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

Timber 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.

Trapped moisture issues

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’ membranes may allow some moisture into the structure due to air leakage during construction, with the combination of high humidity, drying screeds and plasters and cold temperatures creating moist conditions on site, especially if some or all of the build phase takes place over the winter months.

If the construction has limited drying potential to the outside, and the vapour control membrane does not then allow the moisture to ‘back dry’ to the interior when conditions allow during the warmer summer months, the moisture within the structure can become permanently trapped in the building envelope.

Membrane solutions

To address these issues, the most effective specification methodology is to select a membrane that hinders the transfer of moisture into the building fabric during periods of high relative humidity, thereby limiting the root cause.

It’s also essential that the chosen membrane enables back diffusion of any moisture that does enter the structure, enabling it to escape to the inside of the building before it can cause structural damage. This is especially critical in scenarios where the exterior of the construction has limited drying capacity, such as where impervious layers like OSB or plywood are used on the outside.

At pro clima, we have created a term for this technology – ‘hydrosafe’ – and used this as a key element of the development of our airtightness membranes. One of these has recently achieved the Passivhaus Institute’s highest ever rating for airtightness during tests for Passivhaus approval.

Hydrosafe technology ensures that the building envelope is much more forgiving because it provides humidity- variable moisture protection, particularly during the build phase when humidity levels are at their highest due to poured screeds and plastering. During periods of high relative humidity in the winter, when it is wet and cold outside and warmer and drier inside, the hydrosafe vapour check also provides excellent protection against condensation. Then, during the warmer summer months, when the relative humidity is lower, hydrosafe technology provides enhanced back diffusion capacity, enabling any moisture in the structure to dry out to the interior rather than remaining trapped.

The technology is based on building physics data, which has been used to measure ‘hydrosafe values’; a calculation to assess the vapour resistance of a humidity-variable vapour check at an average humidity of 70 per cent. For example, an average humidity of 70 per cent will be present if there is a humidity of 90 per cent inside the building and 50 per cent humidity on the opposite side of the vapour check, adjacent to the insulation between the rafters.

Best practice

As timber-framed buildings continue to increase in popularity in the UK, self- builders are faced with the challenge of achieving a building envelope that delivers the benefits of airtightness without the risks of trapped moisture. There are a host of CPD seminars to support architects and construction professionals in squaring that circle.

Niall Crosson is group technical manager at Ecological Building Systems, and Michael Foerster is engineer head of applications technology at pro clima