If you have installed heavy curtains but not installed a pelmet the curtains will be significantly underperforming.
Normally, warm air will enter the gap between the curtain and the glass from above. It is cooled by the glass and emerges at the bottom of the window causing a significant heat loss. You can check this by placing your hand at the bottom of the curtain and feel the draught. Making your curtains extend to the floor helps, but quality pelmets above the curtain helps even more. The problem is that they are expensive and most people don't like them.
My idea is very cheap and almost undetectable. It is basicallly a piece of plastic attached between the window frame and the top of the curtain rail.
I bought a 30 metre roll of plastic sarking for $6.00 (used by builders under windows to form a water barrier), plus 30 metres of double sided sticky tape from the newsagent. The steps involved are:-
1. Clean the curtain track and top of the wooden window frame. Hopefully they are approximately the same level
2. Attach double sided sticky tape along the top of the curtain track, leaving the paper protector in place
3. Attach double sided sticky tape along top of the window frame, leaving the paper protector in place
4. Cut the sarking to the approximate length
5. Remove the paper protector in sections from the curtain track. Attach the sarking so that it is not visible from the front. Allow it to fold up the wall
6. Run a knife carefully against the wall, trimming the sarking so it sits flat
7. Remove the paper protector in sections from the window frame and push the sarking down firmly
8. Trim of the ends of the sarking
9. Make a slight cut in the sarking and tape next to the curtain track supports so it sits flat