Timber structures: engineered timber Moisture dynamics: the durability of CLT

Moisture dynamics: the durability of CLT

Lewis Taylor summarises the findings from research on moisture dynamics in CLT by BM TRADA and Stora Enso.

Over 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.

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. >>

In the second phase of the research, the drying rates of fivelayer, 100mm-thick CLT panels were investigated; various configurations were tested, including covering the wet outer face of panels with foil to replicate panels covered with rigid foil-faced insulation boards and/or vapour control layers. This test set-up was intended to replicate typical UK construction build-ups for warm walls as well as flat and pitched roofs.

During testing of the covered panels, water in the wet outside face lamination was observed slowly passing through the thickness of the panels to the dry uncovered side, confirming the previously held assertion that panels could dry to the inside. However, with a starting moisture content of 35% in the wet outer lamination, it took almost 16 months for the moisture content to fall to 20%; with higher moisture contents and/or thicker panels, drying could potentially take years. Conversely, uncovered panels that were able to dry directly from the wet face took approximately six weeks for a similar moisture content reduction.

Figure 1: The drying mechanism of five-layer, 100mm-thick CLT panels with (left) and without (right) foil coverings

Long-term durability

A primary consideration to achieve long-term durability of timber structures is to provide a combination of drainage, ventilation and breathability. It is not normally an issue if timber gets wet, providing water can drain away quickly and the timber is subsequently allowed to dry. Slowing down or restricting drying though the use of high resistance insulation products and/or vapour control layers on the inner or outer faces of the panels can slow drying to an extent that the development of fungal decay may become a risk if panels are subjected to adverse conditions during construction or in service.

On the continent, CLT building systems are often paired with mineral wool or wood-fibre insulation products – these types of breathable insulation material are typically beneficial to timber building systems as they allow more rapid drying of the CLT panels if they are exposed to wetting during construction or in service. Figures 2 and 3 show example wall and pitched roof build-ups using mineral fibre insulation products – similar details can be used with wood-fibre insulation boards.

Figure 2: CLT wall with mineral fibre insulation product. Source: Stora Enso

Figure 3: CLT pitched roof panel with mineral fibre insulation product. Source: Stora Enso

The use of these types of breathable insulation products, in conjunction with good overall design detailing and a moisture management plan for the construction phase, will have a significant positive impact on the long-term durability and robustness of CLT structures. n

About the author

Lewis Taylor Senior Timber Frame Consultant BM TRADA

Further information

Further information on this research will be published in 2022. More detailed guidance can be obtained from either BM TRADA or Stora Enso.

Further reading

• Cross-laminated timber: Design and performance, Revised reprint, ISBN 978-1-909594-63-0, BM TRADA, 2019 • WIS 2/3-61 Cross-laminated timber: introduction for specifiers,

BM TRADA, 2019 • WIS 2/3-62 Cross-laminated timber: structural principles,

BM TRADA, 2020