Discussion

The revelation of the good moisture absorption of unfired clay brick will not come as a suprise to the large fraction of mankind which lives in earthen houses, but this article will serve as a reminder to engineers and architects in rich nations of the hidden merit of a cheap and unfashionable building material.

Since quantification of physical properties is required for the approval of building materials in technically regulated societies we offer a simple measure of the buffer potential of a wall surface exposed within a room: the volume of space which has the same water exchange capacity as unit area of the surface under test. This measure of performance is simple and independent of the amplitude of RH variation (within the limits of the normal indoor environment), but it is considerably different from the two already established tests discussed by Roels and Jannsen \cite{roels-jannsen2006}. These two tests are empirical protocols designed to suit simple climate chambers and the customary working hours of their operators. The `Nordtest' protocol specifies 8 hours at 75\% RH followed by 16 hours at 33\% RH. The RH values were surely decided by the availability of cheap saturated salt solutions to control them. The Japanese test `JIS A 1470-1 (2002)' has a more elaborate protocol involving a 24 hour exposure to each fixed RH, abandoning the natural daily cycle in the interest of the operators' convenience. Both these tests define the performance as the weight of water exchanged per unit area per percent RH change, a number which has no immediately understandable significance, though it allows comparison between materials.

Compared with the Nordtest method, the B-value method is similar in its emphasis on performance per unit of surface area, disregarding the materials, the thickness or the intricacy of the wall behind. The B-value is less stringent about experimental conditions. It allows any reasonable cyclic RH variation within the linear range for sorption, which is 10\% to 70\%. We have not checked that an unsymmetrical square wave, as used in Nordtest, gives a similar result to the sine cycle we have used. One would expect a larger B-value from a symmetrical square wave but the shortness of the high RH exposure (75\%) of the Nordtest protocol gives less time for water exchange. However, 75\% RH is close to the inflexion point where materials begin to absorb strongly. A more robust protocol would surely use a lower value for the upper RH. At very high RH, as occurs in kitchens and bathrooms, the humidity is strongly buffered even by materials which perform poorly at moderate RH. The B-value gives a conservative rating for wall performance.

The B-value allows a choice of cycle time appropriate to the job in hand. An obvious use for quick acting buffers is in kitchens and bathrooms, which are unlikely to be in use for eight hours. A much shorter cycle time is appropriate for evaluating concepts for low energy kitchen and bathroom design, directed at eliminating mechanical ventilation. At the other extreme, an annual cycle is appropriate for developing solutions for archive climate control, though it will occupy the experimental apparatus for some time. Finally, we claim that the B-value offers a more intuitive way of presenting the performance of buffer surfaces. The B-value can easily be recalculated to an approximate equivalent to the grams per square metre per percent RH which Nordtest provides, given that a daily cycle is measured.

The few materials we have measured are the commonest immediately available, but they are not the only promising candidates. Cement bonded earth bricks, usually made on site, should perform well, since the cement gel has a very high water vapour sorption. Hemcrete is a lime-sand mixture filled with hemp (\textit{Cannabis sativa}) fragments left over from processing for fibre. This material also is usually mixed and applied immediately on site. Among wood-based materials found in building suppliers there are cement bonded wood chip boards and compressed wood chip boards with similar properties to plywood but with more cut fibres exposed at the surface. We would not expect these to perform better than end grain wood.

An immediate use for buffer materials is in bathrooms subjected to intermittent bursts of water vapour which are currently exhausted through ducts by fans. A test in an unfired brick house built by Tom Morton \cite{morton2005} showed that walls of unfired brick worked as well as mechanical ventilation in reducing condensation. Bedrooms are another target for humidity buffering, since asthmatics sensitive to dust mites need to keep the RH below 60\%.

When one considers the implications of temperature change on a moisture buffering wall the possibilities of humidity control expand in interesting ways. One can envisage a use for perforated unfired bricks in a solar powered dehumidifier in which outside air is passed through sun warmed bricks during the day, to remove moisture. At dusk the air flow is cut and the bricks are allowed to cool. At night, the air flow is switched to pass room air through the bricks, which maintain the low equilibrium RH they achieved during the daytime flush of warm air and therefore dehumidify the room air.