What would be the best choice for window material to keep the outside heat away?

To keep outside heat away, the best choice is double-pane glass with air (or an inert gas) in between. Air is a poor conductor of heat compared to solids like glass or liquids like water [c1][c5]. In a double-pane configuration, the air gap acts as a thermal barrier that significantly reduces heat transfer through conduction and convection [t3][t6]. While glass itself is an insulator, a single pane or double panes without a gap allow heat to transfer more readily through the material [c1][t1]. Filling the gap with water would be counterproductive because water has a much higher thermal conductivity than air, facilitating heat transfer rather than blocking it. Modern insulating glass units (IGUs) utilize this trapped air or inert gases like argon to create a stable thermal break, keeping interiors cool in summer and warm in winter [t4][t9].

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 7: Heat Transfer in Nature > 7.1 Conduction of Heat > p. 91
2. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 7: Heat Transfer in Nature > 7.1 Conduction of Heat > p. 92
3. [3] https://www.energy.gov/energysaver/window-types-and-technologies