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Wednesday, 26 June 2013

Damp and lots of important numbers

At present the Standard BS 5250:2011 is used almost exclusively as the sole test of moisture risk for buildings. This Standard provides advice on the avoidance of internal surface and interstitial condensation caused by the movement of water vapour by diffusion through the building envelop from the interior to the exterior.

BS 5250:2011 refers to BS EN 13788:2002 as the means by which this risk should be calculated using a method commonly known as the Glaser Method which uses vapour pressure differential and temperature conditions to predict occurrences of condensation occurring either within or on the interior surface of building fabric.

So far it all sounds OK - lots of numbers and big scientific words; surely we can trust British Standards?


BS EN  13788:2002 is clear about its limitations and states "This standard deals with critical surface humidity and interstitial condensation, and does not cover other aspects of moisture, e.g. ground water, precipitation, built-in moisture and moisture convection, which can be considered in the design of a building component" (p. 3). And BS 5250:2011 is clear that designers need to also consider "the much greater risk of condensation occurring as a result of air leakage, which transports water vapour through gaps, joints and cracks in the building fabric" (p. 5) as well as the effects of exposure to sunlight, clear night skies, wind and driving rain, particularly in exposed positions subject to high wind speeds.

Whoops, so nothing in the standard about water as a liquid, faults in the walls, wind driven rain, cold nights, sunshine and rising damp!

Also BS 5250 & BS EN 13788 only assess the movement of water (as vapour) in one direction,
from the interior to the exterior, the use of this standard to assess retrofit solutions commonly advises the use of a vapour control layer in combination with insulation to limit and slow the quantity of vapour from the internal space that can be admitted into the building fabric. However, what about water getting in from the outside?

In Wales this is particularly important as it has been known for us to experience wind and rain at the same time. 

The answer is that water coing from outside to in, is just not taken into account. The vapour control layer helps to stop movement of water, but this means that it helps to trap water in the outer layer of the wall. This build up of moisture in the external side of a solid wall can cause a number of major problems like rot in joist ends. Not good!

Virtually all the well known Internal and External Wall Insulation products have been tested using BS 5250:2011. No wonder they all pass the tests and are 'recommended' for use on solid walled buildings. The whole testing regime is not fit for purpose. A major problem one thinks.

There is an alternative standard available to assess moisture risk in buildings, BS EN 15026:2007
Hygrothermal performance of building components and building elements. Assessment of  moisture transfer by numerical simulation. Unfortunately this is not widely used as there is too much vested interest in the status quo.

So, who cares if the products used in refurbishing solid walled buildings are inappropriate?

The companies making and supplying products made to BS 5250:2011? One thinks not.

Probably only you and the people who will live in the house in the future. But does the average person know the difference between BS 5250:2011 and BS EN 15026:2007 and how this applied to products????

The movement of moisture in a house is really important and we should be using the best tests that we can to ensure that we use appropriate materials, but unless tests and knowledge change we shall be left with using products that are 'recommended' and 'comply to British Standards' but that are, in the real world with water, not fit for purpose. This of course will not happen overnight nor without a fight from the big boys in the construction industry, so it is up to you to demand suitable materials both for the health of your home and your family.

Please share this and other posts with friends who live in solid wall homes.

Thanks to STBA Moisture Risk Paper -­ Dr Caroline Rye & Neil May -­ 3rd August 2012

Wednesday, 19 June 2013

Can refurbished solid walls reach 0.3 U?

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, even 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 breathable 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 virtaully 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.

Good luck!

Eco Home Centre blog - THE best Eco Home Blog

Well this was unexpected! Didn't even know that we had been nominated!

Thanks to the voters at Prime Location. Hope that you enjoy reading the content of the blog - there is quite a lot of it mind.

With membership of STBA I hope to be able to spread even more topical research data as it appears. Watch this space.

Wednesday, 12 June 2013

Roof overhang at gable end

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. 

Wednesday, 5 June 2013

We can't even get New Build right!

Just seen that the Low Carbon Hub has been tasked with trying to find out why new build does perform as well as expected.

Research has shown that new buildings that are designed to be low carbon etc, and hence assist with the targets for decreasing the UK's emissions, are up to 100% less efficient than designed.

For many of us in the refurbishment world this is hardly a surprise. The areas in question for New Build are:

Prediction tool
Is the energy assessment tool (eg SAP) sufficiently accurate?

Input data
Have human error or unrealistic conventions caused incorrect input data?

Is the design overly complex, presenting unreasonable challenges for the construction team?
Is the design information adequate in terms of detail?


Are there fundamental construction quality and skills issues?

Materials and manufacture
Do construction materials and building services perform in practice as laboratory tests predict?

Are substitutions of materials or equipment equivalent in performance?

Are post-construction tests and checks suitably accurate and appropriate?

If we are asking these fundamental questions regarding new build in the UK (and this is the type of building that architects, engineers, builders, project managers, surveyors etc, etc are all actually trained in), what chance does the existing stock have when are faced with these exact same issues with even less appropriate tools and no real training for the mainstream professional?

Monday, 3 June 2013

RDE joins STBA

The STBA are a growingly important organisation that has highlighted the need for sustainable refurbishment of older solid walled buildings. The STBA has representation from key organisations like Historic Scotland, CADW, English Heritage, SPAB, UCL, CIAT, ... and now, Rounded Developments Enterprises.

It has been an honour to have been approached to join the Steering Group of the STBA. Much of this is due to the advice we have been giving out in this blog. We feel that it is recognition of the great free advice that you get with the Eco Home Centre when looking at the humble Welsh terrace.

For more advice from the STBA and their research teams have a look at

For a condensed pdf information sheet on Moisture Risk Walls click on this STBA link

For Heat Loss Convention information click this STBA link

Lastly, for their Internal Wall Insulation document click this STBA link