Great interest has been shown over the last couple of decades, particularly within industrialised countries, in developing innovative glazing systems for energy-efficient buildings, either to reduce window heat loss in winter or solar heat gain in summer. In highly insulated assemblies, the constraints placed on the former are due to (i) edge-, frame- and spacer-induced heat losses, and (ii) external and internal surface resistances caused by the range of flow regimes found in the built environment. A thermal resistance network model is used to determine the so-called 'night-time' U-values of different assemblies that are typically measured by hot-box techniques. It treats the problem as one involving complex conjugate heat-transfer mechanisms with unknown interfacial temperatures at the glass boundaries of multiple (single, double and triple) glazed units. The present work focuses on 'advanced' glazing systems that have cavities containing low conductivity gases, together with low emissivity coatings on the boundary glass surfaces. Evacuation of the cavities is examined as the ideal or limiting case. The analysis suggests an order of merit for adopting new design features that place triple-glazed systems and the use of low emissivity coatings well ahead of low conductivity gases in terms of thermal performance.