[Greenbuilding] night insulation for windows savings

[Eli]

I know that movable window insulation saves energy when used at night during
the heating season.  How much and therefore what is the energy savings is
worth when compared to the cost for fuel that was saved.  I want to be able
to show that these improvements will pay for themselves with the savings.  I
need to know what that savings is.

To use a spread sheet to calculate temperature difference, I would need the
average night time temperature.  This may be data intesive.  However, I
would be curious to know if this information exist.  This could be used to
calculate degree night and therefore cost/savings for various components
that are affected by night time temperature diffences.

Average night temperatures could also be applied to cooling conditions to
ventilate cooler Summer night time air to cool a mass in building that would
absorb day time heat in interior.  The average temperature for the night is
what I am looking for.

Otherwise what other methods are out there for figuring savings for movable
window insulation panels when used at night in heating season.

Eli Fishpaw
Architect
Lexington, VA