Low emissivity (low-E) denotes a material's ability to restrict the transmission of radiant heat.  Such materials are designed to help keep heat inside in winter and outside in summer.

R-value measures a material's resistance to heat loss.  The higher the R-value of your insulation, the more effective it will be. 

The U-factor measures the rate of heat loss and how well a product insulates.  The lower the number, the better that product is at containing heat.  It's key in choosing cold-weather windows.

The SHGC - Solar Heat Gain Coefficient - measures how well a window blocks the sun's heat.  The lower the number, the better.   In intensely sunny climates, windows can help keep the sun's heat out of a house.

Image courtesy of Pella.com

Bay, bow, casement, French casement, clerestory, quatrefoil, single-hung, double-hung, sliding, awning, tilt-turn, Palladian, triple-pane, double-pane, single-pane, picture, oriel - windows come in every imaginable shape, size and type. They add natural light to penetrate the thickest building wall. They give a insider a view of the outside. They add architectural interest and air flow to our buildings. They can also be useful in shepherding energy.

In our 21st century energy paradigm, efficiency is the key to window performance. They need to help our homes stay cool in summer and warm in winter.

Insulating existing windows

METHOD 1. Winterize with PLASTIC - cheap & easy

Here's a method of saving on heating costs that can benefit renters, as well as homeowners, with existing single-pane windows.

Lawrence Berkeley National Laboratory says that "window film is the most cost-effective way to upgrade a building's windows."   But be careful to select a film that will not only significantly block undesirable heat and cold, but will transmit at least 70% of visible light.   In addition, this product is not recommended for double-pane windows.  In fact, some double-pane window manufacturers will void their warranty when film is used.

METHOD 3: Thermal WINDOW INSERTS - a.k.a. interior storm windows

These consist of Plexiglas or acrylic in a vinyl or aluminum frame, which is inserted over the inside of an existing window. Some insert manufacturers sell kits for the do-it-yourselfer, others construct to your measurements. One outfit builds an insert that flips from insulator in winter to reflector in summer. Homes owners with single-pane windows, clerestories and/or skylights may find these inserts useful.

METHOD 5. Rebuild old double-hung windows

There is another possibility for owners of old or even historic homes, whose windows bespeak their architectural character, but have become drafty with age. When plastic muntins simply won't do, and the cost of new windows with individually divided glass lights is prohibitive, consider rebuilding your old windows. Here are some pointers.