Abstract

Unfired clay brick, wood, and cellular concrete have been evaluated as relative humidity (RH) buffers for indoor spaces. Their response to a cyclic variation of RH has been measured and reduced to a figure of merit, the `buf' with symbol B. This is defined as the quantity of water exchanged through unit area of surface expressed as the volume of space which will experience the same change in amount of water vapour when exposed to the same relative humidity (RH) cycle. This number is approximately equal to the number of air changes needed to exhaust the buffer moisture reserve in a typical room. Unfired perforated brick has a buffer capacity of 27 for a daily RH cycle, providing significant resistance to RH change caused by an air exchange rate of once per hour. Wood cut across the grain was next with a value of 15, just ahead of massive unfired brick at 10. Cellular concrete was an unimpressive buffer at 7 but worst of all was fired perforated brick with a buffer value of 3 even for a long RH cycle.

However, even the perforated unfired brick reacted slowly to changing RH, having a buffer capacity nearly doubling from a one-day to a four-day humidity cycle then doubling again for a very long cycle. Even for the long cycle, the performance was worse than predicted from sorption measurements made on powdered samples of the brick. If deeper layers of the wall are brought into use by convecting, or forcing, air through channels, the buffer capacity can be considerably increased. A wall, 53 mm thick, of unfired perforated brick with the channels arranged parallel to the surface and ventilated mechanically, has a buffer value of 61. The steepness of the water vapour sorption curve is therefore only an approximate, and optimistic, indicator of moisture buffer performance. For practically useful performance, a wall needs to have a moisture-active surface considerably larger than its area facing the room.