Vapor Retarders/Air Barriers

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Whether liquid or vapor, water in any form will seek its own level, and will travel through any avenue avail able to reach equalization. In a building this phenomenon is called vapor pressure, and it can have serious implications for your house. difference is about half (40°F).

The amount of moisture contained in the air varies with the air temperature; warm air can hold more moisture than cold air. This is the reason that meteorologists refer to the “relative humidity”—that is, the amount of moisture that is contained in the air relative to the amount of moisture it could contain at a given temperature.

Vapor Retarder Requirements: U.S., By Region: White area: vapor retarder to the inside of the wall/ceiling; grey area, no vapor retarder; blue area, vapor retarder to the exterior side of the wall/ceiling; i.e., under exterior siding.

As seasons change and outdoor air becomes colder, the relative humidity of that air drops. At that point, the warmer air inside the house will hold more moisture than the outside air. difference is about half (40°F).

Because of this difference in vapor pressure, the moisture will attempt to migrate through the wall and equalize with the dry outdoor air. This process, the migration of water vapor through materials, is called diffusion.

As the warm, moisture- laden air passes through the wall or ceiling, it may meet a cooler surface within the framing structure. The water vapor will then condense and form water droplets on the material at that point. difference is about half (40°F).

The point in the wall at which water vapor condenses is called the dew point. This moisture migration through the walls and ceilings of the house can produce wet and ineffective insulation, wood rot, mold or mildew, condensation and peeling paint on the house exterior. Vapor retarders/air barriers are intended to retard or bar the passage or diffusion of moisture or air through the walls or ceilings of a house.

The rate at which moisture penetrates or permeates material is a unit of measure called a “perm.” Technically speaking, one perm is one drop of water per square foot (0.1 meter) per hour, per unit of vapor pressure difference. A vapor retarder is a material that has a resistance to diffusion of one perm or less of water vapor.

Vapor retarders were once called vapor barriers, but the term was a misnomer because most materials will permit some vapor passage through the walls or ceiling and thus will not completely bar vapor or moisture passage but simply retard or slow it. A properly installed vapor or air barrier will greatly reduce the flow of moisture vapor and air from outside to inside, or vice versa.

Usually a 4-mil polyethylene film is installed on the warm side of the wall to serve as both a vapor retarder and an air barrier. Materials commonly installed between the sheathing and the exterior siding, such as 15-pound (7 kilogram) felt, Tyvek Housewrap, and paper- backed metal lath, are true vapor/air barriers.

Building Material R-Factors -- Insulation or R-factors of materials in exterior walls for brick veneer construction.

In houses that are more than 30 years old there usually is no effective vapor retarder in either walls or ceilings. This lack of a vapor retarder caused fewer problems in older houses because there was no insulation in the wall cavities—the space between studs or joists—to trap moisture. The moisture passed through until it met the only barrier in the wall: the exterior paint. The most common moisture complaint in older houses is peeling exterior paint. If you own one of these older houses, to provide a vapor barrier you can apply a coat of alkyd (oil) sealer or paint on the interior surface of exterior walls and ceilings. Then prepare and repaint the exterior siding with an acrylic latex paint. This latex paint breathes, or permits moisture to pass through the paint coat rather than peeling the paint.

Early types of batt insulation, common in the ‘60s, sometimes had a face layer of kraft paper or aluminum foil that was intended to serve as a vapor retarder, but most of that insulation was improperly or sloppily installed. For example, both kraft and aluminum foil-faced insulation batts had edge tabs that were intended to be folded together over the face of the framing stud or joist, and stapled tightly to those faces. However, most of that batt insulation was simply stapled to the sides of the joist, leaving open seams between the kraft paper or aluminum facing and the framing. Large holes were cut around electrical outlet boxes; there fore, the faced insulation contained many voids and did not provide a continuous vapor retarder.

Water vapor or air will pass through even a slight gap. For example, United States Gypsum, a major manufacturer whose wallboard trade name is Sheetrock estimates that assuming an interior/exterior pressure differential equal to a 9.3 mph (15 kph) wind, 31 pounds (14 kg) of water could pass in one month through a 1-inch square (2.5 centimeter) gap around an electrical outlet. If you multiply that amount of water by the number of out lets and other leaks in a house, you will see that we are attempting to deal with a serious moisture problem.

The second way in which water vapor can pass into a wall or ceiling is by air leakage, called convection. Because moisture and air can pass through a tiny void, extreme care must be taken when installing the vapor retarder. To provide a continuous shield, all sheets of polyethylene must be over lapped and the seams sealed with tape or acoustic sealant. Also, use tape or sealant to seal joints at any opening cut in the film, such as at windows, doors, electrical outlets, or heating ducts. Workers, especially drywall workers, electricians, and plumbers, must take care to avoid damaging the plastic while installing other finish materials.

No Vapor Retarders; Brick veneer construction w/o vapor retarder.

Retarder Voids: If vapor retarder is not sealed at electrical outlets, windows, etc., air passes into wall cavity and condenses on colder brick veneer.


It is important never to install two vapor retarders in the same wall, because moisture may become trapped between the two retarders and cause extensive moisture damage. For this reason, never install a vapor retarder in a bathroom tub/shower area that will be covered by ceramic tile. The tile itself is a vapor retarder, and installing polyethylene film beneath the tile substrate may result in moisture becoming trapped between the tile and the vapor retarder, with possible deterioration of the tile substrate and failure of the tile job. This is an important point: many how-to texts show illustrations in which a tile substrate is applied over a poly ethylene vapor retarder. This is a serious technical error, and can result in thousands of dollars in damage to the bathroom and tile.


Until the oil embargo in the ‘70s it was common practice to install a vapor retarder only in the walls. The theory was that wall vapor retarders would prevent moisture from migrating through the walls, wetting insulation, peeling exterior paint, and damaging building components. But moisture was allowed to pass by diffusion through the ceilings and into the attic, where it could be exhausted through roof/attic vents. In those days, when we were assured by utility companies that we had a plentiful supply of cheap energy, only 4 inches (10 centimeters) of fiberglass insulation was standard in ceilings, even in the coldest climates. Thus there was little concern that the escaping moisture might be trapped in the thin insulation blanket.

In an effort to promote energy conservation, building codes then were changed to require a vapor retarder in ceilings and walls as well as thicker attic insulation blankets that would trap the escaping moisture. Because water is a good thermal conductor, wet insulation will warm your house no better than wet socks will warm your feet. In climates where there are four distinct sea sons, houses built since 1980 have ceiling vapor retarders.

If you live in an area where there are freezing temperatures in winter, it is easy to check the effectiveness of both the ceiling vapor retarder and attic ventilation. Go into the attic on a subfreezing day and check trusses, framing, and the underside of the roof ply wood sheathing. Even in the best conditions you may see a very light layer of frost on the attic framing. If there is a moisture buildup in the attic you will see a substantial layer of frost on some or all of these components. If there is visible frost in only limited areas, suspect that there is a void in the vapor retarder at those locations. Inspect the area below the frost buildup for any leaks or voids in the vapor retarder, and seal those voids with tape or a sealant.

The code change requiring full vapor retarders has improved the energy or thermal efficiency of houses, but has created new problems. If the retarders are properly installed they can create the effect of living inside a plastic bag. Any moisture that enters the house interior cannot escape by diffusion or convection, and the moisture generated by daily family activities can build up to objectionable or even damaging and unhealthy levels.

One way to defeat high humidity in a tightly built home is to install an air-to- air heat exchanger. This is a device that has a duct that exhausts warm, moist air outdoors and a duct for dry incoming air. The ducts meet in an exchanger where the heat from the exhaust air is passed on to warm the incoming air, so that moisture is exhausted, but the heat in the air is reclaimed. If your contractor advises it, it is wise to install an air-to- air heat exchanger at the time of new construction, or to add one as a retrofit in a house that has problems with moisture. For a further discussion of indoor humidity problems, see Section 6, Humidity Problems.


To check whether your house has a ceiling vapor retarder, go into the attic and pull back the ceiling insulation. If you have no vapor retarder in your ceiling, many texts suggest that you remove the insulation and install strips of poly plastic between the ceiling joists, then replace the insulation. However, as mentioned above, moisture and air will leak through the tiniest hole, rendering the vapor retarder ineffective. Sealing the seams between the plastic vapor retarder strips and the ceiling joists will be difficult. There will be no continuous retarder because the plastic will not cover the wood joists, and there will be two seams to seal between every joist cavity.

If you decide to install a vapor retarder in an existing ceiling, cut the poly strips so they are 6 inches (9 centimeters) wider than the space between each joist. Remove the insulation between the joists and center the plastic strip in the cavity, so the edges fold up about 3 inches (7.5 centimeters) onto each joist. Now use an acoustical sealant to seal the plastic retarder to the joists on either side of the cavity. Carefully replace the insulation between the joists.

A simpler solution, and one that is much easier than working in an attic, would be to remove any ceiling light fixtures, and seal any gap between the wallboard or plaster and the light box with an acoustical sealant. This will reduce air convection and moisture flow through these openings. Then apply a coat of alkyd sealer to all the ceilings in the house. This will act as a vapor retarder from the finished side of the ceiling. Repaint or spray texture the ceilings as desired.


Conventional advice is to install the vapor retarder on the inside of the wall, between the wallboard and the studs. But in warm, humid climates, the outdoor moisture levels may be higher than indoor moisture levels, so the pressure for movement of water vapor may be from the exterior to the interior. Such a warm, humid climate is found in the southeastern coastal area of the nation, from the Gulf of Mexico up to the Carolinas. In those humid areas the vapor retarder must be installed over the exterior sheathing rather than on the interior walls. In a narrow band above the “Humid Climate” is a “Fringe Climate,” where no vapor retarder at all may be needed. Keep in mind that improper installation of a vapor retarder may cause extensive damage to the house structure. As noted in the Introduction, it is best to check with local contractors and building inspectors to learn the conventional building methods for your particular climate.

Adding a ceiling vapor retarder.


In Section 3, Waterproofing an Existing/New Basement, the importance of having a vapor retarder installed beneath the concrete basement floor was discussed. In houses that have a crawl space rather than a basement, it is also important to install a vapor retarder over the bare earth in the crawl space to prevent moisture migration from the soil. Crawl spaces should be built over well-drained soil, but even then moisture may migrate upward to cause a variety of problems within the crawl space.

Venting Soffits: At the eaves, be sure insulation does not block air flow from soffit vents into the attic.

First, install a vapor retarder over the bare soil. Because workers may be required to move about within the crawl space, the vapor retarder should be heavier than the 4-mil retarder recommended for use in walls. A 6-mu vapor retarder is generally considered adequate.

All seams in the vapor retarder should be over lapped by 6 inches (15 centimeters) and sealed. Use either tape or acoustical sealant to seal the seams. At the perimeter of the foundation, fold the vapor retarder up 4 to 6 inches (10 to 15 centimeters) onto the foundation. Seal these edges with acoustical sealant.

When the vapor retarder is in place and sealed, spread a 2- to 3-inch (5- to 7.5-centimeters) layer of dry sand over the entire retarder. The sand will help hold the vapor retarder in place, retard vapor movement into the structure, and help prevent damage to the retarder when workers must move about in the crawl space.

From the crawl space side, install faced insulation batts between the floor joists. The facing should be a vapor retarder of either kraft paper or aluminum foil. The vapor retarder should be installed toward the warm side of the floor. Note that, for the same reasons explained in our discussion of ceiling vapor retarders, this procedure will not provide a continuous vapor retarder under the floor, because the vapor retarder is interrupted by having a joint at each floor joist.

In cold climates, install the insulation with the vapor retarder facing upward; in warm humid climates, install the insulation with the vapor retarder down. If the vapor retarder is facing downward it can be overlapped and stapled over the bottom face of the floor joists, forming a better vapor retarder than when the retarder is installed with the facing up.

As further insurance to help keep the crawl space area dry, ventilation of the space may be necessary. Again, the requirements depend on the area in which you live; in cold climates, year-round ventilation of the crawl space is required to permit moisture to migrate to the outdoors. In warm, humid climates, the outdoor air may be more humid than the air in the crawl space, so the moisture transfer will occur from the outdoors into the crawl space. Check with local building inspectors for ventilation requirements in your area.

No Vapor Retarder: Moisture passes through interior wall and insulation and condenses on cold sheathing. Result: peeling paint, wet insulation.


As noted above, beside diffusion, moisture in the air can move through air leakage or convection. An air barrier bars the infiltration of outside air to the interior of the house, and bars the ex-filtration of conditioned (heated or cooled) air from the interior to the exterior. Otherwise, water vapor in the air will move through the walls or ceiling. In fact, United States Gypsum in their Gypsum Construction Handbook estimates that more than 230 times as much moisture moves by air leaks or convection than by diffusion. So, an air barrier or combination vapor retarder/air barrier is essential to control convective air movement and vapor transfer.

There are three factors that affect the air pressure and its flow from inside out or from outside in. The first is the stack or chimney effect, which means that warm air rises and attempts to exit through the upper walls or the ceiling. In winter, as the warm air rises, it produces an outward pres sure through the upper walls and ceiling, and creates a suction through the lower portions of the walls that pulls in cold exterior air. In summer, an air-conditioned house will produce a reversal of air pressures and flow direction.

Vapor Retarder

The second factor in air convection is wind pressure on the exterior of the house. The outdoor air movement causes a positive pressure on the side of the house that faces the wind (the wind ward side). Thus air will infiltrate from the windward side, causing a suction pres sure or ex-filtration of air on the opposite (or leeward) side of the house. When determining the effect of wind pressure on a house, consider the height of the house, because a two- or three-story house will be subjected to the increased wind pressure at higher elevations.

To minimize the effects of unequal vapor pressure, pay careful attention to ensure there are no voids that permit moisture to be lost through the walls. When building a new house or addition, inspect the vapor retarder/air barrier installation after all other work is done, but before wallboard, plaster, or paneling is installed. Be sure that all seams in the polyethylene film are overlapped 6 inches (15 centimeters) and are sealed with tape or other acceptable sealants. This overlap is especially crucial at the corners where walls meet walls, or walls meet ceilings. All interruptions of the film at windows, doors, plumbing, heating or electrical outlets must be sealed in the same way.

Vapor Retarder in Crawl Space: Cold Climate: In cold climates, a ground vapor retarder prevents moisture from rising to the floor above: insulation has vapor retarder facing upwards.

Vapor Retarder in Crawl Space: Humid Climate:In humid climates the floor vapor retarder is installed face downward.

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