The Great British Stand Off

Submitted by CathHassell on Tue, 03/01/2016 - 09:00

Lucy Pedler | Director | The Green Register

If you are working in the sustainable construction field, perhaps you will be having some engaging debates in the pub with colleagues about the best way to improve the performance of existing buildings. I know I am.

On the one hand (the argument goes), some existing buildings are losing so much heat through the elements we need to install as much insulation of whatever type to stop this wasteful loss of energy.

On the other hand there is sufficient concern about the risks of interstitial condensation when adding insulation to existing properties that some argue that we should not add any insulation at all as it may well increase these risks to an unacceptable level.

It is very hard to know what is the right course of action and there are of course many opinions in between the above two extreme viewpoints. However, not doing anything at all is not an option if we are serious about reducing the environmental impact of leaky old building.

The Green Register runs one day training courses on internal wall insulation (IWI) and we have recently decided to only demonstrate natural insulation products such as wood fibre board (e.g. NBT’s Pavatex: as it seems that the risks of moisture build up is too high with synthetic insulants and can lead to long term deterioration of the building fabric.

But what if even the natural wood fibre boards would be compromised in situations where the existing walls are so wet they might rot the boards? In my eco-development company, we recently had this issue with the conversion of a Bristol stone church hall into four starter homes. The penant stone walls are 450-500mm thick and the hall had been unused for many years. Some gutters had failed, repointing of the walls was overdue and the walls were therefore saturated.

I was advised not to contemplate using wood fibre boards as the walls were so wet (one of NBT’s speakers once remarked: ‘warmth + moisture + natural materials = compost’) and so I had to find a natural IWI material that could withstand these wet conditions and still remain effective. Fortunately there is a material that was just up for the job – a cork and lime plaster product called Diathonite supplied by Ecological Building Systems (

But this was not the only issue to resolve. I knew that we needed to use a natural IWI material but also knew it was very likely that we could not achieve the elemental U-value for the walls required by Building Control. Once again, Ecological Building Systems (EBS) came to the rescue. They referred me to some interesting research that Dr. Caroline Rye from SPAB (Society for the Protection of Ancient Buildings) had carried out that showed the sometimes startling difference between calculated and measure U-values in existing building elements (for a free download see: Dr. Rye’s research showed that in many cases the measured U-value was better that had been expected, sometimes by quite a margin.

Using the SPAB figures, EBS calculated what the U-value of the church hall walls would be with the addition of 50-60mm of Diathonite and I approached the Building Control Officer at South Gloucestershire council with some trepidation…the U-value would be 0.52W/m2K.

I was very pleasantly surprised. Having heard my case for using Diathonite, the BCO was very willing to accept that this solution, whilst not providing the desired improvement in thermal performance, was in fact the safest and most appropriate solution for this particular scenario – her approach was a welcome breathe of fresh air.

The Diathonite installation in all four starter homes was a bit messy but quick to apply and once dry, the lime and cork plaster was given a lovely off white textured finish coat which could be left as is or painted over with whatever type of paint finish required. We are confident that this will be a durable IWI that will be able to work with the existing stone and also provide a comfortable internal environment for the occupiers.

There are still places on The Green Register’s next IWI seminar on 15th March in London