Interior storm windows - thermal study

In an earlier blog post I talked about the benefits of installing interior storm windows to reduce heating bills in cold climates.   You can purchase these double pane plastic film windows for about nine dollars per square foot, or you can make them yourself for around $1.25 per square foot.   Instructions to build them yourself or on my website.   They consist of a wood or metal frame with heat shrink clear plastic on both sides creating a trapped air layer in between.  They fit snugly into the window with highly compressible weatherstripping that prevents air movement through a leaky window.

I have installed these interior storms throughout my workshop because the original windows were cheap, single pane, double hung units that are very leaky.   When I first closed in the building from being an open barn to a heated space I purchased commercial interior storms, and more recently have added my own handmade ones as well.  For every trapped layer of air, an R-value of one is added.  So starting with a single pane of glass, by adding a double pane interior storm there are two trapped air layers creating and R-value of two, and by adding a second interior storm I am upgrading my original windows by an R-value of 4 which is very significant.

Fluke VT04 Visual IR Thermometer

This evening I decided to document the thermal efficacy of these window treatments using my VT04 Visual IR Thermometer made by Fluke.  The temperature outside was almost exactly at freezing and I started by taking a picture of the window with a temperature reading of the glass surface at 31.7°F:

Then I proceeded to take thermal images of the window itself, followed by each of the additional interior storm windows:

By adding my homemade interior storm window I gained 3.6°F and then adding the commercial aluminum framed interior storm window I gained an additional 1.8°F for a total improvement of 5.4°F.  While I adjusted my thermal camera to compensate for the low emissivity of the reflective surfaces, I cannot be sure these readings are entirely accurate, but they certainly convey the concept.

My homemade window is framed with 1X2" primed pine lumber with 3/4" spacing between the panes, while the commercial one is framed in aluminum with only 1/4" between the panes.  Additionally, the air gap between the glass and my window is between one and 2 inches, while the air gap between my window and the commercial one mounted to the surface of the window framing is around 4 inches.  Larger air gaps are less efficient because they can function as a heat pump as cold air flows down at the colder surface and warm air flows up the warm surface creating a circulation.