Why it’s important your home can ‘breathe’

By James Ayres, co-founder and operations director, Lime Green

Water is always around us. It’s in the air, in our houses, in the fabric of their structures and in the building materials we use. Too little or too much water can make a huge difference to the buildings we live in. High levels of water vapour in our buildings, regardless of the temperature, can result in a physically uncomfortable environment, enable fungal or mould growth and trigger respiratory conditions. Low levels can also lead to an uncomfortable environment and are often linked to sick building syndrome.

A conventional approach – involving plastic membranes and finishes – does little to control moisture in our homes and leaves people relying on manual ventilation ie opening windows, doors or trickle vents. In comparison, breathable materials can help buffer and control humidity and allow for a comfortable, natural internal climate to which we are well adapted.

A breathable building
From the moment a building is constructed, water exists as both a liquid and a gas (or vapour) in a building’s fabric – think how much water is used when you paint or plaster walls for example. Over time, there are other sources of water such as driving rain, leaks and, of course, water from moisture generated once the building is occupied.

Breathable materials are vapour-permeable, so they enable water, as a gas, to escape. But with natural insulations – such as wood fibre – breathable materials do much more than this. They can buffer moisture, holding it harmlessly in their micro-pores, which synthetic insulation and plasters lack. This ability to mop-up water, store it harmlessly then wick it away to dry later, makes breathable, natural insulation and plasters the obvious choice for healthy and durable buildings. In comparison, synthetic materials leave water on the fabric surface, leaving a ‘sweaty’ material which is prone to biological growth.

The case for breathability

In 2011, working with both the Society for the Protection of Ancient Buildings and ArchiMetrics, we installed four sensors into the south-facing, external wall of an 1820s terraced house to monitor the actual performance of our insulation materials.

Prior to this, very little research into insulation had been carried out and, the little that had, used guess work with computer simulations. But computers fail to consider real-world performance, over time, in varying weather conditions or to account for human behaviour.

The wall monitored in the research comprised of solid brick, coated internally with a 12mm parge coat of Lime Green Duro. 40mm of woodfibre insulation was applied onto the parge coat, then a 10mm coat of lime plaster – Lime Green Solo – was applied. This was finally coated with mineral paint to retain breathability.

The sensors were installed:

  • Near to the external face of the brickwork
  • Approximately 30mm into the internal face of the brickwork
  • Between the parge coat and the woodfibre board
  • Beneath the internal lime plaster, at the interface with the woodfibre board
  • On the external wall to gather weather data.

For six years, we then monitored the sensors’ data, logging a range of figures every five minutes including saturation margin, temperature, relative humidity and the amount of sunshine.

The results

The data shows clear trends in how the wall and its insulation behave.

On average, installing the insulation internally, leaves the wall cooler as less heat leaks through from the inside of the building. This might suggest the wall would be damper as a result, however, the relative humidity actually dropped, throughout the wall, each year.

There are strong seasonal variations in the wall’s relative humidity.

Lower temperatures and increased rainfall cause the wall to accumulate moisture in winter whereas the spring and summer months see a strong, drying trend. Importantly, this warmer, drying trend outweighs the winter moisture absorption and the breathable fabric ensures water does not become trapped against an impermeable layer.

The saturation margin measures, in degrees, how much warmer the wall is than the temperature at which water gas condenses into liquid. The wall consistently records a good saturation margin – in most conditions, the margin is more than 4°C warmer than needed to stay dry.

In the six years of monitoring the wall, not once did any condensation occur in either the insulation zone or the inner wall. The only liquid water appears to have come from rain soaking into the outer part of the wall.

For more information on the details of the project visit: www.lime-green.co.uk/knowledgebase/the_importance_of_breathability or to find out more about the products used, visit: www.lime-green.co.uk or call 01952 728 611.