On 19 Dec, 16:16, "Dave Baker" wrote:

For a poorly insulated house the savings would be greater but still not very
much. For the saving to be as much as 10% the house temperature would have
to drop by 10 degrees C overnight for which you'd probably have to leave all
the windows open.

Oh dear, ours can easily drop from 22C down to 12C overnight with no
heating (this is with outside temps above freezing, we're in
centralish London, with rarely a frost). Single-glazed sash windows
are probably to blame, along with less-than-optimal loft insulation.