Energy Savings from Acrylic Interior Storm Windows
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Calculate savings from an interior acrylic storm window retrofit
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Questions & answers about choosing, buying, installing or repairing acrylic interior storms, including the anticipated energy savings you may expect.
Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices

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.

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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

Question:

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

Answer:

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.

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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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  SOLAR SHADES & SUNSCREENS
  STORM WINDOW INTERIOR
  STORM WINDOW PLASTIC CHOICES
  STORM WINDOW WEEP HOLES
  SUNGAIN, FILMS, LOW-E GLASS
  SUNSPACE GLAZING for SUNTANNING

Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years. ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in buildings, and other topics.
AAMA, Architectural Aluminum Manufacturers' Association, now American Architectural Manufacturers Association, an " advocate for manufacturers and professionals in the fenestration industry" - website: http://www.aamanet.org/ - Watch out: a search for "what is the effectiveness of storm windows" produced no data although the website suggests that an article is available - web search 06/19/2010.
U.S. Department of Energy, "Energy Efficiency & Renewable Energy - Storm Windows", web search 06/19/2010 - original source: http://www.energysavers.gov/your_home/windows_doors_skylights/index.cfm/mytopic=13490

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"Wonder Windows, Two Let In More Sun, Two Keep In More Heat", V. Elaine Smay, Popular Science, April 1982
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How to Calculate the Energy Savings from Retrofit Interior Acrylic Storm Windows

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