Moisture risk assessments are extremely useful and, in many cases, required when designing and constructing a new building or when retrofitting an existing one. In the former case it is likely that there will be available datasheets or material information for the materials that are proposed to be incorporated into each element of the building. However, it’s a completely different story when looking at a masonry wall retrofit; I would be extremely surprised if a client came to us proposing internal wall insulation on a 19th century solid brick dwelling and handed me a copy of a datasheet or even had a manufacturer name for the existing masonry. It just doesn’t happen as it is not something that was closely monitored or controlled at the time that this building in question was constructed. This poses a challenge when trying to undertake a moisture risk assessment for retrofit projects.

One of the key pieces of information that is required and that can have a huge impact on the results of a moisture risk assessment are the exact material properties that are present in the build-up or detail of interest. Without knowing these, assumptions and educated guesses must be made which can introduce some uncertainty in the results. Think of it like making soup but where you are only given half of the ingredients; those of you who have made soup before will have a rough idea of what ingredients could go into the mixture to make it passable but at the end of your cooking session you could end up with a completely different soup to the one originally intended due to the absence of this vital information. The same is true of a moisture risk assessment; if the exact properties of the masonry are unknown, the assessor can look at photographs and descriptions of the masonry that a client provides, but there will still be a great amount of uncertainty when selecting representative materials to include in the model for the risk assessment. This could lead to the risk assessment indicating that the proposed build-up is high risk when in reality it could be perfectly fine.

Knowing this, it would not be unreasonable for you to question whether there is any value in these assessments at all for retrofit projects. I promise you, there really is. To quote George Box, ‘All models are wrong, but some models are useful.’ Knowing this, there are a couple of methods that assessors use to reduce this uncertainty and so make the assessments useful and as accurate as possible:

The first of these is known as bracketing which is a process whereby a few different materials are chosen to represent the unknown masonry in an effort to encompass the range of possible outcomes. Usually, the specific materials are chosen based on available information such as the building location, photographs and descriptions of the masonry. This can often be used as a sensitivity check too – i.e. it helps us to understand the impact that the certain masonry properties can have on a particular build-up.

The second method and one I will focus on here is material testing. The first testing method is known as Karsten tube testing – this is where the absorptivity of existing masonry is assessed on site using small tubes of water. The rate at which the masonry absorbs this water is measured and an absorption coefficient is calculated. The locations of the tubes can be varied to look at the absorptivity of a range of bricks/stones throughout the building as well as the mortar. As the absorption coefficient is one of the key properties to know in order to understand how the material behaves in the presence of liquid moisture, this gives a nice steer on which database materials to choose for the assessment. 

The second method is more in depth and involves taking core samples from the masonry. This can obviously be more difficult to do if the building has some historical value, but we have done this type of testing on a number of listed buildings in the past and it has proved invaluable to the assessment. When we do this work, we take 50mm core samples from multiple locations in the masonry. These samples are then brought back to our small in-house laboratory and tested to determine some of the key properties including the absorption coefficient, density, capillary saturation (this is the amount of moisture the sample will absorb after immersion for 24 hours), reference water content (this is the amount of moisture the sample will take on at 80% relative humidity) and sorption curves (which look at the relationship between relative humidity and water content for the specific material). Knowing these key properties allows the assessor to select the most appropriate and representative material for the assessment (or indeed create their own material file), making the results much more accurate and reliable.

Undertaking material testing for past projects has really highlighted the diversity of masonry materials – what you see is often really not what you get. Let me give you an example – we worked on a project which involved a large dwelling located in Somerset. We took samples from both the original Georgian section of the building and from a later Victorian addition. The Georgian brick was very soft and quite crumbly, whilst the Victorian brick was hard and well formed. You would be forgiven for assuming as we did that the softer, crumbly Georgian brick would be much more absorptive than the hard Victorian brick. However, this was not the case. In fact, the opposite was true – the Victorian brick was like a sponge, it absorbed the water at a much faster rate than the Georgian brick. This has a knock-on effect that the recommended insulation strategy for the building was different to that originally proposed. This particular project really emphasised the importance of better understanding the existing materials when doing a moisture risk assessment or any retrofit/refurbishment works. That’s why wherever possible, we will recommend that the latter material testing is undertaken prior to a moisture risk assessment, because understanding the material properties is key for this type of risk assessment.

If you would like any more information on material testing, please visit our website or get in touch. We would be happy to talk more about this with you.

Written by Olivia de Sousa Costa

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