Side view of a window with a curtain
Cold outside on the left, a warm room on the right. Arrows show heat leaving through the glass; they shrink as the curtain does a better job.
Outside
0 °C
Room
20 °C
Your window
Single glazing
Older double glazing
Modern double glazing
Your curtain
No curtain
Sheer voile
Lined curtain
Heavy lined or interlined
Fit:
Room humidity:
Length
Sill length
Floor length
Heat lost
Saved vs bare
for a 1.5 m² window
Glass behind it
Condensation
A simple rule-of-thumb model, not a measurement: 20 °C inside, 0 °C outside, one 1.5 m² window. Real curtains and windows vary.