Payback on Guy's Inside Storm Windows

This is an example showing how to calculate the payback period for the double pane interior storm windows shown here.

The detailed plans for the easy to make, double pane storm windows are shown on Guy Marsden's website here.  Other inside storm window plans here...


We will use this Insulation Upgrade Calculator to estimate the payback period for typical 15 sqft window.  I will use my climate and fuel in the example, but you can easily substitute your own climate and fuel to get your savings.

Per Guy's note, the cost of materials for this 15 sqft window would be about $15 ($1 per sqft).


The inputs needed to run the calculator are:

Area of window  -- 15 square feet for this example

Heating Degree Days -- 8000 my SW Montana climate (look up your own here...)


Current R value -- R2 for my current double glazed windowsth (look up your own here ...)


New Total R Value -- R4 for me (R2 current + R2 storms) 


Fuel  -- propane at $2.39 per gallon


The results (see screenshot below) are:

The payback period is less than one heating season (cost = $15 vs 1 year savings of $23)

The 10 year saving is $373 (assuming 10% per year fuel price increase)


The CO2 emissions reduction is 127 lbs per year.

All of the savings would, of course, be correspondingly larger if you did several windows.
For warmer climates, the winter savings would be less, but there would be added savings on AC in the summer.


You can plug in the values for your own situation into the calculator, and see what you come up with


Calculator screenshot for this example:



Gary September 1, 2008