Timber structures Engineered timber
Moisture dynamics: the durability of CLT
Lewis Taylor summarises the findings from research on moisture dynamics in CLT by BM TRADA and Stora Enso.
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ver the past 15 years there has been much talk about cross-laminated timber (CLT) and how this construction material has enabled designers to create taller, longer, wider and more elaborate timber buildings than had previously been possible. CLT has also provided new and exciting opportunities for low/negative carbon and sustainable buildings. CLT has really taken off in the past decade with projects ranging from small houses, medium-rise residential, large school and university buildings to vast commercial office spaces. As with all ‘new’ construction materials, there has been a learning curve, with the industry’s understanding of CLT’s strengths and weaknesses evolving over time. One of the most common areas of debate with CLT is durability (and by extension, moisture). Providing timber and wood-based products remain dry, they will have an almost indefinite life expectancy – modern lightweight timber frame buildings have nearly 100 years of use in the UK, with more mainstream use extending back 60 years. In addition, a large proportion of the UK housing stock has timber pitched roofs, many of which are centuries old. As a result, we now have a good understanding of how these lightweight timber structures behave, and how to design and detail them to achieve a long service life.
Moisture content Timber is at risk of the development of fungal decay if its moisture content exceeds 20% for an extended period of time. In a well-designed and constructed timber frame building or pitched roof, moisture content in service will be between 10% and 14% – well below the fungal decay threshold. While CLT follows the same durability principles as lightweight timber structures, its thickness and the mass of timber used present additional considerations when exposed to moisture. Timber studs, joists and rafters have a relatively large surface area to volume ratio and so typically dry rapidly when conditions allow. CLT has a much smaller surface area to volume ratio and so drying rates can be substantially slower. www.bmtrada.com
CLT drying rates can affect durability
Thermal insulation CLT external walls and roofs should always be designed as ‘warm’ construction, i.e. all thermal insulation is placed on the outside face of the wall or roof panel. By placing the CLT panels within the thermal envelope of the building, panels are in what should normally be a warm and dry environment – ideal for timber durability. In the UK, the most common insulation material to be placed on the outside of CLT has been rigid foil-faced insulation boards (e.g. PIR/PUR/phenolic) installed to walls, flat roofs and pitched roofs. While these insulation materials have excellent thermal resistance and so provide good U-values for a given thickness, the foil facings limit the ability for the CLT panels behind to dry to the outside. Historically it was assumed that any wetting to the CLT panels (either through trapped construction moisture or cladding leaks/water ingress in service) would be able to dry through the panel to the inside of the building.
Moisture dynamics research BM TRADA has been undertaking research work on moisture dynamics in CLT with Stora Enso, a leading global supplier of wood products including CLT. The two-part project looked at both wetting risk during construction and drying rates. This enables us to determine moisture distribution behaviour. >> Timber 2022
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