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Dynamic insulation in through the pores

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letters

Dr Cawthorne's letter (aj 2.4.98) on contraflux dynamic insulation states that this system allows greatly increased rates of ventilation, by de- pressurising buildings internally and thus drawing outside air through a porous building envelope, transferring heat from that envelope to the air in the process.

A related principle has, I believe, been used on a sports centre in Callander, Stirlingshire, designed by Gaia Architects. Known as pore ventilation, this involves pressurising a cavity in the outer envelope in order to force outside air through a porous inner lining, again achieving the heat transfer from fabric to incoming air noted above. Air is then vented to outside using ducts. The advantage of this system, particularly in high humidity areas such as swimming pools, appear to be that ventilation rates are significantly reduced (from 10 down to a potential 2 air changes per hour).

Dynamic insulation often uses cellulose insulation (to bs5803 Part 3 1985), which is made up from recycled newspaper. It has been claimed (eg Jonathan Hines, aj 26.1.95, p51) that this insulation is hydroscopic, ie it can absorb surplus moisture at time of high humidity and without degradation of performance, the humidity to be released later. This property claimed for certain types of insulation is relevant to the current debate between proponents of insulated metal panel and built-up profiled metal cladding systems, to which Paul Hyett refers in his article on the same page as Dr Cawthorne's letter. This debate hinges to a large extent on whether foamed-plastic insulants perform better than those of mineral and glass wool, due to the alleged detrimental effects of small amounts of moisture on the latter.

TIM GOUGH

Austin Winkley & Associates,

London SE1

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