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

To keep outside heat away, double-pane glass with air in between is the most effective choice among the options. Glass itself is a poor conductor of heat [2], but a single pane offers minimal insulation [t3]. Double-pane windows, or Insulated Glass Units (IGUs), utilize two layers of glass separated by a gap [t1][t6]. This gap is typically filled with air or inert gases like argon, which act as excellent insulators because air is a poor conductor of heat [c3][t9]. The trapped air reduces heat transfer by inhibiting conduction and convection between the panes [c3][t1]. While water-filled panes exist in niche research, water has a much higher thermal conductivity than air, making it less effective for standard thermal insulation. Therefore, the air gap creates a thermal barrier that significantly improves energy efficiency and temperature control [t2][t4].

Sources

1. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 7: Heat Transfer in Nature > 7.1 Conduction of Heat > p. 92