Building Regulations in the UK have been interpreted for years as demanding that refurbished walls need to meet a U value of 0.3. Products have therefore been designed using BR443 and BS5250:2011 to comply with this.
Unfortunately, BS5250:2011 states that it is no good at modelling ‘in service’ situations. The whole model is based on water vapour only, no account for liquid water at all. Not so good when you are dealing with moist walls (AKA virtually all walls built before 1919!)
BR443 is the calculation matrix that gives us U values. This, though, has been shown in virtually all cases to be very inaccurate when assessing solid walls. Caroline Rye’s work has clearly shown wide discrepancies with measured in-situ U values with predicted ones from common U value calculators. DECC is taking this very seriously and the STBA has been affecting Government thinking thanks to this key research.
So, given that the tools that we have to use to calculate solid wall U values are, in effect, useless and that the unintended consequences of using non-breathable insulation materials can be as severe as structural failure, what can we do?
Well there is a little used get-out clause in Part L1B of the Building Regulations.
Part L1B states:
Dwellings Exempt from Energy Efficiency Requirements
3.8 Historic and traditional buildings where special considerations may apply
c. buildings of traditional construction with permeable fabric that both absorbs and readily allows the evaporation of moisture
It goes on to say that you should aim to improve the energy efficiency as far as is reasonably practicable and it should not increase the long term risk to the building fabric. It also states that you should make provisions to enable the fabric of the building to ‘breathe’ to control moisture and reduce the decay problems.
The document also points you towards your local conservation officer for advice. So I would recommend sending them the STBA report on Responsible Retrofit first!
Dr Jo Hoppers work on thermal insulation seems to suggest that the thermal bridging associated with wall insulation is such that even the best detailed Passive House refurbishments will be hard pressed to reach U values much below 0.3, so standard installations will be virtually impossible to achieve these types of figures.
However, within this doom and gloom is there a ray of hope?
The U value research by a growing number of people and orgs (inc BRE) show that the U value of these old solid walls are radically better than predicted, so you might already have a well performing wall! The U value of your wall might correspond to the types tested by the STBA / SPAB by Dr Rye, so it is worth checking your wall structure against the research findings. You can then use this to show your Conservation Officer as well.
Using breathable insulations like wood fibre on walls is a lower risk option, but all this depends on the existing wall structure, how it has been altered over time, etc etc. So managing risk is difficult, but certainly using materials that have been assessed and labelled using inappropriate tests raises the chances of ‘unintended consequences’. This subject is one that BRE has been looking at for years and will soon publish its research into.
This post was originally published in 2013, but this is an updated version.