Storm window

Storm windows are windows that are mounted outside or inside of the main glass windows of a house. It may also refer to a small openable flap found in the side window on light aircraft.

On houses they serve as a retrofit on existing windows in order to improve their thermal insulation and soundproofing. Similarly, storm doors (also called "screen doors") allow similar energy savings with less efficient primary doors - and allow a screen for summer ventilation.

Storm windows can be mounted externally or internally; can be made of glass, rigid plastic panels, or flexible plastic sheets; and may be permanently or temporarily mounted. They function similarly to insulated glazing.

Aside from insulation, storm windows provide an additional measure of protection for homes against damages to costly glass panes during inclement weather.

Storm windows are a typical energy upgrade solution for cold climates. They are mostly intended to improve the insulation value (R-value) of existing windows, especially single-glazed units.

Storm windows do not make sense in buildings with energy efficient windows, or in hot or moderate climates; they only make sense in buildings where windows are a major cause of heat loss and lack of thermal comfort.

Storm windows can be very cost-effective, in cold climates. They are inexpensive, and can reduce heat loss by up to 50% or so, increasing the building’s comfort and reducing the heating costs, which is difficult to achieve with inexpensive replacement windows.

They can also reduce exterior air infiltration significantly, and are inexpensive: even the best storm windows - three track exterior windows, with low-E glass – will cost a small fraction of the price of standard replacement windows.

Interior storm windows can produce problems of condensation and be visually obstructive, while exterior storm windows can have a negative visual impact.

The negative visual effect can be minimized by using single line storm sashes, while possible problems of condensation can also be avoided by incorporating vent holes and a sealed fit.

A plastic PVC interior storm panel is particularly inexpensive and easy to install but these panels lose quality over time and may get broken quite easily. Storm windows with Low-E glass and metal frames are a more durable choice, and can provide better insulation.

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Wikipedia