Introduction

Hygroscopic materials are able to influence the relative humidity (RH) of the space which contains them. As the air change rate in the space diminishes towards zero, the RH becomes entirely determined by the water content of the materials, according to the sorption curve of the material. This is the basis of the long established tradition of buffering showcases and boxes containing art in transit. This phenomenon can be exploited also in archives, which are stuffed with hygroscopic paper and which do not need a rapid air exchange because people do not work there - only entering to retrieve and to replace documents. There are several examples of archives whose stable RH is entirely due to buffering by the objects they contain, aided by massive walls and low air exchange. These are reviewed by [Padfield 2007 and Ryhl-Svendsen 2009]. One can say that archives can be satisfactorily operated without air conditioning in the temperate climate zone, though strict adherence to archival standards, for example British Standard 5454-2000, will force air conditioning. The need for constancy of temperature is questioned by [Morten Ryhl-Svendsen 2009].

There are some archives which do not contain much hygroscopic material exposed to the room air. Film archives are a good example, with rows of steel cans isolating the roll of film from the room air. This is a good thing in that it reduces the rate of change of the water content of the film with variation in the room RH, but it is a wise precaution to incorporate a humidity buffer into the archive construction and furniture.

There are currently no standard building materials explicitly developed to moderate indoor relative humidity. However, unfired clay brick is an intermediate industrial product which performs well as a humidity buffer and has a millenia long history of use in building. Before developing a standard theory of buffering for whole interiors, we describe the performance of brick so that the reader has a feeling for the magnitude of the buffer capacity available for moderating RH in spaces with significant air exchange.

Various absorbent materials have been exposed to a fluctuating relative humidity in a reaction chamber (figure xx). The water exchange is measured by keeping track of the water evaporated into the chamber and re-condensed. The chamber is described by [Padfield 1998.2002]. The response of several materials to this RH fluctuation is shown in figure xx and is tabulated in figure xx after normalisation to a square metre of active wall area.

Architects need some standardised figure to represent the usefulness of the materials as buffers. This cannot be a property of the material because it depends on its form: the limiting factor in speed of response is the surface area exposed, so the same material in a labyrinthine format will perform better, or at least faster. The buffer capacity increases at lower temperature, because the water vapour content in the space is lower and less water movement is needed to control its RH.

A unit for buffer capacity

We suggest that the equivalent air volume per square metre of active surface is a good measure of performance, encapsulated in a single number. This is explained in figure xx. A horizontal square metre of active surface is the base for a column of air (strictly speaking a column of space). When the space is subjected to a sudden change of RH, from 50% to 60% for example, The surface will absorb this amount of water vapour. In practice of course, the system will settle at an intermediate RH, but for this formal definition, we define it as the amount of water vapour absorbed, expressed as the equivalent change in water vapour content of this column of air.

This value will change with temperature. It will approximately double with every 10C drop in temperature. The buffer capacity of this material in a cold store will be large.

From figure xx one can deduce a buffer value for unfired brick at 18C of 40, meaning that the change of water content on changing the surface RH from 50% to 60% is the same as adding the same amount of water ot 40 M^3 of space. This number is temperature dependent and it is also time dependent. The value given is the maximum attained when one allows a very long time for equilibration. If the RH is varying cyclicly, as it often will in practice, the performance is limited by the diffusion rate of water vapour into the bulk of the material. This is illustrated in figure xx, where the buffering process is shown at 24 hour, 96 hour and infinite cycle times. The buffer values are 24, 32 and 50 metres respectively.

As the air exchange rate increases to rates suitable for crowds (10 l/s/person), moisture flow from the buffer material becomes diluted by the flow of outside air so the climate becomes less stable. In this article we explore this competition between the injection of outside air and the response of hygroscopic materials within the museum space.

The average RH can be fixed in various ways, depending on the local climate. In warm regions the indoor air can be dehumidified by an underdimensioned dehumidifier operating continuously, relying for constancy of control on the buffer capacity of the hygroscopic materials. In cool regions, heating a few degrees above the running average ambient temperature will reduce the indoor RH from outdoor value which is too high almost everywhere. In rare favoured regions, one need do nothing to change the average RH.

In this article we explore the potential of passive humidity buffering of museum galleries, and ordinary homes. The problem is that the air exchange rate now needs to satisfy the demands of humanity, by removing the carbon dioxide wet exhale. This air exchange rapidly exhausts the humidity buffer capacity of the surrounding walls and furniture. One can only hope to buffer the daily cycle of indoor RH; the annual cycle must be controlled by other means.

The fundamental limitation to RH buffering is the slow diffusion of water vapour through porous materials. Compared to heat movement, which most theorists regard as analogous to vapour transfer, the process is about twnety times slower. A 60 cm brick wall will effectively buffer the daily temperature cycle, so that the indoor surface will remain at almost a constant temperature. For humidity buffering, the active zone is only a few mm.

The active zone can be made thicker by introducing labyrinthine constructions which increase the surface area exposed to the room air. In this article we explore materials and constructions which hold the promise to provide high capacity humidity buffering for a room with an air exchange rate suitable for human habitation, which is about twice per hour. We examine both massive constructions and veneers of active material on pre-existing walls.

Our emphasis is on practical solutions using easily manufactured components, rather than specialised products unlikely to be feasible for a large building project.