The role of absorbent materials in moderating changes of relative humidity
A Ph.D thesis by Tim Padfield
Department of Structural Engineering and Materials, The Technical University of Denmark
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The moderating influence of absorbent materials on the relative humidity in small enclosures has been known, and written about, for ages. The extension of the concept to moderating the relative humidity of large, leaky enclosures like houses has been unaccountably neglected. The influence of the material is diminished by ventilation but a useful contribution remains. In buildings such as museums, where total control of the indoor climate is considered necessary, absorbent materials can be combined with mechanical air conditioning to provide a simpler and more stable control system.
Nowadays we coat most indoor surfaces with almost impermeable paints and plastics. Even the porous materials we build with are not very water absorbent at moderate RH, or react very slowly. Architects and engineers have divided the task of making a habitable building, so it is not surprising that engineers have concentrated on designing walls as bearing structures and screens against the outdoor climate and have completely failed to see how a wall can contribute to the pleasantness of the indoor climate.
This evolution in building tradition has not only hidden the potential for humidity buffering that lies in common materials, but has also generated an array of condensation problems, not only within buildings but in the structure of walls and roofs.
I became aware of this by chance, when I was called in to investigate a particularly dramatic example of water apparently being spontaneously generated by a building. Later, as happens when one completes one job successfully, other examples of sick buildings came my way. Some of these investigations have been published but they are reworked here to form a coherent study of the role of absorbent materials in the regulation of humidity in the indoor air as well as in the structure of walls and roofs.
New experimental data have been collected to quantify the water exchange performance of various building materials. These data come from a unique climate chamber which controls the water vapour flux into a space, instead of the relative humidity in the space, which is the parameter controlled by ordinary climate chambers. This chamber allows direct measurement of the influence of porous, absorbent materials on the indoor climate, whereas the standard climate chamber only allows direct measurement of the effect of the indoor climate on the material.
An assortment of common and unusual building materials has been tested for efficiency in buffering the indoor relative humidity: brick, cellular concrete, wood, earth, lime mortar, gypsum plaster and wool insulation. The best buffer performance among the materials tested is given by wood arranged with the longitudinal direction perpendicular to the exposed surface. The best inorganic material is a specially developed mixture of bentonite, a montmorillonite type clay, with perlite, an expanded volcanic glass. The best standard building material is cellular concrete.
Such materials, used as walls within buildings, will moderate the indoor relative humidity over a period which depends on the air exchange rate. This humidity buffering effect is particularly useful in kitchens and bathrooms where intermittent production of steam is effectively absorbed, to be released later. Effective buffering of the daily variation of relative humidity can be achieved with a layer of buffer material about 2 cm thick on walls and ceiling. A thicker layer does not help, because water vapour diffuses relatively slowly within the material.
In buildings with a much lower air exchange rate, such as archives and stores, the buffering by absorbent walls is so effective that it evens out the annual cycle of relative humidity, without needing help from mechanical air conditioning. Buildings with an air change of about 0.1 per hour will benefit from a thicker layer of buffer: up to 40 cm thick.
In buildings which require both a moderate air exchange rate and a stable relative humidity a symbiotic design, combining mechanical air conditioning with buffering by building material, allows air conditioning of a museum store, for example, using only a small dehumidifier.
Outer walls of buildings which are porous on both sides should theoretically pass water vapour through according to the relative humidity difference across the wall. This means that such a building when warmed in winter will have a higher relative humidity than that calculated from the water content of the outside air, raised to the inside temperature. This is a controversial assertion, which is here supported by indirect evidence from measurements of the microclimate in churches. This matter is presented as a conjecture that deserves further investigation.
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