In the process of air sealing a building, you’ll encounter all manner of leakage paths, including deep holes, cracks, joints between sheets, flat joints and 90-degree joints. The type of sealant you use depends on the type of leak.
Solid blocking and backing: Although it may seem counterintuitive, solid materials are important elements of a continuous air barrier. Most framing configurations include spots where the air barrier must cover a large gap across a cavity. In many cases, fire blocking is required in these locations, and adding a bit of caulk or foam will complete the seal. Rigid sheets are required to cover certain areas that are often overlooked, for example, the area behind showers and tubs. The walls behind fireplaces need to be covered and the fireplace cavity capped when these areas are in contact with exterior walls or ceilings. Generally you can use wood scraps for most of this work, keeping cost down.
Glue: Construction adhesive can be an effective sealant for specific applications, such as fixing wall framing to sheathing and under bottom plates. Because glue is rigid, there is some concern that it may crack as the building shifts over time. However, two advantages of adhesive are its low-cost and ready availability on most job sites. Just be sure to order extra for the air sealing function.
Caulk: Most caulk is designed to fill a joint that is no more than ½-inch deep and ½-inch wide, although products called elastomeric caulks can fill larger gaps. Joints that are the correct width, but too deep, such as the gap between a window frame and the rough opening, can be packed with backer rod first and the remaining space filled with caulk.
Foam: For larger gaps, expanding foam is an effective sealant. Dispensed from canisters through a gun, foam will fill gaps up to a couple of inches wide. However, such large gaps may need to be covered with a scrap of solid wood or OSB first and then foam filled in the remaining gaps. A good example of this is the large hole around a tub drain trap. Take care to fill gaps entirely. Openings around pipes can be difficult to see and suffer from an incomplete seal. One thing to note is that foam becomes brittle over time and may break if building elements, especially plumbing drains and vents, move over time.
Tape: In recent years, construction tape has become an important material for air sealing flat joints. It is often necessary to seal the joint where two framing members or sheets of wall sheathing touch each other. There is little if any gap to fill with caulk or foam. Tape easily spans the gap. Be sure to use construction tape that is very flexible and designed to adhere to wood, such as 3M Flashing Tape #8067.
Ecoseal: Intended for flat joints where framing members and sheet goods meet, this is a low-VOC sealant. This proprietary system is applied by trained applicators just before insulation. Ecoseal is elastomeric and so it will stretch, rather than crack, when the house settles. Despite its flexibility, it is not capable of filling gaps more than ¼-inch wide.
Duct mastic: This paste-like substance is intended for ducts, but solves the dilemma of getting a good seal on electrical boxes. A thick layer of mastic on the back of each box will seal wire penetrations and unused knockouts.