Tip number 131: See Double

Always choose double glazing for windows. The layer of air trapped
inside the glass prevents heat loss into the cold outdoors and will
insulate your house against hear loss in winter. Similarly it will
keep you cooler in summer. The optimum space between two panes is
20mm. A smaller one leads to greater heat loss.

Taken from '1001 little ways to save our planet' by Esme Floyd