Insulation pre-assessments are key to the success of the Green Deal - but most are being carried out using completely the wrong method, warns sustainable building expert Neil May

The Green Deal heralds the UK’s largest refurbishment programme, with the prospect of millions of buildings being retrofitted with some form of internal or external wall insulation.  

But almost every one of the UK’s 6 million traditional solid-walled buildings, which make up about a quarter of our stock, are at risk from well-meaning energy experts and insulation suppliers who are failing to carry out the right pre-assessments, or to build in sufficient flexibility for the future.

Most retrofit insulation solutions are based on a fundamental error in the way they are assessed. This is true of all retrofit, but is most critical in the retrofit of traditional solid wall buildings and particularly with regard to internal wall insulation, which will almost certainly be a major part of the Green Deal.  

We need insulation solutions that allow for three things: less than perfect applications, changing environmental conditions and future adaptations to our buildings

Too many energy experts and insulation suppliers use the Glaser assessment method - a standard static interstitial moisture calculation from BS 5250/ EN 13788 - when it should never be used for such calculations, as it was designed for use in new, dry timber-frame buildings. The Glaser method averages everything to a monthly calculation and is “steady state”. In particular, it omits driving rain from its calculations, which is probably the most significant of all factors in the failure of any insulation system.

An alternative standard, EN 15026, is a dynamic model based on hourly calculations of temperature, relative humidity and driven rain inputs according to direction, which leads to completely different results. It shows that in many places only certain types of insulation will work. And in some places on certain elevations, no insulation solution at all will work. As regards internal insulation, breathable insulation is shown to be very much better in performance wherever driven rain is a significant factor.

Furthermore, the models developed under EN 15026 can make allowance for application error and leakage into buildings. As there is no such thing as a perfect application - and as buildings will inevitably undergo further alteration and repair over their lifespan - it is vital that modelling allows for failure and can assess how a system deals with such failure. In the case of all solid wall insulation, a breathable system performs far better when modelled with standardised building faults.

The dynamic approach of EN 15026 (for which the two approved methods currently are WUFI and Delphin) means that sometimes we cannot use insulation in the way the client or architect would want. But we need to ensure that the application is healthy for the building and the occupants at all times.  

Unfortunately, even with better moisture modelling, human error, over-simplifications and the bits that simply can’t be modelled will all conspire to create other technical problems. Reliance on modelling software can create a false sense of certainty. For this reason, further research is urgently needed to show the limits of the use of insulation in different building types and locations.

What research already shows us is that, in order to protect our traditional buildings, we must build in more resilience and use very safe and buildable solutions. We need insulation solutions that allow for three things: less than perfect applications, changing environmental conditions and future adaptations to our buildings.

On the whole, traditional natural materials in modern forms such as insulation do provide a way forward for improving the energy efficiency of historic buildings, provided that this is done with proper understanding of the building physics and with proper care in application. Before the Green Deal begins, we must achieve this understanding among advisers, installers and suppliers. If we don’t, we will all bear the responsibility for the decay of traditional buildings and the poor health of their occupants.

Neil May is chief executive of Natural Building Technologies (NBT), and founder of the Good Homes Alliance and the Sustainable Traditional Buildings Alliance