In this paper different scenarios for back protection of a canvas painting and their effect on the stability of the relative humidity behind the painting are tested. A painting on canvas, stretched on a wooden frame, was fitted with various styles of back protection and then exposed to a cycle of temperature variation at the back, with the front exposed to a constant room temperature. The painting was also exposed to a constant wall temperature and varying room temperature. The space between the canvas and the back board was fitted with temperature and relative humidity (RH) sensors. The sensors were used to provide the essential single-point data of temperature and RH at the given locations. For more comprehensive understanding of the rather confined space, further numerical simulation (computational fluid dynamics) was adopted as part of the investigation. The computational fluid dynamics was used to understand the natural convection within the microclimate through the depictions of temperature distribution, as well as the corresponding airflow. The unprotected painting suffered a large RH variation at its back, because of the varying canvas temperature interacting with the constant room air moisture content. Effective stabilisation of the RH behind the canvas against temperature variation was provided by a shiny aluminium alloy sheet sealed against the frame. The non-absorbent back board experienced a strong variation in RH, because of humidity buffering of the space by the painting canvas at a different temperature. Either a space or insulation between this back plate and the wall reduced the risk of condensation on the inner surface of the back plate. Insulation will however increase the risk of condensation on the wall surface behind the painting. An absorbent back board de-stabilised the RH at the painting canvas surface by providing a competing humidity buffer at a different temperature. To provide protection against moisture exchange with an unsuitable room RH, extra humidity buffer was placed 3 mm behind the painting canvas, kept close to the painting temperature by insulation between this buffer and the back board. This stabilised RH at the canvas surface but increased both the temperature and the RH variation at the back board and thus increased the risk of condensation on the inner surface of the back board. The RH and the temperature in the narrow spaces between the painting canvas and the wooden stretcher frame were always more nearly constant than in the open canvas area, which suggests an explanation for the widely observed better condition of the areas of canvas paintings which lie close over the support structure. Our conclusion is that a non-absorbent, impermeable back plate gives good RH stability against a changing temperature gradient between wall and canvas painting surface.

The need to keep things cool for durability should be the single most powerful influence on storage design. The simplest temperature control is to moderate the outside temperature by a combination of thermal insulation and heat capacity. The low energy storage building is a lightweight, thermally insulated, airtight building put on top of an uninsulated floor slab laid directly on the ground. The thermal insulation is calculated to even out the daily temperature cycle but to allow an annual temperature cycle which is about half the amplitude, but much smoother, than the annual temperature cycle outside. The winter temperature inside will nearly always be above ambient and so will maintain a moderate RH without need for either humidification or dehumidification. The temperature inside in summer will be below ambient and thus will force dehumidification of the infiltrating air. However, the airtightness of the building allows intermittent dehumidification with low energy consumption, less than one kWh/m3 per year. There now exist enough buildings designed on this principle to reassure curators that highly valued collections can be stored in a space with a gentle temperature cycle and with an RH stability as good as air-conditioning usually achieves.

The chemical deterioration rate of many materials is defined by quantitative theories developed in the late nineteenth century, which explained through molecular kinetics the observed exponential increase in chemical reaction rate with rising temperature.This need to keep things cool for durability is the single most powerful influence on storage design, and has long been practiced, particularly in movie film archives. The other major influence on durability is the chemical activity of water vapour, called the relative humidity (RH) in the conservation literature. Its potency is described by the law of mass action, which for many cases gives a rate of hydrolytic reaction proportional to the RH. Therefore, only a very low RH affords significant protection. Recent advances in building techniques, particularly in thermal insulation and in airtightness, allow us to develop a simple and cheap general principle for museum and archival storage suitable for the collections of materials which centuries of experience tell us are durable in the ordinary climate on earth. The low energy storage principle is simply stated, and equally simply achieved in practice. A lightweight, thermally insulated, airtight building is put on top of an uninsulated floor slab laid directly on the ground, whose immense thermal capacity makes a heavy building unnecessary. The thermal insulation is calculated to even out the daily temperature cycle but not to prevent an annual temperature cycle which is about half the amplitude, but much smoother, than the annual temperature cycle outside. The winter temperature in the store will nearly always be above ambient and so will maintain a moderate RH without need for either humidification or dehumidification. The temperature in summer will be below ambient and thus force dehumidification as warm air of high water vapour concentration filters in. However, the airtightness of the building allows intermittent dehumidification with low energy consumption, less than one kWh/m per year. This energy can be provided by solar panels, covering only a portion of the roof of the building. It can also be provided from the grid through smart me-

We describe a museum storage building which controls its climate by solar heating. The temperature is moderated by heat storage in the ground below the floor while a highly insulated superstructure and good airtightness shield against variation in weather. The relative humidity is kept moderate by solar heating of the attic space through a roof window. The heat is slowly released through the ceiling to the storage space below. This will give a temperature which cycles annually between 10 °C and 25 °C. Fine humidity control can be achieved by use of moisture reactive wall surfaces, such as clay in the form of unfired brick.

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 expressed in a novel unit for describing the buffer capacity, the ‘buf’ with symbol B. This is defined as the quantity of water vapour exchanged with the material, 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 with walls lined with the material. Well ventilated unfired perforated brick, 5 cm thick, has a buffer capacity of 27 m per square metre of surface for a daily RH cycle, providing significant resistance to RH change caused by an air exchange rate of once per hour. One can regard the sum of the buffer values of the wall lining and the furnishing as a virtual (larger) volume to the room, into which water vapour from infiltration and internal generation has to disperse, with a consequently lower RH variation. The buffer value is dependent on air velocity over the surface and on temperature. Wood cut across the grain was second best in performance, 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 well ventilated 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. For a long cycle, represented by a week at a steady RH, the performance approached that predicted from sorption measurements made on finely granular samples of the brick. For practically useful performance, a wall needs to have a moisture-active surface considerably larger than its area facing the room. The buffer capacity can be considerably increased if deeper layers of the wall are brought into use by convecting, or forcing, air through internal channels. A wall, 106 mm thick, of unfired perforated brick with the channels arranged parallel to the surface and ventilated mechanically, has a B–value of 61.

Unfired clay brick, wood, and cellular concrete have been evaluated as humidity 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. The best performance (61 for a daily RH cycle) is provided by a wall consisting of perforated unfired clay bricks, mechanically ventilated through the aligned perforations. The same brick exposed passively with perforations exposed to the climate chamber air had about half the buffer capacity, 27. 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 then, the performance was worse than predicted from sorption measurements made on finely divided particles. The effectiveness of all humidity buffering materials is limited by the slow diffusion of water molecules through the material. The steepness of the water vapour sorption curve is therefore only an approximate, and optimistic, indicator of moisture buffer performance.

In recent years the specification for the climate in museums and archives has become so strict that air conditioning has to be installed, though the scientific evidence of the need for such constancy of climate is very thin. The standards are rather based on what the best available technology can achieve, regardless of energy use and the need for constant surveillance of the equipment. Going to the opposite extreme, some buildings with no air conditioning perform very well, reaching the standard specification in relative humidity and getting close to it in temperature constancy. The key to achieving this stability is to reduce the air exchange rate, to give the structure considerable thermal capacity and to use porous, water absorbent materials for walls. However, the standards are also strict about the need for adequate air exchange rate to flush away internally generated pollutants and to remove patches of stagnant humid air. Stagnant humid air can be avoided by other means, such as ensuring temperature uniformity (which is already embedded in the standard). The usefulness of flushing out pollutants seems to be a good idea, but how much do the pollutants react with the stored materials in their immediate containers before they reach the larger enclosure of the room? The standard setters seem to be in a muddle about both the chemistry and physics of decay processes and about the possibilities inherent in simple changes to the building design, such as rounded corners and absorbent walls. We review the performance of existing buildings and extend our analysis to the design of new buildings.

KEYWORDS: archive, BS5454:2000, humidity, temperature, buffer . SUMMARY: There is now abundant built evidence that archives can sustain a moderate relative humidity and temperature by warming, or by dehumidifying. Four examples of museum storage buildings, of diverse purpose, size and location, show adequate climatic stability without air conditioning. However, it is not possible without air conditioning to conform with the strict temperature limits recommended in the widely used British Standard BS5454:2000, which, though an advisory standard, is often used as a building specification. The strictness of this standard is not supported by evidence for chemical or physical damage if its limits are exceeded, yet deference to it hinders the building of low energy archives which are tolerant of loss of power and neglect.

tween these two extremes is that in the first case the earth's atmosphere defines the state of the paper, whereas in the second case the paper controls its local environment. This chapter is mainly about enclosures so small that the pa- per dominates its immediate environment. To anticipate later calculations, this means an enclosure with more than 100g of paper per cubic metre. Understanding the climate within a small enclosure requires a firm grasp of some concepts in physics and an ability eortlessly to switch between dierent ways of expressing the amount of water vapour in space. These concepts are not part of the standard curriculum of science education, which is surprising, because an understanding of microclimate can enrich our everyday experience, and save us money, and maybe the planet, by allowing us to run our immediate environment more economically. However, my purpose here is more modest: to prepare the reader to judge the vast literature of advice on how to care for en- closed art. Some authorities advocate ventilation by cutting corners (literally), others warn of destruction by locked-in, aggressive chemicals. In this chapter I explain the interaction of paper with water vapour in pure air and in a confined space.

Climate control in archives can be managed very differentlyfrom that in other buildings, but very often standard technology, and reliance on standard specifications is applied. This article is a re-investigation of how best to control the climate in archives, based on the chemistry of decay and the physics of the atmosphere, by at first discarding the pedantic strictness of the archival standards but then paradoxically showing that they can indeed be attained by very simple means.

An analysis of the north European climate and of the preservation requirements of typical museum objects suggests a suitable low energy method of air-conditioning a museum store designed to hold relatively durable materials. Outside air is sucked in when its water vapour content is unusually low while the temperature is raised slightly to give a relative humidity not far below the limit for biological growth. This combination gives a low degradation rate for both objects and the building that encloses them.

A number of questions, which have gained importance lately, require a more detailed consideration of the moisture buffering of the building envelope. These questions include: - How do different envelope components react to the variable indoor air conditions in buildings with temporary occupation? - What humidity control strategies should be employed to preclude mould formation on the internal surfaces of the building envelope? - Can vapour absorbing finishing materials reduce energy consumption and improve conditions of human comfort? - What happens to the building envelope when the indoor environment of an old building is dramatically changed e.g. by opening a restaurant?

Film in sealed metal cans is vulnerable to condensation damage during cooling for storage. Film in cardboard boxes is vulner- able to condensation damage during warming for showing. For several hours there is a temperature dierence between the lm and the inner surface of the container, so water evaporates from one surface and con- denses on the other. Condensation can be prevented by slow cooling and warming, so that there is never more than six degrees temperature variation within the container. A modied lm container, with crin- kled paper leaves between the metal can and the lm, should prevent condensation during rapid temperature change.