HOW TO: Accurate Moisture Risk Assessments

Moisture risk assessment

Karsten Tube Test

One of the key challenges in undertaking an accurate moisture risk assessment is understanding the materials in the building. This is particularly hard when dealing with an existing or historic building. One technique that can be used to help this process is Karsten Tube testing, also called Rilem Tube testing.

The Karsten Tube test is essentially a way to absorb water into a material, usually masonry, in a controlled way such that it can be measured. The rate of absorption gives an indication of the ‘capillary action’ of a material, i.e. how quickly it redistributes liquid water from wetter zones to drier zones. To be pedantic, there are other mechanisms that move liquid water within materials, the main one being surface diffusion. Each tube consists of a small barrel, about 25mm diameter and 30mm long, open on one end; near the closed end a thinner tube intersects. The thinner tube is graduated to show volume of water. The barrel part is temporarily fixed to a wall (other configurations can be had for fixing to horizontal surfaces), with the thinner part vertical, and it is filled with water. Measurements are taken at timed intervals, so that a data set of volume against time can be obtained.

The tubes can be fixed to clean, sound material with white-tack. Pro-tip number 1: if the material is crumbly or just dusty, and it is difficult to form a good seal, consider painting a circle of nail polish onto the material to consolidate it and provide a clean surface to stick to. However, this may not be acceptable in historically sensitive cases.

This method is often used to estimate the properties of historic or existing masonry, but is not useful for open-structured materials such as mineral wool. This test is not without its shortcomings, many of which are discussed in the following article in Construction Specifier.

The emphasis is on the word ‘estimate’. This is a field test, on in-situ materials that are often inherently variable. This is a far cry from the controlled conditions of an analytical lab, and on top of this sample sizes are usually small. Therefore, any results need to be treated with appropriate caution. There is no standardised test method published, and steps should be taken to minimise variability on site. The key points of the procedure are described by our colleague Joseph Little in Historic Scotland Technical Paper 15

For example, a secondary benefit to using nail polish is that an accurate circle can be established (e.g. by drawing on first with a pencil), greatly improving confidence in the measurement of area over which water is absorbed.

This simple test that can be carried out on site can help in several ways. By plotting the results on a graph of volume of water absorbed against time, some qualitative information can be drawn out – “this type of brick is more absorbent than typical for the geographical location”; “there is a wide variation in absorption of the bricks across this building”.

Furthermore, the pattern of water absorption is often visible around the tube, and can give useful information. For example, one would usually expect the mortar to be more absorbent than the adjacent brick, particularly in historic buildings. Pro-tip 2: Not sure what type of mortar you’re dealing with? Drop a crumb of mortar in vinegar, lime mortar will fizz but cement mortar will not! In other cases, materials that are very nonhomogeneous, maybe hand-made brick or some types of stone, will exhibit very asymmetric patterns. Such nonhomogenety may well occur throughout the material and affect the resulting absorption properties.

These observations can help inform strategies for minimising risk when retrofitting, but can also play a role in dynamic hygrothermal simulations using tools such as WUFI. The results can be processed to estimate a material property called the alpha-value. Within WUFI, the alpha value can be used to estimate a moisture absorption function, a key set of parameters for simulation. However, given the uncertainties of the test, it is advisable to instead use the value to pick similar materials from the WUFI database, which have been measured much more scientifically. There is a paradox here in that the highly scientific measurements are representations of materials used in real buildings, which are dirty, inaccurate and non-homogenous in real life. However, the alpha-value is just one of several important parameters that govern the hygrothermal behaviour of a material, some of which are impractical to measure outside of a lab environment.

The key is to remember that this is a risk assessment, not an absolute prediction. Pro-tip number 3: It is good practise to adopt a method sometimes called ‘bracketing’, e.g. picking two materials that describe a range within which the real material is most likely to sit.

There are many pieces of information that can easily be obtained about an existing building which will inform a hygrothermal risk assessment. In all cases, context is critical; there are many cases where Karsten tube testing will not be appropriate. However, where it is applied, it can help gain an understanding of the materials present, as well as guiding more quantitative analysis.


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