Reverse diffusion: the hidden risk

Andy Mitchell from NBT discusses the risk of reverse diffusion in internal wall insulation installations

For both new build and refurbishment projects, ensuring that the building envelope offers excellent levels of thermal efficiency is a high priority. Effective insulation helps to improve building comfort, reduce heating bills and avoid wasted energy.

However, improving the insulation and air tightness levels of the building envelope can lead to other issues. In the case of retrofit internal wall insulation (IWI) in older or heritage buildings, it can fundamentally alter the building physics of the property.

By improving the relative temperature differential between the outdoor and indoor environment, insulation can have a significant effect on the vapour pressure, causing moisture to be trapped within the building fabric. Over time, this can damage areas such as embedded timbers and joist ends.

To address this, the commonly accepted solution is the installation of a vapour control layer (VCL) on the internal wall surface to prevent moisture within the building from permeating the building fabric. However, the effectiveness of this is increasingly being called into question due to the challenges of correctly fitting the VCL.

In reality, the VCL can actually be a contributory factor in trapping moisture that enters the structure from outside – often referred to as ‘reverse diffusion’.

What is reverse diffusion?

If you’ve never heard of ‘reverse diffusion’, you’re not alone. It is also known as ‘solar-driven vapour gradients’ and ‘summer condensation’, terms which can be misleading as they imply that reverse condensation is associated with warmer climates. In fact, the wet winters and warmer summers with prolonged daylight hours that are typical of south east England provide the ideal climatic environment for reverse diffusion, particularly when normal seasonal variations are punctuated with winter sunshine during the colder months and episodes of heavy, driving rain in the summer.

Generally, we expect more rain in winter, which is drawn into the brickwork by capillary pressure, sucking it into the building fabric, despite a vapour pressure gradient pushing it in the opposite direction. In the summer months, increased sol-air temperatures on the external wall surface affect the level of moisture in the wall but only a proportion of the moisture in the structure can be released as vapour due to the very high gradient vapour pressure in the brickwork. As a result, the dew point is located on the wrong side of the VCL, within the internal wall structure, so the very element of the specification that has been installed to address problems with condensation becomes responsible for trapping humidity within the structure.

Thanks to the increased sol-air temperatures outside and the internal wall insulation doing its job of preventing overheating within the internal environment, this creates the ideal warm and wet conditions for mould spores to form, affecting the integrity of the building and the health of its occupants.

What does this mean for specifiers?

The basic issue here is that the critical interface between the insulation and the wall needs to be able to dry out during the summer and, to do this, moisture needs to escape from the building fabric into both the internal and external environment. Over the course of months or years, humidity within the structure can cause serious damage.

CIBSE guidance provides a simplified equation for calculating ‘sol-air’ pressure but this does not account for wind, which can significantly affect the level of moisture entering the building through the brick work – driving rain and splash back as opposed to rain alone – so, ideally, dynamic hypothermal modelling is the best way of assessing risk. It is also beneficial to understand the climate context of the location, combining knowledge of wind, rain and temperatures with average peak sol-air temperatures and clear sky month climate data.

While context and modelling will enable a thorough assessment of project-specific risk, specifiers should begin from a standpoint of viewing reverse condensation as a possibility for any unvented brick or masonry wall build up, and consider it a certainty for projects in the south east and other areas with high levels of driving rain. There are several things that can be done to mitigate that risk, including reducing air temperature absorption with lighter colours on external wall surfaces and protecting the facade from rain and splash back.

It’s also important, however, to consider the properties of the specified insulation and whether, when considered holistically as part of the wall build-up, the insulation could actually aid controlled moisture transfer. For example, woodfibre insulation has naturally hygroscopic properties, reducing the potential for humidity within the building fabric. Where these properties are maximised in a woodfibre insulation that actively exploits capillary conductivity, the risk of reverse condensation and associated damage can be significantly reduced.

Balancing risk

As with any area of specification, neither the VCL nor the insulation specified should be considered in isolation but as part of a full wall build up. Similarly, higher standards of thermal performance must be balanced against the impact of increased indoor/ outdoor temperature ranges on the building physics with steps taken to design mitigation into the specification.

Andy Mitchell is sales director at NBT