Because the price of gas, heating oil, and electricity continuing to increase every year. A typical U.S. home owner spends about $1300 a year on home energy bills. It's become more and more apparent that we need to find ways to save, and reduce our energy needs. Their are two ways of going about doing this. One is conservation. Turning down the the thermostat some more. Putting on a sweeter just to stay warm. Turning off more lights, and sitting in the dark more. The other way is higher efficiency. Another words doing the same task but using less energy.
One of the areas for needed improvement is insulation. Most homes built before 1980 do not have adequate insulation. A good question you should be asking is what is the R- value of my house?
A typical modern home looses about 20-30% of it's heat threw air infiltration. This is caused by air leaking threw holes, gaps, and cracks in the homes envelope. This allows cold exterior air to enter the home. With older homes this becomes a even more significant. So its important that home be more air tight.
Places to look for air leaks are around doors and windows. Use your hand to check for air seeping around door or window seals. You can also look in between the door, and door jam for light. If see light or feel air getting pass the door seal you will need to inspect the seals. Once it has been determined that the door seals are in good condition and the problem still persists. Try push on door to see if the air leaks seal up. If this is the case you may need to inspect the door latch, and door latch striker plate. These can become worn, or may need to be adjusted. Make sure that the striker plate is holding the door fully closed. 
By simple adjusting the striker plate so that it pulls the door more tightly against the door seal. This may fix the air leak. Checking door and window seals for damage. Repair or replace as need to keep outside air out. Another place to check for air infiltration is between the door jam and the wall. This can be done by removing the wood molding that covers the door frame to wall seam. Inside the gap their should be some type of insulation installed. If their is none. Fill this air gap with fiber glass insulation. You can use a ruler or any long slender object to push the insulation into the air gap. Once done check to make sure the door closes properly. Expanding foam insulation can force the door jam out of position, and interfere with the door closer. Replace the wood molding. The same procedure can be done on window frames that are leaking air into the house.
And of coarse their is the good old caulk gun. Caulking around door and window frames is a relative easy and inexpensive process. Caulking should be checked at least once a year. This should be done in the fall before the heating season starts. The reason caulking needs to be inspected is because over time the house has a tendencies to settle. The wood also expands and contracts with aging. This can cause gaps to open up.
Check around light switches, and electrical outlets for air leaks. You can spray foam in between the electrical box and the drywall. Before starting be sure to turn off the power at the circuit breaker box. Start be removing the cover plate only. Inject foam into the wall cavity. Do not inject any foam into the electrical box! If foam does enter the box. wait for it to harden, and use a plastic tool to remove the foam. Note: That on most homes built after about 1985 should not suffer from outside air seeping into electrical boxes. If you live in very old home you may have knob and tube type wiring. Do not mess with this type of wiring. This wire needs modification of any type should be performed by electrician.
Another place to look is the basement. If you look closely at this picture. You will notice that in between the floor beams on the left side that insulation has been installed. This is a relatively easy do it your self project. Also check for air leaks between the basement wall and sill plate. Note:Do not allow excess moister to buildup to occur. If you have a ventilated crawl space under your home. Care should be taken not to allow moister buildup by blocking any air vents.
If you have attached garage. Check the seals on the doors and windows also. This reduces the air turn over in the garage, thus trapping more heat.
When you replace the siding on your home. This presents a good opportunity to improve the insulation on your house. The exterior sheathing can be removed at this time. Addition fiberglass insulation can be added if the wall cavity is not completely full. The backs of electrical boxes can be sealed up with expanding foam. The exterior sheathing can be replaced with foam insulation sheets. By doing this you can further increase the R value of the walls. If you are replacing exterior sheathing with foam insulated sheets, be sure you have adequate diagonal bracing for structural rigidity. Also take note of how additional thickness will affect window and door frames.
Their is a word of caution not to make the home too air tight. Gas appliances such as stoves, water heaters get their air for combustion from inside air. In order for a gas water heater to vent properly. Their must be a source of make up air. This makes up for the exhaust that is going out the chimney. Appliance that have a seal combustion chamber do not need make up air. These can be identified by having a outside air inlet tube and exhaust tube.
See; Zero Energy Home.