Vapor barrier

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Generally, vapor barrier is a term often used when discussing humidity (or vapor pressure) where vapor (e.g. water vapor) is prevented from moving between discrete spaces or volumes. Because no material or installation is perfect, moisture will still cross the surface to be protected; the formal term is therefore vapor retarder. Similarly, air barriers are intended to eliminate air infiltration, but are more accurately entitled air retarders. A single material that is intended to reduce both air and moisture movement is therefore an air and moisture retarder.

For example, the roof, walls, and windows in a house attempt to keep the air inside and outside the house separate. Any gaps (e.g., open windows or open doors, or gaps around these) will not be a barrier to vapor movement. Usually, if air can pass, so can vapor.

More specifically, a vapor barrier is a material that is impermeable to vapor. They are usually made out of plastic, such as low-density polyethylene. Whatever material is used, it must have a permeability rating of 1 or lower. Outside of the tropics, vapor barriers are usually installed on the interior side of a wall where the interior is warmer than the exterior. They protect the wall and insulation from being damaged by condensation. The air inside a structure is warm and warm air can hold more moisture than cold air, according to Psychrometrics. When warm air hits a colder surface, like the inside of a wall, it may release the water it is carrying as condensation. This can cause rotting or mold growth. The situation is reversed - and somewhat less serious - in more tropical locations, since indoor air conditioning seldom produces the temperature difference which would be experienced across the wall on a cold winter's day in a northern climate. The simple rule is that the vapor barrier should be on the warm side of the wall, in order to prevent the moisture from that warm air condensing inside the wall. (As always, consult with your local building codes.)

Another definition of vapor barrier, worded slightly differently:

A vapor barrier is any material, usually a plastic or foil sheet, that resists passage of both air and moisture through walls, ceilings, and floors. They help prevent interior moisture from penetrating into and condensing in unheated attics, basements, crawlspaces, and wall cavities. This is especially important in well-insulated homes, where there is often a great difference in temperature between the air in conditioned space and the air in unconditioned space. The vapor barrier is placed in between the insulation and the conditioned space, usually stapled to the studs.

A vapor barrier must be continuous to work properly. There should be no tears, and you should seal electrical receptacles and switches, windows, and locations where walls meet ceilings and floors. Some people think that vapor barriers are inherently flawed solutions for preventing moisture from entering wall cavities, because you poke them with hundreds of holes when you staple them to the framing studs, and you poke them with hundreds more holes when you install the drywall. Furthermore, there is no way to make sure that a contractor installed the vapor barrier properly once the drywall is installed. Using closed-cell spray foam between framing and closed-cell rigid foam over sheathing is more expensive than fiberglass and cellulose, but closed-cell foam resists air infiltration, is virtually impermeable to water, and makes a vapor barrier unnecessary (since the wall cavities are kept warm).

Vapor barriers have permeability ratings of 1.0 or lower, indicating their resistance to the passage of air (including moisture). Do not use asphalt felt paper as a vapor barrier. Asphalt felt paper has a permeability rating of approximately 1.0, so it doesn’t do a good job of trapping water. There are several alternatives that are far superior.

1 Materials used as vapor barriers:
2 Effects of not having a vapor barrier
3 Where to install
4 Vapor barrier under concrete slabs
5 Vapor barrier in basements or crawlspaces
6 See also

[edit] Materials used as vapor barriers:

Aluminum foil (virtually impervious to water, with a permeability of 0.0001).
Paper-backed aluminum.
Polyethylene plastic sheet, 6 mil or greater in thickness (permeability is 0.06).
Kraft paper, often attached to one side of fiberglass batts. Fairly high permeability vapor barrier, compared to polyethylene. Very useful in climates where drying to the inside of the wall is necessary (during cold/humid weather). Its vapor permeance changes depending on the specific relative humidity.[1]
Vapor barrier paints or vapor resistant paints (for retrofits where you don’t want to remove and destroy finished walls and ceilings, OR for dry basements and crawlspaces).
Certain types of rigid insulation with lapped joints will act as vapor barriers
Most sheet type monolithic roofing membranes will act as vapor barriers.
Glass and metal will act as vapor barriers. This is exemplified in doors and windows.

Many people confuse vapor barriers and air barriers or assume that they are one and the same; this is often a false assumption. Many vapor barriers can act as air barriers but only if they are strong enough to sustain the air pressure loads placed upon them. Air barriers need not be vapor permeable. An example of this is a roof membrane in a flat roof application. In the case of the roof membrane the integrity of the edge condition is critical.

Vapor barriers should be placed on the "warm" side of the insulation. In areas where you heat more than air condition, say north of the 40 degrees latitude, you do place the vapor barrier on the inside or warm side but in areas where you air condition more than heat you should place the vapor barrier on the outside or "warm" side to prevent condensation. To do otherwise is to court moisture buildup in the cavity.

[edit] Effects of not having a vapor barrier

If there isn't a vapor barrier, or if one is installed wrong or damaged:

Humidity and moisture collect in insulated areas. This is especially true for walls insulated with fiberglass, which is susceptible to air infiltration.
Condensation can occur inside walls, especially if the house has impermeable siding (aluminum, vinyl), which traps the moisture.
Condensation lowers the effectiveness of insulation, and ultimately causes peeling paint, mold, and rot.

If you do not have a vapor barrier in some section of your house it is exceedingly difficult to go back and install one because the wallboard must be removed and reinstalled. You should at least increase the ventilation to compensate for the lack of vapor barrier. For example, install ridge, gable, and many soffit vents in the attic.

[edit] Where to install

Install a vapor barrier in any wall, ceiling, or floor that is adjacent to unconditioned space, such as in the exterior walls of a house, or in the ceiling below an unheated attic. In all of these cases, you install the vapor barrier between the drywall and the framing studs, usually stapled across the front of the studs.

You should never install a vapor barrier on the exterior side of exterior walls, between the studs and the sheathing. This will trap moisture in the wall. Instead, using building wrap, which cuts air infiltration but allows moisture to escape.

Installing a vapor barrier backwards can do more harm than good, trapping water where you should not have water. When they are necessary, vapor barriers should always go on the side of the insulation that is warm in the winter. In other words:

In a cool or cold climate, the vapor barrier should be installed closest to the living space, against the warm wall, ceiling, or floor. Faced batts should be installed face down in the attic, and face up in the basement or crawlspace.
In a warm climate, or in a house that uses air conditioning during most months of the year, the vapor barrier should be installed furthest from the living space, against the warmer, open air in the wall cavity, attic, basement, or crawlspace. Faced batts should be installed face up in the attic, and face down in the basement or crawlspace.
In a “lukewarm” or humid climate, such as in the Southeast USA, consult your local building department or the vapor barrier manufacturer. There is much confusion and controversy in these areas of the country, and some people think you should leave the vapor barrier out entirely. Some people think that, in general, vapor barriers are a flawed solution that traps moisture inside walls. If in doubt, check your local codes. File the proper permits, and have your local building inspector sign off on the work.

Things to remember:

If you install wet-spray cellulose, you must allow it to dry completely before sealing up the wall with a vapor barrier and drywall.
Don’t install a vapor barrier between layers of insulation. This will trap moisture inside the insulation, reducing its effectiveness and attracting mold growth. People frequently, but mistakenly, add faced batts to pre-existing unfaced fiberglass batts, or put loose-fill on top of pre-existing faced batts, when adding insulation to the attic.
Don’t install multiple layers of vapor barrier - this will trap moisture between the layers. For example, do not add vapor barrier-faced insulation batts over existing faced batts. If unfaced batts are not available, slash or strip the vapor barrier from the old batts before installing the new ones.

[edit] Vapor barrier under concrete slabs

You should install a thick, cross-laminated, high-density polyethylene vapor barrier under the concrete slab when building a house. Concrete keeps out most liquid water when used as floor, but it is porous, so some moisture will always wick up and transpire into the air. Here is the general sequence of tasks:

Excavate for footings, walls, slab, and sump.

Pour footings. Pour walls, or build concrete block walls. Include foundation vents in the walls.

Install plastic sump in sump hole.
Apply crushed stone before installing the polyethylene vapor barrier. This channels water to the perimeter drain and sump pump.
Install a thick, cross-laminated, high-density polyethylene on top of the stone. Putting sand or stone on top of the vapor barrier and under the concrete creates a catch-basin for water that may remain for long periods, never draining away and gradually wicking up into the basement and creating a damp slab.
Install a built-in perimeter drain to divert water away from the walls. (Even with the vapor barrier, water will often work its way around the edges of the slab and the vapor barrier. The purpose of the perimeter drain and sump pump is to collect this water and pump it away from the house.)
Pour the slab.

[edit] Vapor barrier in basements or crawlspaces

Regarding basements and crawlspaces in general:

You should not install vapor barrier paint on interior foundation walls of damp basements or crawlspaces. If excessive moisture enters the house through the foundation walls, you should remedy the problem at its source - from the outside.
Most of the condensation that occurs in dirt-floor crawlspaces and basements with concrete foundation walls comes directly from the earth, even if it looks bone dry. You should always cover a dirt floor with a vapor barrier, whether the dirt floor is in a basement or a crawlspace.
When you apply vapor barrier to the floor of your dirt basement or crawlspace, you should not install the vapor barrier directly against wood posts and studs. This will trap moisture against the wood, encouraging rot.

How to insulate and install vapor barrier in crawlspaces in cool climates:

First, insulate the rim joists (the joists on top of the foundation) with rock wool batts, closed-cell spray foam, or closed-cell rigid foam, cut to fit. Do not cover the foundation vents.
Install closed-cell rigid foam panels against the “knee wells” (the inside of the foundation walls). Cover with drywall, if they are highly flammable.
Install rigid panels at least two feet onto the floor.
Always cover the dirt floor and the “knee wells” with 6-mil or thicker polyethylene vapor barrier. Overlap seams in the vapor barrier 6 inches, and tape securely. Weigh down the vapor barrier with stones. Cover the insulation with the vapor barrier, and secure the vapor barrier to the band joists.
If the crawlspace is heated, you are done.
If the crawlspace is unheated, another vapor barrier covers the area between the insulation and subfloor. Install closed-cell spray foam or rigid panels between the floor joists. These insulate the floor above, and also serve as your vapor barrier.

How to handle moisture when finishing a basement:

You should never finish a consistently damp or wet basement. You should only finish your basement if it is dry and has been dry for several years. If the basement has any seepage or condensation problems, STOP. Find out whether the moisture is coming from the outside (seepage) or from the inside (condensation from the air). You can do so by sealing all sides of a square piece of polyethylene to the basement wall with tape for a few days and then comparing the area under the polyethylene sheet to the remainder of the basement wall. If the wall underneath the square is darker than the rest of the wall, you have seepage. If it is lighter, you have condensation. If you cannot stop the moisture, you should leave your basement unfinished until you find the solution to the problem. You can still insulate the interior foundation walls – just don’t install studs and drywall.

Advantages of insulating an interior foundation wall:

Reduce noise coming from outside.
Prevent condensation and, therefore, mold (especially during summer when humidity is high and basement walls are cool).
Keep basement warmer in winter.

If you finish your dry basement, and the homebuilder did waterproof and insulate your exterior foundation wall, then you probably should not install a vapor barrier on the interior basement foundation wall, since you will trap moisture in the wall between the tar or concrete sealant on the outside and the moisture blocking paint or water sealant on the inside. This will cause mold and eventual disintegration of the concrete.

If you finish your dry basement, and the homebuilder did not waterproof and insulate your exterior foundation wall, then you probably should install a vapor barrier between the drywall and the insulation, or between the insulation and the concrete basement foundation wall, depending on your climate.

Regardless of whether you install a vapor barrier when you finish your dry basement, moisture will always find its way into the space between the basement foundation wall and the finishing. Even with a vapor barrier, this moisture will eventually wick into your insulation, studs, and drywall, because there is no perfect vapor barrier. Therefore, you should leave a drain plane for proper ventilation. This is a gap of a few inches created next to the basement foundation wall, using furring strips, for example, to allow moisture to evaporate or drip down into the perimeter drain leading to the sump pump. This will help prevent mold, rot, and concrete and mortar disintegration.

Once you properly install the vapor barrier and installation to the basement wall in the dry basement, and leave a drain plane for ventilation, then you can finally install the framing studs and drywall. Keep an eye on the humidity, and increase ventilation if necessary.