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Don't confuse your insulation concepts

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In your recent article 'What is sustainability' (AJ 5.2.98), concerning the design of a sustainable school at Great Notley in Essex, you state:

'The design proposes the use of breathing walls, which both do away with the need for an environmentally unfriendly membrane and have important implications for the ventilation of the building.'

Since 1991 I have been closely involved in the research and development of heat and mass transfer through porous insulation in building fabrics, particularly 'breathing walls'. This term is normally used to describe a timber frame envelope containing a porous insulant (often cellulose fibre) sheathed on both sides of the studwork with fibreboards of differing vapour permeabilities. It is intended to allow the regulated flow of moisture from the inside to the outside of the envelope under a vapour pressure gradient with a reduced risk of condensation.

Extensive monitoring, laboratory testing and computer simulation in government-funded and commercial research have all shown that in practice the amount of both water vapour and bulk air transfer through this kind of envelope (as commonly constructed) is negligible, the majority of water vapour and air being transferred through normal air infiltration and ventilation. While breathing walls as I have described them here often provide a well-insulated and perhaps an environmentally benign construction method, they do not contribute significantly to the ventilation of buildings.

They should, however, be seen as completely separate from another method of heat and mass transfer through building envelopes called contraflux dynamic insulation, which does have significant implications for the ventilation of buildings.

In this concept, the interior of a building is slightly de-pressurised and air is drawn through an air-permeable insulant in order to transfer heat that would normally be lost by conduction through the insulant to the incoming ventilation air. The heat energy gathered in this way may then be reclaimed by a heat exchanger operating on the outgoing air passing through an exhaust flue.

With this method, greatly increased rates of ventilation are possible without occupant discomfort while reducing the conducted heat loss through the building fabric with minimal condensation risk.

Both breathing walls and dynamic insulation may use timber frame and cellulose insulated envelopes, and it is perhaps this that has led to some confusion. I very much hope that this fascinating area of envelope design continues to be explored, but to assist in doing so it would be helpful for the architecture profession to differentiate between the fundamentally different concepts and adopt a consistent nomenclature for them.

DR DOUGLAS A CAWTHORNE Barrow upon Soar Leicestershire

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