Sealing Up Your Home's Leaks
Your house's walls and windows-what building professionals call the building envelope or shell-are supposed to prevent air from moving through them.
June 16, 2001
Article and photographs by John Krigger, director of Saturn Resource Management, based in Helena, Montana
|Your house's walls and windows-what building professionals call the building envelope or shell-are supposed to prevent air from moving through them. In the winter time, the building envelope works to keeps the heated air inside the building and prevents the cold outside air from coming in. Likewise, in the summer, the envelope should keep the cooler air inside your home and should block the hot outside air from seeping in. Unfortunately, most houses are not built as well as they should be, and air leaks inside from the outside, and vice versa.|
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
Finally, consider if and how you need to treat the following common problem areas.
Attic and basement stairways
Recessed light fixtures
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.
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
Caulk and Mastic
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.
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
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.
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.
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.
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?
|Article and photographs by John Krigger, director of Saturn Resource Management, based in Helena, Montana|
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