Suspended particle device (SPD) switchable glazing can change optical transmission from "opaque" state to "transparent" state in the presence of an alternating current (AC) power supply. It can be applied to control internal temperatures in buildings. Thermal characterisation of both SPD and same area of a double-glazing sample was accomplished using an outdoor test cell in Dublin, Ireland. The overall heat transfer coefficients (U value) were calculated for both systems from the experimental data. The average U values for SPD and double glazing samples were found to be 5.9 W/m(2) K and 2.98 W/m(2) K, respectively. Addition of double-glazing to this SPD switchable single glazing offered a U value of 1.99 W/m(2) K. (C) 2015 Elsevier Ltd. All rights reserved.