Air sealing a home is generally considered the most cost-effective way to reduce space heating and cooling energy use. It also improves comfort, and indoor air quality. Achieving a high level of air tightness requires more than just filling visible holes, it requires several well thought out steps. First, identify the thermal boundary. Next, ensure that there is a continuous air barrier along the entire boundary.
The key word here is continuous. The secret to continuity is to identify the large sheets of existing material, such as the exterior structural sheathing or the interior drywall, and then to bridge the gaps – or joints – between them in order to completely seal the boundary. Next, select the best air-sealing method for your project. Several good solutions exist for creating a continuous air barrier. Some are based on skilled use of materials, such as air-tight drywall (ADA) and adhesive attachment (gluing) of the structural framing to the sheathing.
Others, like SIGA membranes and ZIP System, are proprietary and therefore slightly more expensive. And a newer process, called Aerobarrier, has achieved very high levels of air sealing.
Of course, there is a cost to any advanced air sealing method. Some approaches use less expensive materials, but require more labor and greater skill. Proprietary systems reduce labor, but have a higher purchase price. Every building professional must balance these tradeoffs to arrive at the desired air sealing goal with an acceptable cost. One point to keep in mind is that air sealing approaches generally cost less for a given amount of energy reduction than do other building systems, such as heating and cooling equipment or solar panels. Your energy model will be a useful tool to find the right balance for your situation.
Set an Air Tightness Goal
Based on energy modeling, set a goal for air leakage in air changes per hour under 50 Pascals of pressure (ACH50) using a blower door test. Fifty Pascals of air pressure is the common standard used on blower door tests. A typical airtightness goal for a cost-effective zero energy home would be below 2.0 ACH50.
Sealing the Exterior Wall Surface
Conveniently, the outside surface of most modern homes is covered with large sheets of structural sheathing (plywood or OSB). Exterior sheathing can become a continuous air barrier by laying a solid bead of construction adhesive around the perimeter of each sheet as the wall is assembled. Another continuous bead should be placed below bottom plates before walls are tipped up. These measures lock the sheathing to the outer face of the wall framing members and to the bottom plates, blocking otherwise major air leakage pathways. Framers already have tubes of construction adhesive on hand, although they will need more than they usually use.
To maintain continuity, the air barrier must be carried from the exterior sheathing to the ceiling drywall. The wall’s top-plate serves as the bridge. A gasket or Ecoseal on the inside face of the top plate will connect the ceiling drywall to the air barrier system.
Sealing the exterior has a major advantage. It seals most of the building including many of the framing connections inside the house. Even so, pay special attention to the continuity of the air barrier around windows, doors, bottom plates, band joists, penetrations, and the joints between roofs, walls, and floors. Special care should be taken where multiple framing members occur together, such as built-up posts and rough openings. Air leaks where framing members meet can be significant and worsen as the framing dries and shrinks. Make sure all exterior sheathing is securely nailed. Seal any holes or breaks. High quality tape, such as 3M Flashing Tape, is very flexible and adheres well to wood, which makes it great for covering joints and gaps.
In double-stud walls framed with separate bottom and top plates, there is a gap that must be covered with plywood. Use strips of plywood or OSB to span the top plates as well as the window and door openings.
This approach of using sheathing as an air barrier is considered by most builders to be the lowest cost, although it is labor intensive.There are proprietary exterior sealing systems, too. ZIP Systems supplies special sheathing and sealing tape to span the joints. The sheathing has a weather barrier layer attached and ZIP offers a line of insulated sheathing, too. SIGA provides a wide range of sheeting and flexible tape that functions as both an air barrier and weather barrier.
SEALING THE OUTER WALL
Sealing the Inner Wall Surface
In most homes, sheets of drywall cover the walls and the joints are covered with paper tape and drywall compound. This creates an additional air barrier inside. One key location to air seal is the gap where drywall touches the top plate of the wall framing. To seal this location, staple a gasket to the face of the top plate before drywall is installed. A material commonly used for this purpose is the strips of “sill-sealer” intended as a moisture break between a concrete foundation and a mudsill. It’s thin and compressible and works well as a top-plate gasket, too.
It’s also possible to use either drywall adhesive or Ecoseal. While drywall adhesive costs less than Ecoseal, adhesive must be installed during the process of hanging the drywall and it is difficult to confirm that it’s done correctly. Ecoseal has the advantage of being a separate process that can be monitored. Because Ecoseal stays soft, it will adhere to drywall even after several days.
SEALING THE INNER WALL
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: 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. In other locations, rigid sheets are required to cover certain areas that are often overlooked, for example, the area behind shower units, under tubs and below stairs. The walls behind fireplaces need to be covered and the fireplace enclosure covered when these areas are in contact with exterior walls or ceilings. Thin sheets of rigid insulation or Thermoply are commonly used. Or, you can use OSB or framing 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. Once cured this glue is rigid, prompting 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 work.
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 a backer rod or stuffed with fiberglass insulation 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, larger gaps may need to be covered with a scrap of solid wood or OSB first and then foam applied in the remaining gaps. A good example of this is the large hole around a tub drain trap. 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. Where 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 or SIGA tape.
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.
Aerobarrier: Aerobarrier can seal an entire building in a few hours using it’s proprietary aerosol sealant. This process offers the advantage of hitting very low air leakage targets and maintaining predictable results across many projects. Read more details in Is Aerobarrier the Future of Air Sealing?
Penetrations through the air barrier complicate air sealing, so minimizing penetrations can reduce time and money spent on air sealing. Here are some things that can be done to reduce penetrations:
- Avoid recessed lighting in insulated ceilings. Use track lighting, pendants, or recessed cans placed in soffits that are within the conditioned space to keep the air barrier intact. Some new LED ceiling lights offer a look similar to recessed lights without a large hole in the ceiling. Each LED fixture requires a ceiling style electrical box, which will need to be sealed, but this is much easier than with the old-style recessed light.
- Eliminate bathroom exhaust fans by installing an energy efficient ERV/HRV ventilation system. Be sure they are sized properly and meet local code.
- Choose a ductless heat pump (mini-split HVAC system) since penetrations for refrigerant lines are much easier to seal than those for large ducts.
- Avoid plumbing in outside walls wherever possible. Try to locate sinks and other fixtures on interior walls. Only hose bibs should be installed on the outside walls.
- Kitchen range hoods can be eliminated by using an ERV or HRV vent in the kitchen, provided it is permitted by code. It is important to locate the ventilation exhaust point at least six feet from the range to reduce the accumulation of grease in the ducts. The ERV/HRV exhaust in the kitchen should be sized properly to meet the airflow required by code.
All HVAC equipment and ducts should be inside the conditioned space. Ducts can be built into chases or soffits so that they are inside the thermal barrier.
Sealing Electrical Boxes
After all electrical boxes and wiring have been installed, seal them with spray foam or duct mastic. Be sure to cover all wire penetrations and unused knockouts. Once electricians have installed plugs and switches, the boxes should be sealed to the drywall with caulk or a foam gasket.
Airtight Electrical Boxes
Airfoil airtight electrical boxes and Lessco airtight electrical boxes – although considerably more expensive than conventional single boxes – are preferred in some applications. They are important to use in vaulted ceilings and other vulnerable locations where a standard box cannot be sealed easily. Boxes in flat ceilings often can be sealed from the attic space, but if access is not available, airtight boxes would be appropriate in flat ceilings, too. Airtight boxes in exterior walls may be useful if the air leakage target is below 1.0 ACH50. They are not necessary on interior walls.
Sealing Windows and Doors
Window and door frames should be sealed to the wall frames with caulk, foam or flexible tape depending on the size of the gap. Casement and awning windows are preferable from an air leakage standpoint because the sash presses against the gasket when closed. This forms a better seal than a sash that slides across the weatherstripping, such as a single-hung window or slider.
Attic and Crawl Space Access
When possible locate attic hatches and crawl space access doors in places where they will not penetrate the air barrier. For example, the attic hatch can often be located in a garage or gable end wall. Crawl space access can be placed in an outside wall below the level of the insulated floor. Place weatherstripping around all openings.
Double Entry Doors
If entry doors are exposed to cold winds, two doors can be installed to form an air lock. These double-door entries reduce drafts and increase comfort, often leading to lower thermostat settings. Double-door entries may not be cost-effective in all homes, but if the home is well designed, they can create a useful space, such as an entryway with a coat closet, or a mudroom.
Three-Point Latching Mechanisms
While three-point latching mechanisms for doors are considerably more expensive than ordinary latches, they hold the door tightly against its weatherstripping and resist warping. They are especially important where double-door entries are not feasible, and where the doorway is exposed to cold prevailing winds or intense sun.
Air-Tight Vent Covers
Where external vents are used – such as for a clothes dryer – select vent covers that are as airtight as possible. An added advantage of using an air-tight vent cover is that it will help keep rodents and other pests out of the home.
Air Sealing Checklist
Using a simple checklist is helpful for systematically documenting and sealing every possible air leak during blower door tests. All potential air leakage sites identified on the checklist and in the design should be added to the scope of work of the appropriate subcontractors to be sure a thorough job is done. Use the ENERGY STAR National Rater Field Checklist to identify areas that need sealing and use the excellent air sealing diagrams from Green Building Advisor.
Blower Door Directed Air Sealing
Blower door directed air sealing involves an air leakage test early in the course of construction. It is most often used as an educational process so contractors and crews can learn the finer points of air sealing. When their knowledge and skill level has reached a point where the crew can achieve consistently low air leakage results, only the final test may be needed.
For successful blower door directed sealing, set up a blower door test after the plumbing and electrical systems have been roughed in. Also, the ceiling drywall needs to be installed and sealed ahead of the normal construction sequence. Installing a ceiling plane creates a pressure boundary for testing while the potential leakage sites on the outer wall can still be fully accessed for locating and sealing any leaks. Exterior sheathing should be sealed as much as possible, but neither insulation nor the drywall are installed on the walls. Once the obvious gaps have been identified and sealed, the blower door should be run almost continuously to discover additional leaks using a smoke stick or your hands. All detected leaks should be sealed and rechecked while the blower door is running.
A final blower door test should be conducted after all fixtures, finishes and trim have been installed. This last check for leaks gives the final ACH value of the home. Be fanatical, systematic and persistent in finding and sealing every leak, while learning, check your progress with the blower door as you go.
Blower Door Test Challenges
Like most instruments, blower doors have a margin of error. Several factors can affect the test results, including wind, barometric pressure, and differences in operating procedures. At the level of air tightness needed for a zero energy home, these variations can be significant. In an attempt to better standardize the test, the same door should be used each time. Avoid testing on days when wind speeds exceed 15-20 mph. Extreme weather conditions and large shifts in barometric pressure should also be avoided. Placing the blower door in the door to the garage with the overhead door open may reduce exposure to wind. A multipoint test at various test pressures can be conducted and the curve can be calculated, which will give a more accurate reading. The Minneapolis Blower Door Tectite software is a helpful tool for multipoint testing.
Attention to Detail
Whatever the materials and techniques used for air sealing, the most crucial technique is to take your time, pay close attention to detail, and double check everything. With care, practice, and experience, high levels of air tightness can be achieved at minimal cost.
For more on best practices for sealing air leaks see Tape It? Seal It? Glue It? Sealing Weather Barrier Seams. Detailed illustrations of air sealing procedures and problem areas can be found in the ENERGY STAR Thermal Enclosure System Rater Checklist.