Assessing damage in strawbale walls

By Judith Thornton and Emma Appleton

A few weeks ago, Emma Appleton and I went to do some investigations at a strawbale house where the inhabitants had reported a musty smell. The concern was that the bales were damp and degrading and that this is what was causing the smell. Rotting walls are the worst nightmare of every owner of a strawbale building. Before we started we weren’t entirely sure how to do a damp survey, so this blogpost discusses what we did and what we learnt in case others need to carry out similar surveys.

We went armed with the ability to carry out 3 types of measurement:

  1. Surface moisture measurement using a conventional damp meter from Protimeter
  2. Drills and sample bags to collect clay plaster from walls, which we could then weigh, dry out, and reweigh in order to calculate the moisture content of the clay.
  3. Once a hole had been drilled through the clay plaster, direct monitoring of bale moisture content using a Protimeter balemaster. 

The first step was to work out if the conventional damp meter was telling us anything useful. In short, it wasn’t. These meters measure electrical conductivity, and if the main source of electrical conductivity in a material is water, then they provide useful information (e.g. the moisture content of a bit of timber), even if they’re not actually calibrated for the material in question. However, if there are other sources of electrical conductivity in a material (e.g. due to its chemical composition), then this can dwarf any change in conductivity resulting from moisture content. In the case of clay plaster it very quickly became apparent that we could totally change the reading simply by varying the pressure of the probe tips against the wall. Much has been written about the unreliability of these measurements in other types of damp survey (e.g. here), so it’s not too surprising that it didn’t tell us anything useful in this context.

Conventional damp meter that measures electrical conductivity

Next up was the collection of clay plaster. After a small scrape with a trowel to take off the wall glaze, we used a 10mm drill bit to drill through the depth of clay. Whilst doing this, we had a cone of paper taped to the wall, which guided the clay dust into a sample bag for further testing in the lab.

Drilling holes and collecting clay dust (left), collected samples bagged and ready for weighing (right).

Once we had a 10mm diameter hole in the wall, it was time to push the balemaster probe into the bales. We’d marked off particular depths on the probe with a marker pen so we knew how deep we were into the wall. The depths we chose to take recordings from were:

  • The junction between clay and straw bale
  • Inner region of bale
  • Centre region of bale
  • Outer region of bale
  • Junction between bale and outer surface

Bale probe in the wall. A reading of – – – means less than 5% moisture content.

In the lab

The clay sample bags were sealed, weighed, and then reweighed after a couple of days drying in an oven at 50 degrees to drive off the moisture. The moisture content of all the samples we took at the site were less than 2%; this is what we would expect for dry clay plaster. If the plaster has been applied very thickly (e.g. if wall straightening and strimming has not been carried out with sufficient attention to detail prior to application of the initial base coat), you can use this technique to assess whether the clay has dried out sufficiently and/or is causing the bales to become damp.

Have a hypothesis

Once we’d worked out how to do measurements, the next thing to establish was where to take measurements and why. Having been present for parts of the build process, we had some idea of potential sources of moisture ingress during the vulnerable period after the wall is up but before it has been rendered or clad. We therefore focussed on these areas. Your hypotheses will depend on what you know about the building; they might involve footings, blown away tarpaulins, leaky roofs, cracks in render, faulty flashings around windows and unexpected rain storms. Whilst you could do a survey using a random approach to sampling, you would find yourself needing to drill a lot more holes.

Our results

To everyone’s relief, the moisture contents of the bales in these walls (10 separate sampling points) was very similar to when they went in (under 15%). The readings were higher immediately adjacent to the lime render on the outside of one of the walls, but still within acceptable limits. On this particular building we may well return and repeat these measurements in 6 months time to see if the lime rendered wall needs to be monitored regularly.

How wet is too wet?

The commonly recommended maximum moisture content for strawbale walls is 20-25% as measured by a balemaster probe. However, ‘how wet is too wet’ is a surprisingly complicated question, and one that I will cover properly in a future blogpost. If you were to get a reading over 25% you would want to investigate it as a matter of urgency, focussing on the following:

  • Removing any source of moisture into the area
  • Drying the wall out (e.g. by applying heat to drive out moisture)
  • Ensuring sufficient air flow in the area

If the moisture problem had been occurring for more than a couple of months, you would then want to drill some cores through the plaster so that you could visually assess the straw for damage. It will be pretty obvious at this point if the straw has degraded significantly (it will be dark and smelly). If it has got to this point, then full bales may need removing and replacing. But it is perfectly possible for bale walls to survive transiently high moisture levels without degrading significantly.

Lessons to learn

Make sure that the moisture content of the bales is measured onsite before they are used in the building. Every strawbale builder knows the importance of starting with dry bales, so give your client peace of mind by measuring and recording moisture contents. This could also help with long term liability issues if problems arise due to maintenance issues. 

If you’re called on to do a damp survey for a strawbale house, bear the following in mind.

Find out as much as you can about the construction of the building and what sources of damp might have existed at any point. This will allow you to have some starting hypotheses about where to begin your investigations; you don’t want to be sampling every bale, but you do want to be certain that you haven’t missed any particular areas of concern. Don’t forget the common external sources of damp that exist in all buildings; insufficient drip details on window sills, poor guttering, plants growing against the wall.

Consider internal sources of moisture; bathroom and kitchen ventilation are vital and basic behavioural measures like shutting the bathroom and kitchen doors will allow moisture to be extracted directly rather than relying on the vapour permeability of the walls.

For an excellent summary of moisture in buildings, see this youtube video.

Conventional damp meters will not give you useful information about moisture content of clay or lime rendered walls. Balemaster probes work on the same principle, but are calibrated for straw and come supplied with a robust probe for jamming into bales.

Think about what probing at different depths might tell you; it would not be surprising if the outer part of a wall has a higher moisture content if it’s lime rendered. We sampled a ‘fairly sure this would be the driest point in the house’ point (behind a wood stove) to help us feel confident that we were measuring something meaningful. The previous week I had stuck the probe into a bale that we knew to be wet, for the same reason.

Probes should be sent back for calibration every couple of years.

Thoroughly document your findings including photos/plans of where you probed and having a proper labelling system, so that you could go back at a future point in time to see if the moisture contents in particular locations are better or worse. Make sure you record the indoor and outdoor temperatures and relative humidities in case you are in the mood for doing some calculations at a later date!

Final thoughts

Overall, the process was relatively non-invasive. A smear of plaster back over the holes we’d drilled in the walls, a bit of a vacuum to get rid of the dust, and we were done. Depending on how many samples you needed to do, you could expect the whole process to be done in a day, with a little more work on weighing once the clay samples were dry.

This project was carried out with support from BEACON. BEACON is funded through the European Regional Development Fund (ERDF) by the Welsh European Funding Office (WEFO).

In memory of Neil May, who understood more about moisture in buildings than most of us could ever hope to.

2 thoughts on “Assessing damage in strawbale walls”

    1. Good question, we don’t know! The smell was very apparent when the building was depressurised during an airtightness test, but the residents have only noticed it once since. We will continue to monitor the building periodically.

      I am of the view that ongoing monitoring moisture contents is important whenever we build with biodegradable building materials.


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