Rigid approach to damp-proofing

Grant Terry, head of brand development for Marmox (UK) Ltd, advocates a structured approach to dealing with damp and insulation issues

Despite theories that global warming could turn our latitudes into some sort of sub-Saharan desert, the past few months have reminded us that the UK remains – for the present at least – a temperate climate given to long wet spells with generally high humidity levels. For this reason, the latest revisions to the Building Regulations – due to come into force in April – not only strengthen the requirements for cutting heat loss through the introduction of Fabric Energy Efficiency Standards (FEES), but also continue to emphasize the need to address the problems of moisture ingress and condensation risk.

It is important to realise that the latest edition of Approved Document L to the Building Regulations – being a major step change on the route to zero carbon living – is now so challenging that it requires a holistic approach to the effectiveness of the building envelope; in refurbishment and conversion situations as well as new-build work.

Meanwhile, traditional damp proof courses and membranes are installed to protect against moisture rising from the ground on which properties are built, or are introduced above openings to direct wind driven rain back out of the cavity. There are other risks, however, which the specifier or self-builder needs to be aware of and that ideally they should address at the design stage.

The construction methodology needs to respect interstitial condensation, otherwise viewed as the tendency for the ‘dew point’ to be reached within the make up of the wall, roof or floor. Furthermore, with buildings being made far more airtight in order to cut heat loss, there is more chance of moisture laden air being trapped within structures, leading to problems of condensation and mould growth. The Building Regulations actually permit the use of solid wall construction, which can be achieved using very thick aircrete blocks carrying an insulated render system, but most dwellings rely on twin-leaf construction with either a partially or fully filled cavity. This approach to achieving the necessary U-values for walls is popular whether the building structure features a steel or timber frame as the alternative to a traditional blockwork inner leaf.

Residential properties designed under the 2013 Building Regulations must have a calculated Dwelling Fabric Efficiency Rate (DFER) not greater than the Target Fabric Energy Efficiency Rate. This TFEE figure is expressed as the number of kilowatt hours of energy consumed annually, per square metre of floor area. These are shown in Table 1 (below). These targets are now so demanding that in addition to fully filled extra wide cavities, designers are also likely to have to include a thermal laminate across the inner face of the exterior wall to improve performance.

These are commonly specified in the form of a standard plasterboard with insulation layer on the back, but these have the problem of being susceptible to impact damage and deterioration due to dampness. They are also normally screwed in place across a backing of battens, which leaves a cavity that can be prone to the collection of condensation.

The more robust and reliable alternative is to utilize an extruded polystyrene based material which, unlike the type of white polystyrene frequently seen as packaging, features a closed cell structure that is totally water resistant. The core of XPS will ideally feature a thin but very strong facing of fibreglass reinforced polymer concrete that facilitates bonding direct to blockwork. The inner surface can also accept direct decoration or even be wallpapered over if an appropriate lining paper is used.

There are XPS cored lining boards available that carry the recommendation of the Energy Saving Trust, which are easy to install and that can make a valuable contribution to the overall performance of any element to the building envelope. They will remain resistant to both moisture ingress and condensation problems throughout the life of a property and can even be employed for the tanking of wet rooms, which would help make a bathroom fully accessible to less able bodied people – in line with the Lifetime Homes standard – and importantly these very versatile and user-friendly boards, available in different thicknesses, are just as appropriate for refurbishment and conversion projects.