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Interior Storm Windows


Homeowners are usually interested in lowering their energy expenses. However, they are not willing to pay large costs or sacrifice the appearance of their home to do so. Interior storm windows provide a compromise by effectively increasing the energy performance of a home by updating single-pane windows at significant cost savings over window replacement. They are accepted in historic and other homes whose owners want to retain the original windows or do not wish to alter the home’s architecture with exterior storm windows. In addition to energy benefits, interior storm windows can improve sound resistance, decrease condensation, and reduce ultra-violet (UV) light damage to furnishings. . .

. . . Interior storm windows can be used in historic homes, because they have little effect on interior or exterior architecture. Interior storm windows can increase the overall thermal insulation of the window and provide energy savings during heating and cooling seasons. Interior storm windows can also reduce exterior sound transmission through windows. Many manufacturers offer interior storm windows with UV protection, which reduces the amount of UV rays transmitted through the windows. Storm windows mounted on the interior can reduce condensation on windows during cold weather by prohibiting warm, moist interior air to contact the cold exterior window.

Energy-Efficient Interior Storm Windows, Toolbase Services, NAHB Research Center

For the most part, interior storm windows offer greater convenience than exterior storm windows. They are easier to install and remove and often need less maintenance because they are not exposed to extreme weathering. Interior storm window designs may also complement interior decor. Since they reduce air infiltration better, they also reduce energy loss more than exterior types.

Interior types inhibit air infiltration the most because they seal airtight to the primary window. This creates a superior dead-air space relative to exterior storm windows, which require weep holes. As mentioned above, exterior storm windows require weep holes to allow any rain that may be driven behind them or water vapor from inside the house that condenses in the space between the primary window and the storm window to run out of the window. If an interior storm window is sealed tightly to the primary window, the insulation value of the window increases by about R-1 for ordinary glass or plastic. If the glazing has low-e properties then the R-value increase could be as much as 2.

Storm Windows, Energy Efficiency and Renewable Energy, U.S. Department of Energy

Generally, an interior-mounted storm unit preserves the visual qualities of historic windows better than an exterior one. There are unobtrusive, high-performance, commercial quality interior storms intended to be jamb-mounted rather than affixed to the casing. Some of the interior storms are side or top-hinged, although the more common styles are double or triple-track units. Where fixed windows are appropriate, single or double panels attached to a subframe can be used, thus saving on initial costs while still allowing occasional removal for cleaning and maintenance. Condensation may be more of a problem with interior rather than exterior storm applications, particularly with residential buildings in extremely cold weather climates; however, the problems encountered in buildings with interior storm applications can be minimized with most windows if weatherstripping, caulking, and weep holes are part of the upgrading process and there is good quality construction and installation work.

Fisher, Chuck. Rehabilitating Windows in Historic Buildings: An Overview (GSA 08500-03), Historic Preservation Technical Procedures, U.S. General Services Administration

If the configuration of the inside of the window permits it, the installation of a second internal window will cut down heat loss, and provide some measure of acoustic insulation as well. Often, the presence of internal shutters will prevent this (shutters themselves should be kept in working use because they themselves will cut down air infiltration and can be regarded as 'compensating measures' in any heat loss calculations required under Building Regulations).

Proprietary systems exist, usually in aluminium framing, and can be made to fit a given opening. The idea is not new; some houses in the past were fitted with a second double hung sash window, or with solid panels which had counterbalancing weights and dropped down into a space below the window. In modern secondary glazing, whether for sash or casement windows, the internal meeting rails should always coincide with those of the window it serves.

The reflection caused by secondary glazing is fairly obvious from outside; this could, in theory, be reduced by using the non-reflective glass which is now commercially available. As most secondary glazing for sash windows is made from light metal framing (in order to be as unobtrusive as possible) and generally does not have spiral balances to assist lifting, there are limitations on size, due to the weight of the glass.

Wrightson, David. The Conservation and Renewal of Timber Windows, Public Information Leaflet, Cathedral Communications, Wiltshire, England [PDF file]

A low "tech" and inexpensive alternative is to install polyethelene plastic sheets or films on the room side of the window. You can find "do-it-yourself" kits in hardware and home improvement stores, which are plastic films that you tape over the window. Using a hair dryer to shrink-wrap the film around the window helps to tighten the film and improve its transparency. Such kits are used once and then removed at the end of the season.
In general, plastics are most economical for people with small budgets or who live in apartments. The primary advantage is to reduce air leakage into and out of the window. While inexpensive and relatively easy to install, they are easy to damage. Also, some plastic films may significantly reduce visibility and degrade over time when exposed to sunlight.

Storm Windows, Energy Efficiency and Renewable Energy, U.S. Department of Energy


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