Timber structures Engineered timber
Moisture management Nick Clifford and Lewis Taylor reflect on lessons learned through consultancy work on the importance of accounting for timber’s hygroscopic properties when building with engineered timber.
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odular timber building products such as crosslaminated timber (CLT), structural insulated panels (SIPs) and closed panel timber frame cassettes offer significant time savings during construction. Compared to steel, masonry or concrete buildings, the primary structure can be erected quickly using prefabricated or modular timber systems. However, these non-timber forms of construction are largely unaffected when exposed to the prevailing weather unlike prefabricated timber building systems that can absorb and retain rainwater due to timber’s hygroscopic properties. Once the building is complete, timber components will dry out due to these same hygroscopic properties, but if they are enclosed at a very high moisture content, drying can take a sufficiently long period of time that fungal decay becomes a risk. A greater consideration of moisture management is therefore needed for prefabricated timber building systems, starting at the design and project scheduling phase, following through to the construction site. Through BM TRADA’s consultancy work, we have been involved in several projects where ineffective planning and moisture management has led to time-consuming and expensive drying works during construction and defects in newly completed buildings. These types of defects can undermine confidence in timber building systems.
Moisture uptake is much greater at panel perimeters than across the surface. Moisture assessments should focus on the higher risk zones.
Considerations Trapped water within pre-insulated closed panel walls and standing water on flat roofs are particularly concerning; avoiding these issues should be considered in the design, construction and phasing of timber buildings. Small buildings with short build programmes are typically low risk, but large multi-storey residential buildings and extensive commercial and education buildings are at a higher risk due to the extended nature of the build programme. Despite closed panel timber frame walls being factory wrapped with breather membrane and vapour control membranes, there is a risk of water penetration into the panel around window openings or at the top. Traditional open-panel timber frame will also be exposed to wetting during construction, but the open nature of panels allows water to quickly drain away, drying the timbers when conditions allow. However, factory-fitted insulation and vapour control layers to closed panel kits can hold or trap moisture within the panels resulting in a greater risk of elevated timber moisture content.
Reducing risks
Standing water on CLT during construction should be minimised whenever possible.
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The risks of trapping water within wall panels can be reduced by ensuring that windows and roof coverings are installed as soon as is practicably possible once the timber structure has been erected. Robust lapping of membranes around window openings and at the top of panels can also help to reduce risks along with consideration of vertical phasing and timely engagement of follow-on trades. >> Timber 2021 Industry Yearbook
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