Builders often refer to the exterior of a home as the "envelope" or the "shell." Sealing the envelope or shell against air infiltration (air leaking into the house from outside) and air exfiltration (air leaking from inside the house to the outside) helps reduce your energy expenditure for space heating and cooling. Besides, no one likes to live in a drafty house.

In this article, we'll show you a variety of ways you can seal leaks and improve insulation to make your home cozier and more energy efficient. Once you've sealed and insulated the weak areas, the work doesn't stop there. We'll show you how routine cleaning and water conservation can increase energy efficiency and save you even more money on your energy bills. Begin the improvement process with the following basic sealing guidelines to help you secure your home's exterior.

Testing for Leaks

Technicians use a "blower-door test" to accurately measure air leakage in houses. The test involves sealing a portable, frame-mounted fan in an exterior doorway to the house. Any known openings to the outside, such as the fireplace flue; bathroom vent fans; and the flues to the water heater, furnace, or boiler are temporarily sealed.

After the sealing and setup is complete and the blower fan is switched on, it is possible to measure with precision how much air is entering the house through all the various "unintended" cracks, gaps, and holes in the exterior envelope. Using devices called smoke pencils, technicians can pinpoint areas where air is entering the house while the blower door is in operation.

While every home is different and each has its own set of leakage points, there are areas where infiltration shows up repeatedly in blower-door tests. These often include the seam between the top of the foundation wall and the wood framing that runs above, around, and through doors and windows; along baseboards; through electrical receptacles and switches mounted on exterior walls; and around fireplaces, laundry chutes, attic hatchway doors and pull-down stairways, whole-house fan installations, and pipe and wire chases. A homeowner can go a long way toward increasing energy efficiency by locating and sealing up as many of these entry points as possible.

You don't necessarily need to have a blower-door test done on your home in order to locate the unsealed areas that are leaking air. Knowing that these points of air entry have been routinely and consistently identified in other houses gives you a start on where to look for gaps and cracks in yours. A windy day outside can be helpful in this endeavor. Wind can push air into the house through unseen and unnoticed holes to the point that you can feel the air movement.

Checking Exterior Sheathing

Before plywood and oriented strand board (OSB) were invented, homes were built entirely with solid board lumber. The exterior was sheathed underneath the siding with wide boards that, over time, shrank and cracked. All these cracks -- and the many others inherent in most homes -- are pathways through which air can enter or leave a house. Sidewall sheathing is covered with siding, so all those cracks that appear in and between wide boards on older homes are hidden and inaccessible.

Air enters the sheathing through cracks in the siding; around windows and doors; and through other openings in the exterior envelope that include kitchen and bathroom vent fan louvers, dryer vents, holes bored for air-conditioning lines, electrical and gas service wires and pipes, along the underside of the lowest course of siding, and through other holes. Any time you can find and seal a crack on the exterior of a house, you go a long way toward reducing air infiltration and exfiltration on the inside.

Because of the large size of plywood and OSB sheets, there are relatively fewer seams in the sheathing on newer homes. And the use of products like house wrap on new construction has further reduced air infiltration. Consequently, most new homes are more airtight than older ones. But although the sheathing might be more airtight in a newer home, there are still many places where air is getting in and out. Finding and sealing those leakage points not only reduces drafts and energy usage, but it also helps keep out insects and other pests.

Filling Holes Around Lines

HVAC system installers need to bore a large hole through the exterior wall of the house in order to pass refrigerant lines through to the compressor outside. Most take time to caulk the hole around the lines, but the caulk fails over time, often leaving a gap where air (and insects) can infiltrate the house. A few minutes spent with a caulk gun will close the gap and shut off the flow of outside air into the house.

Caulking the Exterior

Some people find that once an older home has been freshly painted, they suddenly feel warmer or "cozier" inside during the winter. That may be because the painter who worked on the house took time to caulk cracks, gaps, and other holes in the home's exterior "skin." While minute gaps around doors and windows might not seem as though they could possibly add up to much, under certain conditions it is surprising how much air they can let into and out of a house.

Consider a windy day. Wind drives air into gaps and around obstructions. Add rain to the mix, and you've got the recipe for both water and air infiltration. So caulking pays off in regard to both energy savings and building preservation. That's why you don't need to wait until it's time to paint to caulk visible openings on the exterior of your house.

Securing the Perimeter

The wooden framing in most homes rests on top of a solid concrete or concrete block foundation. In homes built before 1980 or so, the lowest section of wood, called the "mud sill," rests directly on top of the concrete. While the connection is secure from a strength standpoint, in terms of eliminating air infiltration, things could be much better. The problem is the rough and variable surface of the top of the foundation wall. While there are many areas where the wood presses down tightly, other areas may leave a gap through which wind can enter.

The gaps, which collectively might add up to a hole the size of a basketball in the exterior envelope, can usually be sealed with either caulk or cans of spray foam. This procedure, which can be done either on the inside or outside of the house (depending on which offers the best access) requires that you first brush away the dirt and cobwebs from the concrete and wood so the caulk or foam will stick to both surfaces.

From that point on it's just a matter of aiming the caulk tube's tip or spray foam applicator tube at the gaps and gunning them full of caulk or foam. It's a job that doesn't have to be neat or precise, just thorough. Once you're finished, you will have stopped up one of the leakiest places in the home.

In newer homes, the gap between the mud sill and the top of the foundation wall is filled with a thin, compressible length of foam material. The foam creates an airtight seal that does not need remedial caulking or foaming. However, it's worth checking along this area anyway, as occasionally the foam sealer didn't get placed exactly where it should have been. Also, the top of the foundation wall might be too uneven for the foam to fill the gap, someone might have forgotten to put it in place, or it might stop short of the corners. In any of those cases, a shot of caulk or foam can quickly remedy the problem.

In the next section, we'll discuss some basic sealing techniques you can use indoors to help make your home more energy efficient.