To investigate the effects of moisture movement on thermal efficiency, an experiment was performed on a low-slope roof system containing permeable insulation. Five replicate test panels of the roof system containing a variety of vapour retarders were evaluated in a climate simulator. Describes the test procedures. Presents the experimental results. Uses a numerical model to help explain the findings. Summarises the conclusions.

KEYWORDS: moisture, thermal insulation, heat flow, performance, testing, calculating, vapour barriers, experiment, inclined, roofs, climate chambers, efficiency.