Pitchfork locking door (C) Daniel Friedman Energy Savings from Acrylic Interior Storm Windows

  • Calculate savings from an interior acrylic storm window retrofit
    • Factors in storm window energy savings: what makes storm windows effective or ineffective in saving heat?
    • Films to increase solar collector efficiency
    • Window Glazing Energy Products: What are the Differences in Function & Use Among Low-Transmission Films, Low-E glass, Coated Reflective Films & High Transmission, Low Emissivity Films or Reduced-Iron-Content Glass?
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  • Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices

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Interior stormw indows, acrylic: this article discusses the energy savings from retrofit interior acrylic plastic interior storm windows to control heat gain, heat loss, heat transmission. We list and discuss choices among window glazing energy-saving products.

Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Our page top photo shows a troublesome exterior storm window retrofit in Hyde Park, NY - we expect to find eaks into the walls of this building!

Readers interested in constructing storm windows should also see STORM WINDOW PLASTIC CHOICES. Readers interested in reducing un-wanted window glazing heat gain or heat loss should also see SUNGAIN, FILMS, LOW-E GLASS.

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How to Calculate the Energy Savings from Retrofit Interior Acrylic Storm Windows

The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.

  • Q&A on Interior Storm Window Savings - how to calculate the energy savings - PDF version, use your browser's back button to return to this page

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Storm Window Savings

Leaky window on a silo (C) Daniel FriedmanQuestion:

Is there a fast way to calculate savings from an interior acrylic storm window retrofit? - Franklin Ellingwood, Honeoye NY


The Architectural Aluminum Manufacturers Association (AAMA) in the 1980's developed and printed many nomographs for exterior storm windows. These should also apply to an interior storm window retrofit.

A "rule of thumb" that is popular among building inspectors is that each layer of glazing on an existing window will cut heat loss through that window by about 1/3.

Our photo (above-left) shows a leaky interior window installed in the sloping exterior wall of a silo converted to living space in the Hudson Valley of New York. An interior storm window won't help much if the main window is in such poor condition as this one.

Where an interior storm window is particularly appropriate is for an energy improvement retrofit over casement or awning windows. Because these windows open by swinging "out", an exterior storm window can't be installed. Many older casement and awning windows provide for an interior storm window that fits inside the movable sash - but that won't do anything to reduce heat loss if the sash or window frame is leaky.

Watch out: the energy savings effectiveness of any storm window, installed inside or outside, can vary enormously. Here are some factors we have observed in the field [DJF]:

  • Is the exterior or interior storm window properly installed, mounted, sealed so as not to be leaky?
  • If the exterior storm window is a double-hung unit, or a "triple track" storm and screen unit, are the storm windows actually closed properly? We often find that a forgetful building occupant has left storm windows partly open, or that an individual window was broken and removed entirely.
  • If the exterior storm window is a double hung unit, is the right sash in the "up" and "down" position? The outermost sash should be "up" on an exterior mounted double hung storm window so that wind-blown rain won't enter the window.
  • Are there drafts around the storm window, around the window frame itself? This problem may be more common on older homes using sash weights and a rope and pulley system to raise and lower sashes.

According to the U.S. Department of Energy, the most effective way to improve a home's energy efficiency is to install new energy efficient windows. [See WINDOW / DOOR ENERGY EFFICIENT, DOE]. But where your budget does not allow that costly improvement, some types of storm windows are a good option, such as for people living in apartments. The DOE continues:

Even though storm windows add little to the insulating performance of single-glazed windows (that are in good condition,) field studies have found that they can help to reduce air movement into and out of existing windows. Therefore, they help reduce heating and cooling costs.


For the most part, interior storm windows offer greater convenience than exterior storm windows. They're easier to install and remove; they require less maintenance because they're not exposed to the elements; and, because they seal tightly to the primary window, they're more effective at reducing air infiltration. Interior storm windows also are often the best choice for apartments and houses with more than one floor. If you can afford exterior storm windows, you can probably afford some newer, more energy-efficient windows, which will be a better investment.

Glass pane types offer better visibility and longer life than plastic pane types, but glass is heavy and fragile. In general, plastics are most economical for people with small budgets or who live in apartments. However, while inexpensive and relatively easy to install, they are easy to damage. Plastic panels, such as Plexiglas and acrylics are tougher and lighter than glass, but may scratch easily. Some may turn yellow over time as well. Some plastic films may significantly reduce visibility and degrade over time when exposed to sunlight.

Wood, aluminum, and vinyl are the most common storm window frame materials. There are advantages and disadvantages to all types of frame materials. Although very strong, light, and almost maintenance free, aluminum frames conduct heat very rapidly. Because of this, aluminum makes a very poor insulating material.

Wood frames insulate well, but they weather with age. They also expand and contract according to weather conditions. Wood-frame storm windows installed during the winter may not close easily during the summer, and those installed during the summer may fit loosely in the winter. They can also be quite heavy and thicker than metal frames. This can make storage difficult, reduce the view out the window, and reduce the amount of natural light in the room. Wood frames also require the most maintenance. There are, however, aluminum- or vinyl-clad wood frames that reduce maintenance requirements.

Vinyl frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. They, however, may expand and warp at high temperatures, and crack in extremely low temperatures. Also, if sunlight hits the material for many hours a day, colors other than white will tend to fade over time.

For information on using nomographs see Solar Age 12/84, p. 48. For information on the nomographs, contact AAMA Technical INformation Center.

For more up to date information about the performance of films to increase solar collector efficiency, see SOLAR COLLECTOR FILMS

The question-and-answer article above quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.


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