Solutions to the Problems

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effectively reducing the Fv	Multiple panes with energy films and coatings	Numerous coatings have been developed for glass panes that reduce the amount of radiation that can be transmitted through them. One such highly successful coated glass is called Low-E glass. See Diagram 6 below for further details on the manner in which Low-E glass functions.
	Controlling air leakage around the window	Utilize weather-stripping, caulking, and screw-type closures on windows with ability to open. Inspect the weather stripping and caulking for cracks and/ or peeling annually.
	Use materials with low thermal conductivity for the frame and other structural components.	Wood is fair, wood buffered with thermal breaks (such as extruded polystyrene) is better, fiberglass is also fair, but must be completely sealed on all surfaces. See Diagram 7 for an example of thermal breaks in a window frame.
	Take advantage of window placement (passive solar heating) by facing the majority of windows within 30 degrees of south.	Planting deciduous trees in front of these windows will help prevent overheating during the summer and yet have nearly full solar heating during the winter.
	Consider shutters	Shutters, insulated, un insulated, permanent, temporary, seasonal or nightly, all will help retain radiant and conducted heat lost during the night.
	Utilize blinds,	Drawing the curtains at night can save up to 10% on ones annual energy bill ( 7. How to Insulate Your Home and Save Fuel) because it adds insulation to resist heat transfer by conduction, it changes the radiant surface (especially if the window side of the blinds are made with mylar or other shiny polished metal layering. ) If the blinds are designed in such a manner as to be air-tight, they will also reduce heat flow by conduction (here it is important that the material used has a low permeability to air.
	Relocation of low activity areas	To reduce the felt effect of radiant heat loss, avoid placing sofas, chairs, etc. close to windows, instead set them back a small distance to retain the aesthetic view, (See Diagram #9 below) but also effectively reducing the View angle (Fv) in the Stefan-Boltzman Equation for Radiant heat loss. (See Methods of Heat Transfer page). (1. Northern Comfort)

A Note on Solar Gain:

"The area of south windows should be moderate: large enough to admit much solar radiation on sunny day, but not so large that heat-loss on cold nights is very large or heat-gain on hot summer days is very large. Typically, the area should be about 5-10% of the floor area." (8. Super Insulated Houses and Double Envelope Houses)

Since Radiant Heat loss accounts for a large component of the energy transfer through a window, reflective coatings, known better as Low-Emissivity or Low-E coatings reduce this radiant heat loss. These coatings are rated by their emissivity, a value between 1 and 0, with 0 being a perfect reflector and 1 being a perfect black body emitter.

Low E coatings have a disadvantage however, in that they tend to decrease solar transmission as the Emissivity is reduced, thus reducing the benefit of passive solar heat gain. Low E coatings tend to have an apparent green metallic tint, caused by the nature of low emissivities being associated with polished metals. (See emissivities table on bottom of Methods of Heat Transfer page)

Thermal Breaks are often constructed of extruded polystyrene rigid foam insulation, which helps to reduce the fringe effect of the conductive framing elements.

Note: The optimal distance between two panes of glass is 3/4", this generally increases the R value of the window by 1-1.25 for air filled windows and 1.5-2 for argon filled windows. It should be noted that the R value of the window will marginally increase as the temperature decreases, (due to the increasing density of the medium filling the window reducing convective currents). (1. Northern Comfort )

The method of thinking shown on the right takes advantage of the cooling due to radiation by placing a high energy, high heat activity (children's play area, weightlifting bench etc.) close to the thermal sink of the window.

Keep in mind that this is not a complete list of ways to avoid excessive heat loss through a window, merely the tools and some basic ideas to understand the process at work and how to go about countering them. There is still much active research in the field of Arctic window technology, with some windows approaching R values of 9 or 10 (hr)(ft)(ft)F)/BTU. Utilizing these tools and the formulae provided, anyone can secure their home from the striking heat loss of inefficient windows.