When you bought your home, you probably were not wondering whether the building envelope was airtight. Nor did you appreciate what a big effect the airtightness of your home's envelope would have on your energy costs. You are not alone. Just as the general public doesn't understand the benefits of airtight building shells, most builders and subcontractors do not understand how to achieve airtight building shells. What this means is that most homes are full of air leaks. Some of the leaks are obvious; some are more obscure. To find and fix the very small leaks, you would probably have to hire a professional. However, a handy homeowner can seal up the obvious leaks. Here's a strategy to follow, and a list of the materials that work best.

In deciding how best to create an airtight home, the first question to ask is, What type of climate do you live in? In climates with cold winters, it's preferable to seal air leaks from inside. Air inside the home is more humid during the heating season, so moisture tends to travel from indoors to outdoors-and it's better not to have moist air getting into your walls. Stopping air movement by creating an air barrier on the inside surface of exterior walls and ceilings prevents warm, moist air from migrating into the attic and wall cavities. In warm, humid climates where central air-conditioning is used, the air barrier is more useful on the building's outside surface. These are just general recommendations because other factors, such as which surface has the most seams, also exercise a strong influence on where to create the air barrier. To get a complete analysis of the best way to air seal your home, you would want to consult a building scientist or home performance contractor.

Seal the Biggest Leaks First
Sealing air leaks, or "air sealing", would be easy if all air leaks were direct leaks. You could just look for daylight shining through the building shell. Closing up these gaps should certainly be a priority as sealing all the visible cracks in the building's interior and exterior walls will definitely slow air leakage, but these gaps should be considered as just the first stage in a larger air sealing plan. Start with sealing large air leaks at the building's surfaces-around doors, windows, and where two walls meet--with permanent airtight patches. Then move on to plugging major leaks around plumbing, wiring, and flues-if you can get to them. Joints in forced-air heating and air conditioning ducts are usually cost-effective to seal-but don't use duct tape. (For more information on ducts, see "Those Wild Ducts in Your Walls".) Holes in attics, under floors, and above suspended ceilings are usually accessible, but other large air leaks within the wall cavities may not be so easy to get to.

Finally, consider if and how you need to treat the following common problem areas.

Attic and basement stairways
Think of a stairway as a rectangular hole in one level of your home that leads to another level. If a stairway leads to an area that is not heated or air conditioned, such as an attic or basement, then particular care needs to be taken to make sure that this hole is plugged. An insulated and weather-stripped hatch cover or door can be installed at the top of the stairs. Or, the stairway's walls and the stairs themselves can be air-sealed and insulated.

Recessed light fixtures
Recessed light fixtures, especially cylindrical ones called "cans," are often a direct leak through the air barrier. Typically installed in soffits, cathedral ceilings, and suspended ceilings, recessed light fixtures connect the heated or air-conditioned space, known generally as conditioned space, to unconditioned spaces, such as attics or roof cavities. When these fixtures contain incandescent bulbs, they must be ventilated by holes in their shell to purge heat from the fixture. This ventilation is a code requirement to prevent fire hazards, but unfortunately it allows air to move through the walls of your home.

Not only do these fixtures exchange air between conditioned spaces and building cavities, recessed light fixtures also can allow warm, moist indoor air to reach cold roof decking, causing condensation. The best remedy is to replace the fixture with a similar fluorescent fixture, which produces only a quarter of the heat and doesn't need venting. You can also build an airtight box to cover the fixture, but precautions must be taken to prevent fires.

Chimneys
Chimneys are designed to move lots of air. Many older homes, especially large ones, have multiple fireplaces and chimneys. Any fireplace chimney has the potential to be a massive air leak if it has a damper that is leaky or wide open, or if the chimney simply has no damper. Unused central heating chimneys also can steadily pull conditioned air from the home. Inflatable pillow-like bags or flexible foam plugs are effective for sealing fireplace chimneys. The tops of unused chimneys should have metal caps to prevent water from eventually damaging the home.

Remodeling Opportunities
Since sealing the less accessible regions can be crucial, air sealing should be a primary consideration in any remodeling project. (For more information on incorporating energy efficiency into remodeling projects, see No Regrets Remodeling.) In fact, air sealing can be more efficient if it is done in conjunction with the renovation, because contractors may need to use destructive methods to access air leaks. For example, contractors may have to drill through interior wall surfaces to blow insulation into walls or drill through flooring to insulate a rim joist area between floors. Renovators can patch, repaint, and install new flooring after air sealing.

Repainting presents an opportunity to caulk all the cracks in the wall and ceiling surfaces for both airtightness and visual appeal. Recarpeting or installing a new floor presents the opportunity to seal the floor-wall junction-often a source of considerable air leakage. Installing high-density wall insulation can be timed with siding replacement if there is room within the wall cavities. This is also a terrific opportunity to install a vapor-permeable air barrier paper under the new siding.

Choosing the Appropriate Material
For those leaks that you feel competent to tackle on your own, you will need to figure out which of the many available air sealing materials are best for sealing up which kinds of holes. The most common types of air-sealing materials and their applications are discussed below. The choice of air-sealing methods and materials depends on:

• the air leak's size, shape, and location;
• how accessible the leak is;
• the air-sealing material's compatibility with the existing wall materials; and
• the air pressure that the patch or sealant will have to resist.
  

Caulk and Mastic
Applied as a gel that flows at room temperature, these sealants fill small gaps and cracks in the walls. Selecting a caulk involves consideration of: the substrate materials (the materials around the crack), the gap size, and weather exposure, and an understanding of how walls or wall sections may move.

Caulking can stop both air leakage and water leakage. Caulking and mastic seal any consistent cracks that remain relatively stationary. For sealing joints in metal, glass, and plastic-materials that move significantly with temperature-the sealant must be very flexible and have good adhesion. Many glass installations use gaskets- a flexible material designed to seal a gap between two less flexible materials-instead of caulking, because the gasket will accommodate more movement while maintaining a seal. Exterior caulks must resist ultraviolet radiation, moisture, and chemicals in the air.

Caulk is applied with a caulking gun It is applied to seal joints and cracks that measure 1/8 to 3/4 inch wide. Common caulks, like acrylic latex, siliconized acrylic latex, and butyl, are adequate for gaps less than 3/8 inch between wood and other common building materials. Siliconized acrylic latex is adequate for stationary gaps less than 3/8 inch outdoors on low-rise buildings.

Larger gaps and moving joints require one of the moderate-performance caulks, such as butyl rubber and polysulfide rubber. These moderate-performance caulks have good adhesion and flexibility, but they are not as weather-resistant as polyurethane and pure silicone, which offer superior weatherability, adhesion, and flexibility. When in doubt about selection, especially when replacing failed caulk, use a high-performance caulk like polyurethane or silicone. Polyurethane has the best adhesion of common caulks.

Caulks are sold in a wide variety of formulations. Read the specifications carefully, especially when choosing a caulk for joints bordered by different substrates.

Installing Caulk
Exterior joints, designed to resist movement and weather, require careful preparation. Priming both sides of the gap is sometimes necessary. Backing-usually flexible, smooth polyethylene-foam rod-is advised for gaps larger than 1/4 inch. The backing should not adhere to the caulk, because adhesion may tear the bead or the backing during joint movement.

The gap's width and the caulk's consistency affect: the preferred angle of application, the size and angle of the tube's tip opening, and the installation technique. Pulling the gun toward you usually gives adequate results. However, pushing the caulking gun forces the caulk into the gap more powerfully, if that is necessary. Pushing the gun is a little more difficult because the caulking gun tends to block the technician's view of the newly laid caulk, making it more difficult to control the quality of the emerging bead. The technician should know the desired shape of the bead and should look carefully at the caulking bead and at a cross-section of the gap to ensure that the caulk fills the gap and the installation technique is working.

The cut angle of the tube's tip is important to the bead's shape and its penetration into the gap. The angle should generally be between 50° and 75° from the long axis of the tube. This angle allows the technician to tip the caulking gun at 50° to 75° to the line of the crack. This range of wider angles sends the caulk more directly down into the crack. Cutting the sharp point of the tip back at the opposite angle from the main cut is also helpful. This second cut will help achieve a healthy bead that rises slightly above the joint, giving extra bulk to resist cracking and tearing.

Interior caulking is a little different than exterior caulking. Interior caulking usually suffers little or no movement, and priming and filling are usually not necessary for gaps up to 5/16 inch. Use backer rod for larger cracks. Avoid smearing the bead with your finger unless it is necessary to smooth it. Smoothing the caulk smears it into a wider area and makes it thinner and more likely to crack. The most important aspects of interior caulking are the appearance of the bead and its penetration into the crack.

Film or Flexible Sheeting
Film sheeting is used to cover holes leading to hidden areas-suspended ceilings and unconditioned crawl spaces, for example. Flexible sheeting often is used for permanently sealing large air leaks that are hidden from view. Polyethylene film, along with its cousin, cross-linked polyethylene paper, are the most common types of flexible-film sheeting. These materials vary in thickness from .003 to .008 of an inch. Common polyethylene film is also a vapor barrier. Cross-linked polyethylene isn't a vapor barrier, but instead is designed as a breathable air barrier that lets water vapor pass through. If patching a vapor barrier, it's important to use a vapor barrier material. If there is no existing vapor barrier, it may be better to use patches that aren't vapor barriers.

Films are the easiest air-sealing materials to cut and fasten, and they work well when the pressure difference across the leak is small. Staple films into nearby wood. Use caulking or construction adhesive for additional adhesion and permanent sealing. Some types of caulk and construction adhesive will hold the flexible patching materials without staples.

Thin Panels
Rigid materials are the best for sealing large air leaks permanently. A rigid patch resists pressure and mechanical damage better than a patch made of more flexible materials. Rigid patching materials include plywood, lightweight steel or aluminum sheeting, rigid insulation, and rigid plastic sheeting. Thin panels, like 1/4-inch plywood or wood paneling, also are used to cover surface penetrations leading to hidden areas. Thinner gauge materials are easier to cut and fasten.

Gaskets
Gaskets seal joints between metal and glass in window systems and serve as thermal breaks between conductive building materials. Other gaskets are designed to seal between building materials in the construction of an air barrier.

Tapes
Tapes can be effective air-sealing materials when used exactly as the manufacturer specifies. Contractor's tape is a versatile product that can seal a variety of materials. Air-barrier tape is made by the manufacturers of air-barrier paper to seal the paper at its seams. Tapes are more prone to failure than are other types of air-sealing materials because their thin adhesives are prone to drying and failure from material movement.

Tapes often fail to permanently seal ducts because they may have been applied to dusty surfaces, or because their adhesives heat-dried and failed. High-quality foil tapes are available that adequately seal metal ducts and duct board. However, even the highest quality tapes will eventually fail from air pressure or gravity pull. Butyl-rubber-backed foil tape is the best type of duct tape. It sticks well to clean metal duct surfaces, but it is very sensitive to dust and requires careful installation.

Adhesives
Adhesives are used to adhere films, thin panels, gaskets, and tape. Thin adhesives, like those used on duct tape and plastic self-adhering weatherstrip, require very clean and very smooth surfaces to adhere. Adhesives applied in thicker layers are appropriate for most rougher or textured building surfaces. Common adhesive will bond to a variety of materials. Other adhesives are formulated for specific purposes like gluing foam insulation to concrete.

Some caulks and sealants have good enough adhesive qualities to be used as adhesives. These include polyurethane foam and caulk, duct mastic, and siliconized acrylic-latex caulk.

Hand Stuffing
You can stuff foam rubber and fiberglass insulation into voids to reduce air movement through the void. Stuffing and filling materials are used in cracks, crevices, and cavities whose interior surfaces can't be sealed due to lack of access and too many seams. By filling the cavity's whole void, you can retard leakage through penetrations and airflow through the cavity. Fibrous insulation isn't an air barrier and must be packed densely enough to provide significant resistance to air movement. Enclosing fibrous insulation in a plastic bag can provide a much more effective air barrier than the insulation by itself.

Space Fillers
Space fillers perform the same function as hand stuffing materials described above, but they are installed with mechanical help from compressed gas or an insulation blower. Polyurethane foam in squirt cans is a space filler most often used for small cavities. Blown cellulose and fiberglass insulation are space fillers for larger spaces. Density is crucial for the success of blown insulation used as a leak stopper.

Liquid plastic foam fills large and variable-sized cracks very effectively. Urethane-based foam, which comes in expanding or non-expanding varieties, is superior to caulk for filling large cracks with varying width and depth. It is destructive to skin and fabrics, so use appropriate protective measures. Polyicynene foam is used as a sprayed or injected insulation and is also a good air sealer.

Cellulose and fiberglass loose-fill insulation are good air sealers for inaccessible building cavities providing air-leakage pathways. Cellulose is superior to fiberglass because it packs tighter and has smaller fibers that are driven into small gaps during installation. However, cellulose can absorb a lot of water from leaks and high humidity. Technicians can seal areas where they can't even crawl or reach by using fill tubes to blow tightly packed insulation into the cavities.

Time to Call in an Expert?
Caulking and weatherstripping are effective tools to seal obvious air leaks around doors and windows. Indeed, these materials, along with insulation, are the first line of defense when trying to stop unwanted air leaks. However, applying caulking and weatherstripping while neglecting larger hidden holes ultimately will have little effect on heating or cooling costs. If you find that your air sealing efforts are not paying off in lower energy costs, it may be time to call in a home performance expert to identify and treat the more difficult, hidden air leaks.