Tackling timber humidity risk

Niall Crosson of Ecological Building Systems and Michael Foerster of pro clima discuss the need to ensure that unforeseen moisture trapped during timber builds can be released, avoiding damage over time


imber construction is undergoing something of a renaissance in the UK, and it’s easy to see why. Not only does timber provide design flexibility, it can also help to fast track your self-build project. And of course, it also offers a sustainable approach to creating new homes, utilising a renewable material that boasts both low embodied carbon, even post processing, and carbon lock-up through replanting. However, timber construction is not without its challenges. In the wet UK climate, keeping structural timbers dry during the build phase of any project is virtually impossible. Rain, atmospheric moisture and high humidity are inevitable onsite, even during the summer months. As the project team races to get the building watertight, this moisture can become trapped, causing issues with damp and mould. The dark, dank conditions of damp timbers enclosed within a building’s structure also create an ideal breeding ground for spores, which can form over time, potentially leading to rot over the course of the building’s service life.

Climate is not the only culprit. Moisture

from the drying of plaster and screeds can also lead to high humidity during construction, contributing to moisture- related damage to structural timbers and potentially reducing the service life of the property.

The good news is that these risks can be avoided with effective site management, and if appropriate materials are used within the building fabric during construction to enable gradual release of moisture to the outside. Indeed, specification of advanced construction materials and a best practice approach to quality control can ensure your new timber home is thermally efficient, moisture free and more comfortable thanks to improved air quality.

november/december 2018

Snug Within was designed and built to Passivhaus standards © Graham Drummond


While the vapour trapped may only represent relatively small amounts of moisture, its impact can be significant over a longer period of time. For example, trapped moisture can lead to issues with damp insulation, affecting the building’s energy efficiency because the insulation does not perform to its designed capability. Around 90 per cent of all building failures are due to issues caused by moisture in some form, some of which may have penetrated the structural timbers before the building became watertight. Often a vapour control layer (VCL) is seen as the catch-all remedy for managing moisture issues within a property but, if moisture has penetrated the structure during the construction phase, this can actually trap the moisture within the wall or roof build up. This is because all ‘vapour check’

While the vapour trapped may only

represent small amounts of moisture, its impact can be significant

membranes may allow some moisture into the structure due to air leakage during construction, with the 35

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