If only conductive heat loss was significant for a house then by what percentage would you lower the heat loss fi the temp was reduced inside from 70 to 60f and outside temp 20f

I think the conductive heat loss is proportional to the DIFFERENCE
between the inside and outside temperatures.  In other words, if it's the
same temperature inside and outside, then no matter what that temperature
is, no heat flows through the walls of the house in either direction.

You said it's 20° outside, and you turn the thermostat down from 70° to 60°.
So you'd be reducing the DIFFERENCE between the inside and outside
temperatures from 50° to 40°.

From 50 to 40 is a decrease of (10/50) = 20%.  So your heat loss ... and
the amount that gets added to your heating bill ... becomes 20% less for
each hour that the inside and outside temperatures stay like this.

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