The effect of air tightness on RH buffering and control

David Thickett, Phillip Fletcher, Andrew Calver, Sarah Lambarth

It has been known for over thirty years that air exchange rate is fundamental to the hygrometric performance of display cases. However, the expense of commercial testing services and the unavailability of methods had severely limited testing. The availability of relatively inexpensive equipment and the use of simplified methods has resulted in at least a fifty fold increase in the number of air exchange rate measurements undertaken in the UK. Coupled with developments in measuring technologies for relative humidity, which has allowed more widespread and more accurate monitoring, a corpus of data now exists to assess the effect of different air exchange rates on RH buffering and control within display cases and storage enclosures. These results demonstrate that in most instances where attempts to control the hygrometric performance of enclosures has failed, air (or moisture) exchange is the key variable. The potential major drawback of tightly sealing showcases, besides cost and time, is the concentration of off-gassed products from objects, dressing or construction materials. The carboxylic acids are by the far the most widely reported culprits in adverse effects on objects inside showcases and storage enclosures. Diffusion tube based measurements are ideal to determine carboxylic acid concentrations inside such enclosures and a body of such data has now been acquired. Methanoic acid emissions from paint have been shown to follow existing models. Ethanoic acid concentrations from MDF were found to increase dramatically at air exchange rates below 0.5.