The causes of air movement in hidden indoor micro-environments: measurements in historic bookshelves

The use of ventilation holes in small micro-environments has been proposed as a mechanism to improve the environmental conditions of moisture and temperature within bookshelves. The National Trust, for example, has used these mechanisms to encourage air movement behind books as a possible strategy to reduce the risk of mould growth. It is believed that including ventilation holes as a passive design solution to promote airflow within micro-environments could prevent decay from occurring in archives of historic buildings. This paper investigates the mechanisms that cause airflow behind bookshelves using field measurements in three National Trust historical libraries. The measurements indicate that small but measurable velocities, up to 4 cm/s, can be generated passively behind bookshelves. Air movement in such confined micro-environments is caused by a combination of natural convection, caused by temperature differences between the interior and the exterior of the bookshelf(stack effect), and forced convection due to drafts in the surrounding environment. While in some cases one mechanism prevailed, both mechanisms may be present simultaneously in most cases. Finally, this paper proposes that air movement as a result of the stack effect in these micro-environments can be predicted with a simple mathematical model, which offers a good fit with the experimental data in the cases where temperature differences promote air motion.