UK housebuilding is facing perhaps an unprecedented set of challenges. It needs to boost output by about 100,000 homes annually to meet the government target of building 1.5 million in its five-year term.
Given the government has also set net zero carbon targets and that construction and buildings in use account for 25% of UK emissions, the building sector is concurrently having to shrink its carbon footprint.
The Future Homes Standard requires higher levels of build quality, fabric efficiency and overall energy performance. Cutting construction waste is another environmental focus for builders, as is the carbon profile of individual building materials.
Timber-based building is clearly seen as a significant part of the way forward. It’s recognised as potentially up to 25% faster than traditional masonry construction. At the same time, timber is lower carbon to produce, process and transport than energy intensive building materials, notably steel and concrete.
The government has set up the public-private sector Timber in Construction Working Group and tasked it with drawing up a roadmap for developing timber-based buildings in the UK. Government and industry, it concluded, need to work together ‘to promote use of safe, best practice, high performing timber construction products that will provide the most benefit in supporting delivery of the net zero pathway.
Weather resistant barrier installation challenges
A product that plays a critical role in timber frame performance, and application of which has implications for construction productivity, is the weather resistant barrier or WRB. Sometimes site fitted, although becoming more often offsite fixed, these are not just vital for protecting timber products and assemblies from the elements during construction. They prevent water ingress in the completed building, while most are vapour permeable too, allowing any intermittent internally generated moisture to escape the building envelope.
Weather resistant barriers – Plastic membranes
These are relatively cheap and combine water resistance with vapour permeability and can be fitted on- or off-site, fixed in place with plastic capped nails or staples. They take expertise and experience to fit effectively. Membrane sheets must overlap accurately, and joint tape effectiveness can be affected if running over the nails or staples. They can tear and snag during installation or transportation and add a construction site process, potentially compromising timber frame building’s build speed.
Weather resistant barrier – Building paper membranes
Building paper is a long-standing WRB. It is kraft paper impregnated with asphalt. Also requiring experience to fit, compared to plastic membranes it is heavier and less flexible, so more of a job to handle on site. It can become cumbersome in poor weather. It is mechanically fixed to the building shell and can rip during fitting, especially if wet.
Lower surface traction also makes it harder to fix to complex surfaces, and it can dry out and become brittle. Its performance is also not up to modern WRB standards. While giving good water resistance, it is not vapour permeable.
Weather resistant barrier – Peel and stick solutions
Peel and stick, or self-adhered WRBs are a technical step up on plastic membranes but more expensive. Like other site fixed solutions they add a site process and require experience to fit. Peel and stick products adhere poorly to wet surfaces and if primer isn’t used, adhesion can deteriorate in cool weather. Adhesive primers should always be used with OSB given its uneven surface.
Further considerations include vapour permeability variation between products, so the right one for the job needs to be selected, and fluid applied flashings must be compatible with the particular make.
Weather resistant barrier – Fluid-applied WRBs demand experience
Fluid-applied WRBs are another step up in technology and price. Still requiring skill to use effectively, they are claimed to deliver better water resistance and airtightness than plastic membranes and peel and stick WRBs, and are vapour permeable. They are brushed, rolled or sprayed on to the wood surface,
It takes about two days to apply the WRB coating to an average house and it’s a two-person job. Fluid-applied WRBs are advised to be used in warm weather, with the coating and hoses potentially requiring heating in cold conditions.
Thermally fused weather resistant barriers – the straightforward solution
Next generation technology products such as Arctek® Dryshell™ represent a different approach to WRB application. Unlike most of its predecessors this approach does not add a further site job in timber frame construction. It is a proprietary resin formula overlay and is an integrated solution, applied to wood-based panels in the producer’s factory. Application in a quality and climate managed environment ensures consistent, uniform all-round coverage.
It means that the water resistance wood panel is already protected when it gets to site. It is vapour permeable, preventing build-up of interstitial condensation in the envelope, and it’s proven to improve air tightness, with joints between panels simply taped on site, with zero waste to be disposed of.
This labour-saving technology enhances the inherent quick-build benefits of timber frame and de-skills the whole WRB application process.
Contact Arclin’s expert team to find out more about how they can support your future timber building projects.