The thermal performance of an integrated collector storage solar water heater (ICSSWH) is numerically examined using the numerical software Fluent 6.3. As this solar system presents the disadvantage of its high night thermal losses, a new strategy helping to reduce these losses is proposed. A particular system in which the storage tank is covered with an outer glass tube is studied and a parametric study is conducted in order to evaluate the optimal air spacing gap between the water tank and the covering glass tube which minimizes this system's heat losses. A 3D computational fluid dynamics (CFD) model interpreting this modified system is so developed and its thermal performance is compared to that of the noncovered tank based ICSSWH. The analysis of these solar systems daily performance shows that the modified ICSSWH is able to generate more thermal output and presents the advantage of its lower thermal losses. Regarding the night operating, this covered tank based ICSSWH is shown more effective in retaining higher temperatures for longer period, resulting so in lower night thermal losses. Results also suggest that the best performance corresponds to the lowest air gap spacing (L = 0.005 m).