This report contains the results of a series of U-value measurements on a roof structure insulated with a variety of insulation systems; results of thermal performance measurements carried out using non-steady state temperature conditions and a brief review of the possible methods for determining the thermal performance of insulation systems based on multifoil products.The thermal transmittance measurements were carried out using the NPL rotatable hot-box facility of a section of roof, insulated with four different insulation systems i) ACTIS Tri-Iso Super 10 multifoil, ii) Celotex TUF-R GA 3050 expanded polyurethane, iii) Knauf Rafter Roll 32 glass fibre, iv) and four off, 25 mm deep air cavities with low emissivity surfaces. The U-values of the four insulated roof systems were measured at four orientations which were i) the test element mounted vertically with horizontal heat flow, ii) at forty five degrees and iii) horizontally with upward heat flow and iv) mounted horizontally with downward heat flow. The measurement at forty-five degrees was repeated with a small airflow allowed into the air cavity between the tile membrane and the insulation. The U-values measured for these insulated roof structures are in agreement with values that were predicted from classic steady state heat transfer theory, which was demonstrated by calculating the U-value for each roof type using 2D finite element analysis, and air cavity thermal resistance values calculated using the procedures specified in BS ISO 15099.Temperature cycling measurements were carried out on a low-density glass fibre specimen and an air cavity insulated with ACTIS Tri-Iso Super 10 multifoil insulation. The energy used per 24 hours (in Watt-hours) obtained from the temperature cycling measurements show the energy required to cycle the low density glass fibre and the air cavity insulated with ACTIS Tri-Iso Super 10 were within approximately 17% of each other with much of the difference resulting from differences in thermal resistance and the effect of temperature difference on the thermal resistance of air cavities.A brief review of the various methods of evaluating the thermal performance of multifoil insulation products has also been carried out.