Longevity of construction materials and components is crucially important in line with environmental objectives such as waste reduction. Phil O’Leary, Technical Manager at BM TRADA, explores how moisture dynamics can affect the durability of cross-laminated timber, covering the latest research from BM TRADA and Stora Enso.
Cross-laminated timber (CLT) has become a popular choice for designers over the last 15 years. Spanning residential buildings to educational and commercial spaces, the market growth of CLT can be attributed to its usability in complex timber structures. Modern lightweight timber frame buildings have been used for nearly 100 years. Plus, a large proportion of the UK housing stock has timber pitched roofs, many of which are centuries old.
CLT has also created exciting opportunities for low/negative carbon and sustainable buildings. However, as with all construction products, there has been a learning curve as the industry’s understanding of CLT’s strengths and weaknesses has grown. One such area where new knowledge has been uncovered is in how moisture can compromise the material’s durability.
CLT Durability
Durability of construction materials and components is particularly important as more people talk about sustainability. The benefit of timber and wood-based products is that the lifespan can be almost indefinite if they remain dry. That said, timber is at risk offungal 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. On the other hand, CLT has a much smaller surface-area-to-volume ratio and so drying rates can be substantially slower, which can affect durability greatly.
Insulation Considerations
CLT external walls and roofs should always be designed as ‘warm’ construction. This means 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 – creating the ideal environment for timber to endure.
The most common insulation material placed on the outside of CLT in the UK is rigid foil-faced insulation boards (e.g. PIR/PUR/phenolic). It is installed to walls, flat roofs and pitched roofs. While these insulation materials have excellent thermal resistance and provide the required U-values, the foil facings limit drying of the CLT panels behind.
Developing Understanding
Moving forward requires learning from previous experience, of which data forms a large part. For this reason, BM TRADA and Stora Enso have conducted research to better understand the moisture dynamics in CLT. The two-part project looked at both wetting risk during construction and drying rates, information which can be used to determine moisture distribution behaviour.
The drying rates of five-layer, 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%. This means drying could take years for higher moisture contents and/or thicker panels. On the flip side, 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
Creating Room to Breathe
To achieve long-term durability of timber structures, a primary consideration 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 breathable insulation materials 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.
The use of breathable insulation products and systems, 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.
Planning Ahead
CLT will no doubt play a key part in creating greener, low carbon buildings. To do so, an understanding of how to effectively protect CLT from moisture during construction is vital. Where moisture does pose issues, appropriate remedial action must be prioritised. This should also be taken into consideration at the onset of any timber construction project, particularly when allowing sufficient drying time to prevent fungal and structural decay.
For more about BM TRADA and its range of timber services, please visit www.bmtrada.com/timber-services