The blog

Roger Hunt is an award winning writer and blogger specialising in sustainability, old houses, housebuilding and traditional and modern building materials. He is the co-author of Old House Handbook and the companion volume Old House Eco Handbook.

Breathability, airtightness, ventilation

Breathability, airtightness, ventilation

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.

Breathability

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

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

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.

The goal

What we need to achieve is a breathable old building which is as airtight as possible and is equipped with controlled ventilation.

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SPAB Working Party

For the last 25 years conservation experts and volunteer heritage enthusiasts have come together to join the annual Working Party run by the Society for the Protection of Ancient Buildings (SPAB). I went along to join them and created a video about the Working Party at Sullington Manor Farm near Storrington, West Sussex. They were working…

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Fire in old buildings

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SPAB Working Party

For the last 25 years conservation experts and volunteer heritage enthusiasts have come together to join the annual Working Party run by the Society for the Protection of Ancient Buildings (SPAB). I went along to join them and created a video about the Working Party at Sullington Manor Farm near Storrington, West Sussex. They were working…

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Every year, many of the estimated 450,000 listed buildings in the UK change hands on the property market. In England and Wales these properties are designated Grade I, Grade II* or Grade II having being deemed to be of historical, cultural or architectural interest. All buildings built before 1700 Tweet

Environmental Pocketbook

Environmental Pocketbook

If you’re going to invest in just one book on sustainable, low carbon building I’d strongly suggest that you make it The Environmental Design Pocketbook. Now in its second edition, this useful volume by Sofie Pelsmakers should be essential reading for architects, designers, developers, planners, students, clients and anyone else involved in the construction and operation of buildings….

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The devastating fire at the Grade I listed, 18th century National Trust mansion at Clandon Park, Surrey, once again highlights the need to do everything we can to protect old buildings. Whatever the size of the building, there are simple measures that can be taken to reduce the risk of fire, ensure early warning of a…

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The need for fresh air and light in buildings is something I’m often talking and writing about because it’s central to creating a good home, but the theme is nothing new. I was reminded of this when I recently visited the King Edward VII Estate, near Midhurst, West Sussex. Here, the former sanatorium is being…

Building lime knowledge

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Lime, in the form of mortars, renders, plasters and paints, is a key component of old buildings and essential to their repair – or at least it should be. Today lime-based materials are also emerging into the mainstream and being used within low carbon construction systems, employed in everything from homes to superstores. All this…

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A blocked drain is not a pleasant thing to wake up to. What’s worse is the realisation that it’s something that can generally be avoided by doing what I’m always talking about: maintenance. The drainage system is easily forgotten because much of it is hidden away underground but, as with any element of a building, it…

Building remembrance

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Visiting Blood Swept Lands and Seas of Red, the poppy installation at the Tower of London, reminded me that the built environment frequently plays an important part in both remembrance and memory. Each of the 888,246 ceramic poppies that flood the moat of the Tower depicts a death in the British forces in the First…

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