistance to the panels, and its permeability prevents build-up of interstitial condensation within the building envelope.