Storm windows save heat, reduce drafts, increase comfort and markedly reduce frosting and sweating of the inner windows.  However, the belief that a good storm window will eliminate all condensation is not true.  The basic purpose of a storm window is to provide insulation.  Its influence on condensation, when beneficial, is a by-product of its insulation value.  Storm windows do save heat.

Glass is a poor heat insulator; so poor that the thickness of the glass is on no importance in holding heat in the house; but a layer of air clings to each side of the glass and air is a very good insulator.  With no wind the blanket of air on each side of the glass is equivalent in insulating value to 3” of brick.

Wind cuts down the thickness of this blanket.  In a fifteen mph wind, the blanket of the windy side is only 1/4 as thick as when there is no wind.  For this reason more heat escapes from the house through the glass on a windy day.

When storm windows are installed, the wind is prevented from reaching the inner glass so that instead of one full blanket and one thin one, there are three full blankets and one thin one to help insulate the window.

In calm weather, of course, there are four full blankets in place of two.  Besides the loss of heat by direct transfer through the glass, there is a leakage of air through the cracks in any operating window.  This leakage amounts to about 175 cubic feet of heated or cooled air per hour through an average size window of the very finest and most expensive manufacturer.  Lots of moisture-laden air can escape to the space between the prime and the storm window.  In a plain unweatherstripped window of average fit, this leakage can amount to more than 1,000 cubic feet per hour.  The installation of storm windows with movable sash will drastically reduce this leakage but it cannot be expected to entirely eliminate it.

One of the chief reasons for reducing air leakage is to reduce drafts and the resulting “cold spots”.  A large part of the draft felt in a room is due to the cooling of the air passing the windows.  This cool air is heavier than the warm air in the rest of the house, so it falls to the floor and flows across the room.  The colder the window, the colder and stronger the draft.  Moving air feels colder than still air and so it is necessary to keep a drafty room at a higher temperature.  This, of course, is a waste of fuel to say nothing of the discomfort.

Condensation on windows is due to warm moisture-laden air being cooled to the point where it can no longer carry its moisture.  If the temperature of the glass is above freezing, the moisture condenses as dew.  If the temperature is below freezing, frost appears.

Condensation results from two things: 1) Moisture and 2) cooler temperature.  In a home these are represented by a high relative humidity in the warm inside air contacting a cold surface such as glass or metal.  In the absence of storm windows, the glass of the house can get very cold in severe weather, resulting in a heavy deposit of frost which later melts, causing damage to curtains, woodwork, and plaster walls.

Installation of storm windows will reduce condensation on the inner windows in all but very extreme cold weather, provided the humidity inside the house is kept at a reasonable level.  In fact, condensation on a window fitted with a storm window is a danger signal, warning that the humidity in the house is high enough to cause unseen condensation within the house walls where it can rot out the wood structure of cause cracking plaster and peeling paint.

Condensation on the inner surfaces of the storm window is an entirely different problem.  The moisture, which condenses on the cold glass of the storm window, comes from inside the house through cracks in the inner window.  It is impossible to entirely eliminate this leakage and it is impossible to eliminate the resulting condensation.  Therefore, try to locate the source and eliminate the excess humidity.

Except in severe weather good weather-stripping of the inner window plus adequate ventilation of the space between the windows to the outside wall keep this condensation below the level of a nuisance.  This ventilation should be sufficient to dilute the moist air leaking through the inner window with dry air outside to cause rapid movement of the air in the space between the windows.

Usually the weep holes in the storm windows are sufficient and this will cause no appreciable loss of insulation value.

Condensation will often occur in the mornings on the sunny side of the house due to the sun warming the woodwork or masonry between the windows and evaporating moisture, which has previously condensed there.  This moisture again condenses against the cold outer glass of the storm window.  While it may be annoying to be unable to see clearly through a frosted storm window, there is no danger of damage when this frost melts, as it is really outside the house where the masonry and the painted wood surfaces are not as subject to damage from water.

To summarize:  Storm windows save heat, increase comfort and reduce but do not eliminate condensation.  They do, in all but extreme cases, transfer condensation from inside to outside the house.