Mass-timber components exposed to precipitation get wet, which could lead to moisture damage. Therefore, the goal of the doctoral thesis of Villu Kukk, an expert of the Nearly Zero Energy Buildings Research Group of Tal Tech, was to establish technical criteria regarding moisture for the design of CLT external walls, the implementation of which would ensure the air-tightness and moisture safety of the building, considering the moisture related to the structure of CLT, its production technology, and moisture during construction.

Eight years of interest

How did he arrive at the topic? ‘I first heard about the new ‘engineered timber’, called cross-laminated timber or CLT in short, after graduating from my engineering studies in 2014; I also learned that it can be used in timber high-rise buildings. The product seemed like an exciting solution and I wanted to learn how this material, which had only come to the attention of the construction sector, will perform in our climate in regard to moisture,’ Kukk disclosed. ‘At first, I studied this topic as a Master’s student in the wood technology programme of TalTech and then continued as a Doctorate student in the Department of Civil Engineering and Architecture.’

In order to analyse the air-tightness and hygrothermal properties of CLT external envelopes, Kukk gathered data from laboratory and climate chamber tests and field measurements. Based on the results, he prepared and validated simulation models which were subsequently used for determining the hygrothermal criteria for data analysis. For example, the risk of mould developing on the surface of CLT and wind barrier was one of the assessment criteria of hygrothermal efficiency.

Internal humidity affects the development of cracks

According to the main conclusions of the thesis, change in internal humidity affects the development and size of cracks in CLT significantly. This in turn leads to considerable air leaks in panels. A single internal layer in a CLT panel is insufficient for preventing air leaks through cracks in panels. Therefore, a 3-layer panel needs a separate layer of an air barrier. If a panel has five layers, then it could be considered to already contain an air barrier.

However, an exception occurs when the installation of CLT panels and the construction of the building continues without proper protection from the weather so the likelihood of panels getting wet is high. When the panels get wet, their moisture content becomes significantly higher than in case of changes in internal humidity. The panels losing a greater amount of initial moisture when the building is first commissioned leads to larger cracks on the surface of the panels, which increases the risk of bigger air leaks; therefore, 5-layer CLT panels can only be considered air-tight if their low initial moisture content during construction is ensured.

Spring constructions have lower risk of mould

The main factors when ensuring good hygrothermal properties of a CLT external envelope include sufficient dry-out capacity while guaranteeing low initial moisture content during construction. Another important fact – installing CLT panels in spring leads to significantly lower risk of developing mould.

‘Mould grows on the surface of timber when the conditions are favourable – the temperature must be above 0 °C and relative humidity at least 80%. Spring comes right before summer and the rising outdoor temperatures help the timber dry out faster,’ explains Villu Kukk. ‘Therefore, if the mass-timber constructions are installed during precipitation and the wood has become wet, the so-called built-in moisture dries out faster in spring and in the beginning of summer. This reduces the risk of mould, i.e. the possibility of favourable conditions for mould are less likely to develop.’

According to Kukk, the effect of the time of installation is higher if the CLT in external walls is covered with layers which allow water vapour to escape, such as thermal insulation or air barrier. ‘When using layers with higher vapour tightness, the time of installation is less important, and if timber gets wet, the risk of mould could be high,’ Kukk adds.

Which mistakes should be avoided when constructing CLT buildings?

‘As a result of the research, I came to the conclusion that moisture safety must be prioritised when designing and constructing CLT buildings. If possible, panels should be installed while protecting them from the elements (for example, a temporary tent could be used to cover the construction site) and the design of the joints of the building should protect the cut edges of panels from coming into contact with precipitation at the site of the installation. In addition, systematically monitoring the moisture safety during the entire construction process is important – for example, the moisture content of timber in spots which have become wet should be observed,’ Kukk recommends.

Read the doctoral thesis of Villu Kukk ‘Hygrothermal Criteria for Design of Cross-Laminated Timber External Walls with Ventilated Facades’.