Abstract Based on available correlations relations are found for the local maxima's and minima's in heat transfer as the gap spacing is varied in flat plate solar collectors. These relations can shorten the task of selecting an optimum gap. A criterion is proposed for evaluating the use of alternate mediums in the enclosed space. It is shown that the use of heavy gases such as Argon can result in a 34 per cent reduction in heat losses. Nusselt number correlations of a single gap are extended to a two-cover system. It is found that by using two-covers there is an overall saving of more than 50 per cent in convection losses. It is also found that heat transfer rates in the laminar and turbulent regions are relatively insensitive to the internal spacing of the covers but reduces on changing from the mid-way position in the initial regime. A new type of two cover system is proposed in which the upper space is partially evacuated and it is shown that heat losses can be reduced by 85 per cent on a one-tenth reduction of pressure. Design relations for calculating cover spacings and heat transfer coefficients in this system are derived.