The importance of breathability – real-world data bylined by James Ayres, co-founder and operations director, Lime Green @lime_products

James Ayres, co-founder and operations director at Lime Green – UK-based manufacturer of healthy materials for beautiful buildings – cites pioneering data evidencing the importance of breathability for buildings.

Working with both the Society for the Protection of Ancient Buildings (SPAB) and ArchiMetrics, we installed four sensors into the south-facing, external wall of a two-bed, 1820s, Victorian home. These sensors monitored the performance of our breathable natural insulation materials, recording data every five minutes for over six years.

Most of the existing research into insulation performance is carried out using computer models, but this doesn’t factor in real-world effects such as weather and humidity variations. As the research is a world-first, Historic England was very interested in the results and part-funded the project.

The need for real-world data:

There are a number of basic, unrealistic assumptions made by computer model which creates this fundamental need for real-world data. These models assume:

  • Any insulation is perfectly installed

When any insulation material or Vapour Control Layers (VCL) are installed there will always be some gaps which lead to vapour permeation.

  • Humidity only flows from the internal environment to the external environment

In certain weather conditions, the opposite will be true.

  • Temperature of external-facing walls will be equal to the temperature of the external air

In some conditions, such as direct sunlight, the temperature of an external wall will be much higher than the external air temperature. This can cause moisture to be ‘pumped’ in the opposite direction to the one predicted.

  • Water is ignored

A lot of the moisture found within a wall will come from driving rain or leaks and not from condensation. Computer models assume all moisture is condensation.


A breathable insulation system

Unlike a typical insulation system, which consists of plasterboard fixed to foam insulation, we installed woodfibre onto a Lime Green Duro parge coat. We then plastered internally with a permeable plaster, Lime Green Solo, and applied a coat of mineral paint to retain breathability.

The four sensors were placed near to the external (S4) and internal (S3) faces of the brick wall, onto the parge coat (S2) and at the plaster’s interface with the woodfibre (S1).

The following data shows the performance of the wall over the most recent period (Nov 2017 – Jan 2019).


Saturation margin

Saturation margin (01 Nov 2017 – 30 Jan 2019)

The saturation margin indicates how much colder an area would need to be before condensation would occur. A figure below 0°C indicates there is condensation.

S1, S2 and S3 all showed no evidence of condensation. S1 is influenced by internal humidity, meaning it buffers excess moisture before releasing it later. The evidence of no condensation at S2 is a crucial finding as the computer model had predicted this would be a high-risk zone for condensation.

The saturation margins at S2 and S3 are shown to be stable with slow variations indicating that any moisture present here will take longer to escape.

The saturation margin data for S4 did indicate moisture presence at various points throughout the period. This is likely to be caused by penetrating rainwater during periods of wet weather.


Relative humidity

Relative humidity (01 Nov 2017 – 30 Jan 2019)

Relative humidity represents the amount of moisture present within the air as a percentage of how much it can hold before saturation occurs.

As mould has the potential to occur around 80% humidity and higher, a temperature increase would see the relative humidity level drop and mould risk reduced.

S1 often showed a relative humidity which is less than that of the internal environment, demonstrating how rapid changes in the internal air are buffered by the lime plaster.

Again, S2 and S3 both remained stable. Due to their position within the wall and distance from both the internal and external environments, any vapour movement will be incremental.

S4 varied a lot throughout the period as it’s heavily influenced by the external weather.


This research proves that breathability works, as it shows how natural materials enable vapour to escape a building’s fabric, preventing the build-up of mould or condensation.

Across the six year period monitored, the wall steadily dried out and no condensation was detected. With this result, we’ve shown that insulation, be it for new builds or historic retrofits, requires careful thought which considers many factors which are ignored by many computer programmes.

For more information how breathable materials could work for your project, visit or call 01952 728611.

%d bloggers like this: