Conference Agenda

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Session Overview
Session 01: CLT & moisture
Wednesday, 25/Aug/2021:
10:30am - 11:15am

Session Chair: Prof. Targo Kalamees, Tallinn University of Technology
Location: Room 1 - Room 082, Building: 116

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10:30am - 10:45am

The design of CLT envelopes in terms of hygrothermal performance

Villu Kukk, Jaan Kers, Targo Kalamees

TalTech University, Estonia

This study analysed variable parameters such as thermal conductivity and vapour resistance of materials, initial moisture content and a thickness of a CLT panel and dry-out capacity to evaluate the hygrothermal performance of CLT external walls. The study focused on the CLT external wall insulated with 300 mm vapour permeable glass wool insulation and CLT with a thickness of 100 mm exposed to the indoor environment. The results of the work were obtained first from the field measurements, after which the simulation model was created and validated, and the hygrothermal performance of the wall was analysed by a stochastic approach. The results of this showed that the CLT dries out quickly and safely in a wall assembly with a high water vapour permeability, even with the large range of initial CLT MC (13-25%). When an additional airtight layer with high vapour diffusion resistance (Sd of 244 m) is added between the insulation and the CLT, the dry-out capacity of the CLT decreases significantly and there is a high probability of mould growth on CLT surface. Parameter analyse using partial correlation coefficient showed that in the case of high water vapour permeable wall the material properties had no significant effect on the results of mould growth evaluation and vapor diffusion through the wall was dominated by boundary conditions. In the case where the vapour- and airtight layer was added, a significant impact had the initial moisture content, which caused the moisture accumulation between the barrier and CLT. The risk of mould growth can be prevented when the vapour resistance (Sd) of the airtight layer is reduced to 1.5 m in case where initial CLT MC is up to 25% and in the case where initial MC is up to 20%, the vapour resistance of an airtight layer must be reduced to 3 m.

10:45am - 11:00am

Experimental analysis of moisture uptake in CLT end-grain exposed to free water

Kristo Kalbe1, Alvar Annuk2, Aime Ruus2, Targo Kalamees1

1Tallinn University of Technology, nZEB Research Group, Estonia; 2Tallinn University of Technology, Tartu College, Estonia

The end-grain section of a cross laminated timber (CLT) panel is the most vulnerable part to wetting, however research is scarce regarding capillary absorption of CLT.

In this paper we present the results of a laboratory test of CLT end-grain moisture uptake and dry-out characteristics. We put twelve five-layer CLT test details (TDs) with an initial moisture content (MC) of 12% into direct contact with dyed water from the end-grain edge. Other end-grain edges were covered with liquid plastic to mimic the effect of a larger panel and the sides of the TDs were left untreated. Drying of the TDs was done in four groups: 1) indoor and 2) outdoor (autumn weather) with all sides exposed and 3) indoor and 4) outdoor with the wet sides against another CLT detail.

Fibre saturation point was quickly reached in the bottom part of the TDs in both surface and middle layers. However, MC did not increase much in middle layers that were more than 30 mm away from water level. During the two-week drying period, it became evident that there is no drying potential in the autumn outdoor environment, even with wet edges exposed to air. Drying was also slow in the indoor environment for the TDs which were in contact with another CLT detail. This suggests that moisture dry-out is very complicated for constructions joints where the CLT end-grain is covered, such as the exterior wall to foundation or intermediate ceiling connection. Moreover, the results indicate that although the wetting of CLT end-grain does not reach far into to the structure, the dry-out of CLT details is not expected in cold and humid outdoor environment once the CLT has become wet. Construction time dry-out is only expected when mechanical drying is applied.

11:00am - 11:15am

Built-in Moisture in Cross-Laminated Timber Roofs - A Field Study

Jørn Emil Gaarder1, Trine Dyrstad Pettersen2

1Norwegian University of Technology and Science (NTNU), Norway; 2Takprodusentenes Forskningsgruppe (TPF), Norway

Cross-Laminated Timber (CLT) elements have a growing popularity resent years due to i.a. low carbon footprint, low weight and efficient construction time. However, the elements are sensitive to moisture and prone to organic growth if not treated properly or if used incorrectly. After the structural CLT shell is in place at a building site, it stands exposed to the elements for a period before the weather screen is in place, and moisture uptake from rain may accumulate if the CLT are not temporarily protected in some way. Roof slabs are particularly exposed, as they have a large area of exposure and the horizontal orientation doesn’t allow rainwater run-off. The efforts made to protect CLT-roofing elements by Norwegian contractors vary widely, as there are few guidelines and little experience with potential consequences over time. Hence, built-in moisture in the CLT may occur when the vapour barrier is erected on top of the CLT roof elements, making the construction sensitive to mould growth and rot decay. In close cooperation with TPF (a Norwegian interest group for compact roofs made up of about 50 manufacturers of roof products), a field study was conducted in order to investigate the conditions of compact roofs with CLT elements after some years in service. The study includes inspection and moisture measurements of CLT elements from the exterior side in 10 building projects 1-9 years old from two regions of Norway. The contractor of each project were interviewed in order to assess the extent of climate exposure and protection measures during construction. The results indicate a correlation between moisture content, building age and exposure level during construction. There is a clear indication that the drying time for built-in moisture in CLT roof constructions are slow. The need to keep built-in moisture to a minimum is therefore paramount.

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