Secondary glazing (or internal storm windows) can be used as a functional alternative to retrofitted insulated glazing units (IGUs) in existing domestic single-glazed window frames. This work investigated the thermal transmittance of four secondary-glazing products - plastic film, magnetically attached plastic sheet, plain and low-E glass. Thermal transmittance measurements were made on a typical New Zealand single-glazed aluminium window, to the method of ASTM C 1199 using a guarded hot box (GHB). Four different secondary-glazing systems were then retrofitted into the window and the thermal transmission measurements determined. All four of the secondary glazing options provided significant thermal improvements to the single-glazed window (0.15 m(2) K/W), with R-values from 0.34 to 0.57 m(2) K/W. Each was also modelled in Window 6 software, with the results found to be within the GHB measurement error. The performance exceeded the expected performance of double glazing retrofitted into the existing framing due to the secondary glazing effectively insulating the framing. (c) 2012 Elsevier B.V. All rights reserved.