What is the Thermal Envelope?

A home's thermal envelope is its exterior shell: walls, foundation, floors, ceiling, windows, doors, and roof.
 
A thermally enveloped house does not allow hot air in or cold air out. Most houses, however, are as pourous as Swiss cheese. Ignoring the holes that allow for such leakage will waste energy, descrease the home's comfort level, and increase your utility costs. 
 
Cold and hot air can escape from your home in a variety of places. The following are the most obvious areas:

1. The chimney. A poorly designed chimney can allow 14 percent of the air escape through the chimney.
2. The attic. The older the home, the poorer the insulation.  Attics in Arizona can reach 140 to 150 degrees F in the summer.  Penetrations for electrical wires, poorly insulated can lighting, and unevenly distributed insulation allow air to filter from the home to the attic and vice versa.
3. Windows. Windows account for 14 percent of the air lost in a home.
4. Light fixtures. Any place in the house that has an electrical fixture built into the wall is suspect. Wires have to travel through the walls. Because wires and walls are solids, they cannot occupy the same space. That means holes have been cut. Make sure that those wires are thick enough to plug the holes. If they are not, caulk or putty can seal them.
5. Doors. Poorly sealed doors account for 10% of the air lost in a home (assuming that your door isn't wide open all of the time).

Creating a thermal envelope is a worthwhile investment. It may be something that is easily put aside for a later date, but the faster you fix it, the faster it will save time, money and energy.