When it comes to old buildings and making them energy efficient there’s one very important point to understand: old buildings work in a different way to modern buildings.
Old solid wall buildings – whether medieval timber-framed houses or Edwardian terraces – are designed to allow a degree of moisture penetration into their structure. But, and this is the important bit, they remain in equilibrium because of the ‘breathable’ nature of the materials they were built with, the open fires that were burnt within them and the draughts that entered through gaps in the fabric. Modern buildings are the exact opposite, they rely on keeping water out through impermeable surfaces and membranes.
So far so good, but it’s also vital to understand that breathability and air leakage must not be confused – they’re two very different things.
In its strictest sense, breathability is the water vapour transmission rate, or the speed at which vapour passes through a particular material or construction. In a practical application, breathability is a combination of three important properties: vapour-permeability, hygroscopicity and capillarity. If waterproof membranes and inappropriate materials, such as cement based products, sealants and modern ‘plastic’ paints, are employed in an old building it’s similar to what happens when we wear a plastic mac: we get very damp and sweaty, and we feel uncomfortable and unwell, because the moisture from our bodies cannot escape. In an old building this trapped moisture will inevitably lead to rot and decay.
Air leakage or ‘air permeability’ is NOT breathability. It’s the uncontrolled movement of air (draughts) through joints and gaps in a building’s fabric and can be a significant source of heat loss.
When we’re improving the energy performance of any building we need to make the building envelope as airtight as possible. At the same time we need to remember that the more airtight we make the building, the more important it is to ensure there’s good controlled ventilation to avoid condensation, mould growth and ill health.
Ventilation generally reduces relative humidity (RH) – the amount of water vapour in the air relative to the maximum amount that can exist at that temperature – by swapping internal damp-laden air for fresh air from outside. This assumes that the external RH is lower than the internal RH – this is not always the case on a damp, humid day. In this instance, a powerful extractor fan will do more good than opening a window. Ideally RH should be between 40 and 60 per cent for good internal air quality. Once RH is above 73 per cent, mould and dust mites will thrive and pose a serious threat to human health. Sharp fluctuations in humidity can also be damaging, leading to dimensional changes in the structure – particularly joinery – and shakes opening up in timber.
What we need to achieve is a breathable old building which is as airtight as possible and is equipped with controlled ventilation.