IHBC Yearbook 2017

R E V I E W 35 WALL INSULATION AND MOISTURE RISK COLIN KING Britain’s housing stock is a key part of our national infrastructure. Efforts to improve its energy efficiency are now being extended to ‘hard-to-treat’, solid wall properties. With more than seven million solid wall dwellings in Great Britain, there are very significant energy savings to be made although, as Colin King explains below, there are significant risks too. The current focus of attention in the UK building stock is towards the hard- to-treat or ‘HTT’ properties, those with a solid wall construction rather than a cavity between the two wall elements. They were often built using natural local materials held together with mortar – typically lime-based – and other fine local aggregate to bind it together. The HTT properties have rightly been left alone until recent years. Their inherent moisture movement capacity (breathability) makes the introduction of modern materials risky if the process is not carefully considered, approached with a keen eye on the location of the building and undertaken with a degree of caution. Until recently, the mainstream construction industry considered the performance of these buildings to be poor in comparison with more modern buildings constructed of cementitious or timber frame materials. However, recent and ongoing research indicates that the U-values of these older structures can be up to 30 per cent better than previously assumed. This further reduces the potential for savings from the introduction of internal or external wall insulation. When undertaken correctly, external wall insulation is an important measure in helping to reduce heat loss through solid wall buildings. It can also improve the health of occupants, protect buildings from adverse weather and, sometimes, improve appearance and value. However, it is easy to get it badly wrong, generating a much smaller reduction in heat loss than anticipated. Changes to the physical state of the structure can also cause damage to the building fabric (for example through mould, dry rot or wet rot) which can subsequently affect the health of the occupants by adversely affecting the indoor air quality. This is made more likely by current industry practice which rarely considers the assessment of adequate ventilation pre- and post- insulation. One would think that the Building Regulations would be sufficient to cover this eventuality, but unfortunately Part F of the Building Regulations offers no real insight into safe or adequate levels of ventilation during the retrofit process, merely defaulting to a ‘make it no worse’ stance. However, without testing air permeability pre- and post-installation, it is almost impossible to decide whether it really is ‘no worse’. In most of the cases I have surveyed and investigated, ventilation was never on the radar of the workforce or procurement process. The answer is to consider a more risk-conscious approach to the decision-making process and one that is focussed on moisture risk. When considering moisture risk in a built environment context it is vital that the presence of water is considered in in all its different states (ice, liquid water and water vapour). All buildings and their inhabitants require a level of moisture to be healthy. Efforts should always be made to balance moisture by a combination of fabric protection, heating and ventilation –whether planned or through infiltration such as draughts and gaps in the structure or through physical vents. Solid wall or HTT buildings were constructed to deal with moisture differently from modern buildings and so need to be treated differently. In particular, their construction materials let moisture (as rain and humidity) in and out without the resistance of barriers or cavities. Again, looking at the regulations it would be reasonable to assume that they are written to protect the fabric of the building and the health of the occupants. However, this is not always Poorly detailed external wall insulation resulting from a failure to extend the roof-line of this traditionally constructed building