Saturday, January 24, 2015

Staying warm with interior storm windows

Small interior storm
window installed
Maine, which has the oldest housing stock of any state in the US has a lot of leaky old buildings heated with oil boilers.  Needless to say these buildings are expensive to heat and waste a great deal of fossil fuel .

Several years ago my friend Topher came up with a simple design for a double pane interior storm window that doubles or triples the insulation value of windows while also preventing all air movement through the window.  Through the auspices of our group called the Midcoast Green Collaborative we set up workshops throughout the state to teach people how to make these very simple windows.  Since then we estimate more than 10,000 of them have been deployed around the state and many more in the world at large.

They are constructed from a 1 x 2 wood frame with heat shrink plastic film affixed to an inner and outer surface to create a double pane window.  Highly compressible weatherstrip foam tape is adhered to the outer edge so that when the window is inserted into the window frame a complete seal is created.  Each trapped air layer has an approximate R-value of 1.  If you assume a single pane window which has an R-value of 1, by inserting an interior storm window you are trapping 2 additional air pockets resulting in an R-value of 3.  This dramatically reduces the heat loss through exterior windows.  In most homes in northern cold climates these windows pay for themselves in the first heating season.

The cost to build these interior storm windows is approximately a $1.25 per square foot.  This means that they can be constructed for a cost of $15-$20 per window.  Generally they take two people 30 to 40 minutes each to build and do not require any significant skills or special tools.

If you live in a cold climate and want to reduce your heating bills these interior storm windows represent an extremely cost effective way to do that.


Very detailed assembly instructions on my webpage

Midcoast Green Collaborative webpage
Basic two-page instruction sheets (pdf format)
Thermal study of a window with interior storms added

Instructions are also available in Charlie wings excellent book: "The Visual Handbook of Energy Conservation".   Here is my review of the book.

Topher's website


  1. I wonder how your foam insulated frost walls are holding up? Would you change anything? Rigid mineral wool rather than foam? Insulated Frost Walls

  2. rj - the blue foam is not degrading - if that was your concern. I would not change a thing, given the documented 11F improvement in interior wall temperature!

    Did you also see this page:

    1. Excellent. I had not seen your other page. The biggest aha for me was 10" concrete is only R1.


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