Niall Crosson, group technical manager at Ecological Building Systems and Michael Foerster, engineer head of applications technology at pro clima, discuss the need to ensure unforeseen moisture trapped during the build phase can be released from the building envelope as part of a viable approach to improving energy efficiency through airtightness


aximising the airtightness of a new build structure can provide

significant benefits in reducing the energy consumption associated with heat loss. It’s a ‘fabric first’ philosophy that has moved beyond the specialist arena of Passivhaus design and into mainstream sustainable construction best practice. However, specifiers must be mindful of

the risk of trapping moisture within the structure as this could not only outweigh any energy saving gains due to issues with damp, mould and the formation of spores that will lead to rot over time, it could also compromise the designed thermal performance by compromising the insulation. Consequently, it’s essential to specify a

humidity-variable vapour check that will enable any moisture trapped in the structure during construction to back-dry following the build phase.

HOW DOES MOISTURE OCCUR? Rain, atmospheric moisture and high humidity are inevitable during any construction project and this can often lead to moisture becoming trapped within the structure. Moisture from plaster and screeds can also lead to high humidity during construction, which can also contribute to moisture-related structural damage. Around 90 per cent of building failures are due to issues caused by trapped moisture and the vapour control layer (VCL) can sometimes be responsible for trapping the moisture within the wall or roof build up because all vapour check membranes may allow some moisture into the structure due to air leakage during construction. If the vapour control membrane specified does not allow the moisture that has penetrated the structure during construction 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. This is particularly pertinent if vapour resistant materials, such as OSB or plywood are applied.

30 WINTER 2018/19 | ENERGY MANAGEMENT While the vapour trapped may only

represent relatively small amounts of moisture, its impact can be significant over 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.

HYDROSAFE SOLUTIONS To address the issues that can result from moisture penetration to the building structure during the build phase, 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 of the problems. It’s also essential that the chosen membrane enables back diffusion of any moisture that does enter the structure through unavoidable leakages, moist building materials and flank diffusion effects, enabling it to escape to the inside of the building following completion of the build phase. Pro clima has created a term for this

technology: ‘hydrosafe’ and has used this as a key element in the development of its airtightness membranes, including its Intello Plus, which has recently achieved the Passivhaus Institutes highest ever rating for airtightness during tests for Passivhaus approval. This technology ensures that the

building envelope is much more forgiving than specifications that include a

Image 1:

The BBA and Passivhaus approved pro clima Intello Plus range ensures a reliable airtight seal while its Hydrosafe technology preventing moisture from becoming trapped in the structure

standard vapour control layer or vapour barrier, because it provides humidity variable moisture protection, particularly during the build phase when humidity levels within buildings will be 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, this 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 tangible

Image 2:

Niall Crosson, group technical manager

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 opposing side of the vapour check adjacent to the insulation between the rafters. The German building code standard (DIN 68800-2) requires that a humidity variable membrane’s vapour resistance should be greater than 7.5MNs/g and less than 12.5MNs/g. This is the hydrosafe threshold and specifiers should ensure that humidity variable vapour checks, such as Intello Plus, performs within this range to deliver effective protection from moisture retention in the structure and all the associated problems it can cause, especially at the building phase and in particular in our more humid UK climate. Ecological Building Systems and pro

Image 2:

Michael Foerster, head of applications technology

clima have teamed up with the BBA to host a series of CPD seminars to help specifiers navigate those challenges. For more information visit their website.


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