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Monday, 18 May 2015

Getting solid walls to meet Building Regs

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.

Friday, 15 May 2015

The importance of a good overhang!

In Wales we have had a tendency to design our houses with the minimal amount of overhang on the roof. The tiles sit very close to the walls. This has meant that anything else that we add to the depth of the walls loses precious protection for the wall plate. So rendering and over cladding often has the unintended consequence of making the inner wall more liable to water ingress. Where the over cladding is water proof this can easily trap wind driven rain / overflow from blocked guttering into the structure.

I was in a house last week where the fascias have been twisted and are now potentially channelling water into the building. Another building around the corner from the centre is also suffering after the owners installed external wall insulation. Here the builders had put one of those awful plastic covers over the top of the insulation to 'provide protection' against rain. Surprise, surprise a few months later they were back extending the roof to do the job properly! Wind and rain are a potent force in Wales!

The junction between roof and wall is one of the weakest in a house and so we must be very strict about how we protect walls from the elements. Extending the roof line may seem like a costly addition to any work on the external walls, but if you are doing work on them it may actually save you money if you get it done at the same time. Certainly it would have saved the people around the corner a pretty packet on their scaffolding costs!

If you look at houses on the continent, most have large roof overhangs. This helps to protect the building fabric, provides shading against the summer sun (and with climate change we might even get some of this at some point!), gives additional shelter for windows etc. Learning a lesson or two from the near continent is always a good idea.

Note: this post was first published in 2013. Check through our whole blog for other interesting articles.

Friday, 8 May 2015

Vapour Control Layers in Insulation

The pressure for energy saving in solid walled buildings is real and tangible. Green Deal and rising energy bills are encouraging us to insulate our walls. Where we have external features / facades that we want to keep we are forced to choose internal wall insulation (IWI).

BS5250 (the British Standard) for moisture control in buildings recommends a series of potential interventions (but crucially it also says that this is NOT a recommendation for breathable walls - which are virtually all the solid walls). One of the main recommendations (and hence common practice) is to install a Vapour Control Layer on the warm side of the insulation.

If there is no water in the wall then this works fine, but it is now becoming common knowledge that virtually all the UK's solid walls must be seen as 'moist'.

This is starting to create problems for the vast majority of the solid walls fitted with VCLs or non-breathable insulations due to Reverse Condensation (or Summer Condensation). This is a phenomenon that occurs when external temperatures and hence humidity levels are greater than the internal environment. Higher external pressures associated with temperatures forces this moisture laden air into the wall and when it cools and hits a water proof membrane or insulation the vapour condenses and runs down the back of the barrier. This concentration of moisture deep into the wall structure also means that joists and other wood is more at risk of damage.

The answer is to carefully read BS5250 and listen to what it says, which is, for breathable walls take professional advice from specialists. The best to provide this are bodies like National Trust, CADW, English Heritage and Historic Scotland. Thankfully all these sources of specialist advice are members of the STBA (as indeed RDE is) and their advice / case studies etc are available from

The best advice recommends using a fully breathable insulation (wood fibre for example) with a lime / clay plaster and a breathable paint finish (like our Earthborn Claypaints or Auro Lime Paint). 

So beware of advice and common working practices that are based on half-true information.