Interior Storm Windows

Interior Storm Windows

Interior storm windows offer energy savings comparable to replacement windows but at more than half the cost. Our rustic cabin has some very old period windows that add charm but also leak heat. Also, we live on an island with no window supplier. To us, the only alternative for energy conservation was to build our own interior storm windows.

In a given home the average losses of energy are shown in Figure 1.

interior storm windows
Figure 1 Infrared image: yellow to red areas show heat loss from given sources

The U-factor

Firstly, interior storms windows are airtight inserts you place inside your window. When referring to windows, the lower it’s U-factor, or it’s insulating ability, the better. A typical single glass pane window has a U-factor close to 1 (Btu/h·ft²·°F). Whereas a high-performance double-pane window can have a U-factor of 0.30 and triple-pane window 0.15. These windows would cost upwards of hundreds to thousands of dollars to replace, with the added carbon footprint to produce and transport them.

The R-value

Secondly, a more common term is the R-value which refers to the resistance of the window to heat conduction and is the inverse of the U-factor (R = 1/U). A single glass window pane has an R-factor of about 1. Adding a double-glazed replacement window would triple that whereas an interior storm window could increase the R-factor to 2.84 at about 25-50% of the cost of replacement (“Energy-Savings and Period Too”, 2009). Most of all, an interior storm window can lower heating bills by 20-30% (“Interior Storm Windows”, 2016).

Interior storm windows made from repurposed materials

Thirdly, they can be made from repurposed glass or plastic, with and without low-E coatings, and the frames can be made from repurposed metal or wood. Low-E coatings are metallic or metal oxide materials that reflect light and reduces radiative heat loss from the warm interior to the cold exterior; hence, low-E coatings reduce the U-factors. Polycarbonate and acrylic are clear, light, sturdy plastics that are often used, however, they are expensive. An 8 mm polycarbonate or a single pane glass interior window can reduce the U-factor by about fifty percent (Culp et al., 2015). They have an airtight seal created with the use of magnets or a compression, materials such as foam or rubber weather stripping.

Lexan acrylic picked up from the side of the road repurposed for interior storm window

Interior storm windows increase the window surface temperature and will reduce the potential for condensation forming.

Furthermore, air leakage is another consideration. Over time, due to degradation of weather seals and warpage of material, windows will leak air and heat. In our old windows, air leakage could be 3.0 cubic foot per minute (CFM)/square foot. Installing a tightly sealed interior storm window could reduce this factor to 0.7 cfm/square foot (Culp et al., 2015). 

Essentially, interior storm windows, also called, energy panels or invisible storms are easy to install and increase insulating ability of existing single pane windows comparable to full replacement, but at a fraction of the cost. They also allow preservation of  the rustic esthetics of our cozy seaweed cabin in the woods.


Culp, T.D., Widder, S.H., Cort, K.A. (2015). Thermal and Optical Properties of Low-E Storm Windows and Panels. US Dept. of Energy.

Polson, M. (2009) Energy Savings, and Period Too. Old-House Interiors15(3), 70–74

Interior Storm Windows (2016). Old-House online. Retrieved on 2019/07/08 from