According to the EPA sealing air leaks and adding proper insulation to your facility can add up to 10% savings to your annual energy bills. To answer this question let's first define a few things. "A building envelope is the physical separator between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer. The three basic elements of a building envelope are a weather barrier, air barrier, and thermal barrier."
Weather Barrier Controlling Rain
Air Barrier Controlling and maintaining airflow within the building via energy management. Also controlling external air from infiltrating
Thermal Barrier Maintaining heating and cooling via insulation.
Next lets define typical components of a building envelope and typical air leaks:
Doors lack of proper weather stripping and doors left open are common practices we find during assessments. A door is typically 7'x 3', which is 2,100 cubic square feet of conditioned air flowing outside. A poorly insulated door perimeter, or a door with gaps in weather-stripping, make a big difference in building integrity. If you think about it air will look for ways to escape. Our job is to minimize building integrity as much as possible. This is certainly one way.
Windows A typical finding for us with windows are weaker single-pane windows. We also find poor insulation around the perimeter, cracks, poor shading, lack of tinting or more. Think about the amount of windows you have within your facility. The more efficient they are the stronger the integrity of the envelope. Perhaps you do not have the budget to replace windows, you could consider adding window storms to strengthen the efficiency. Other options are new dual or triple pane windows (argon-filled), thermal tint, proper shading to minimize thermal gain and more.
Foundation A strong foundation will minimize compromises from lower parts of the facility. A thermal scan can identify compromises.
Walls A thermal scan can also identify weaknesses (lack of insulation) in walls. Check your plug outlets, these too are areas where weaknesses are found. Add insulation around outlets, add insulated (energy star) outlets, or add plug cover to existing outlets.
Roof/Ceiling Insulation in the attic, located above the ceiling of the house or the floor of the attic separating it from the roof is very important. There are different levels of insulation based on US location that can be found here. Insulation partnered with radiant barrier for hot environments will maximize protection.
Examine the picture from Energy Stay above. The picture identifies other area; that compromise the integrity of the building envelope. Historically we spend most of our time investing on the internal components of energy management, and ignore the envelope. However this is just as important, and as we can see a strong building envelope will minimize energy use.