[Editor's note: When this article first appeared in print, the title was offered tongue-in-cheek; we didn't really expect it would be the last word. Fast forward 25 years and we are still getting the same type of questions from readers. Over that time period, the building code has evolved, although it still has a ways to go on the topic of vapor retarders (see "Avoiding Wet Walls," May/2017). The biggest change in the code is that air barriers are now required, and in most places in the U.S., the air barrier must be verified with blower door testing. In this article we have made a few changes to reflect current code. What hasn't changed, and remains in this article, is the basic science of how water vapor moves through building assemblies and the guidance, expressed in as straight-forward a way possible, on how to manage it.]
We receive more questions each year about vapor barriers and their close cousins, air barriers, than any other single topic. In an effort to clear up any lingering confusion about these pesky membranes, we’ve assembled this list of the most common queries along with the clearest answers we could provide.
Q. What is the difference between a vapor barrier and an air barrier? Between a “barrier” and a “retarder”?
A. A vapor barrier is a material or coating that significantly reduces the passage of water vapor by diffusion through wall, ceiling, and floor materials. In general, its purpose is to keep moisture from getting into exterior wall and ceiling cavities, where it can condense on framing members and sheathing.
Diffusion is the transfer of moisture through tiny pores in building materials. The amount of diffusion through a material depends on the material’s permeability and the vapor pressure that pushes the moisture through. It’s a scientific fact that moisture will always move from areas of higher temperature and relative humidity (higher vapor pressure, to be precise) to areas of lower temperature and humidity. The greater the vapor pressure difference, the greater the push.
In cold climates, the vapor pressure is always higher indoors during the winter, so the moisture wants to move towards the outside. In hot, humid climates, the reverse is true — the moisture usually wants to move indoors if the building is air-conditioned.
In climates with both heating and cooling, the vapor flow changes with the seasons and weather, but it’s usually the strongest during colder weather when it’s heading from the inside to the outside.
An air barrier is a different animal altogether. It blocks the passage of air through leaks in the walls, ceilings, and other building components. Because air often carries a lot of moisture, air barriers and vapor barriers work together to keep moisture out of building cavities. A single material such as polyethylene often performs both tasks, and hence is often called an air/vapor barrier.
Most codes require a vapor barrier of one perm or less. Researchers, however, have found that air leakage generally moves a lot more moisture than diffusion. Therefore the air barrier is much more important than the vapor barrier. So if you’re concerned about really keeping moisture out of your wall and ceiling cavities, you ought to be paying close attention to the air barrier. That means using 1.) a continuous, airtight membrane, and/or
2.) a lot of caulk, foam sealant, and gaskets to block up any significant leaks (see “Air Sealing Details”).
As to the difference between a “barrier” and a “retarder,” there is no practical difference, only a semantic one. A material is frequently identified as a "barrier" when below 1 perm, and as a "retarder" above 1 perm. But almost no barrier material is perfect. At best, any material with a perm rating will only slow down the passage of moisture or air, but will never stop it completely. Only a material, such as aluminum foil, with 0.0 perms will completely stop vapor transmission.
The bottom line: You need an air barrier in all climates. The colder the climate, the more you need pay attention to the vapor barrier, as well.
Q. Does a vapor barrier need to be installed perfectly to be effective?
A. No. At a given vapor pressure, the amount of diffusion you’ll get is directly proportional to the surface area that is exposed to the moisture. So if you have only 90% of your walls covered with poly, your vapor barrier will still be 90% effective.
This is not true of an air barrier, however, since air seeks the path of least resistance. Under pressure (air pressure, that is, not vapor pressure) a great amount of air can leak through even a small crack in a building — just as a lot of air can rush out of a small hole in a balloon.
How fanatical you need to be about vapor barrier installation and air sealing depends on 1.) how moist the interior of the building will be, and 2.) how cold the climate is.
An office building that is unoccupied at night and where no cooking or bathing takes place generally has pretty low indoor humidity levels, whereas a small house where three teenage children take two showers a day will likely have quite high levels.
As for climate, a house in northern Minnesota is much more prone to moisture problems than one in southern California. So you’ll need to pay attention to local codes and practices, or talk to an experienced energy consultant.
The bottom line: Don’t knock yourself out with the vapor barrier, but do a thorough job with the air barrier in all climates.
Q. Should the vapor barrier go on the inside or the outside of a building?
A. In [climate zone 4C, 5 and above ] code requires you to put a vapor barrier on the inside. That’s because the strongest vapor flows are during cold weather from indoors out.
Vapor flows from the outside in are significant only in very hot, humid climates (CZ 1 and 2). In those areas, there is a great deal of controversy over whether you need to install the vapor barrier on the outside or leave it out altogether. Leaving it out is probably the most prudent course, provided you do a good job sealing the building against air leaks.
The bottom line: Put the vapor barrier on the inside in cold climates and leave it out in hot, humid climates.
Q. Is paint a vapor barrier? How about black paper, housewrap, and plywood?
A. A material’s perm rating indicates the ability of moisture to diffuse through the material. The lower the perm rating, the better a material’s resistance to moisture diffusion. In general, codes call a material a vapor retarder if it has a perm rating of one or less.
From the table above you can see that a couple of coats of oil-based paint can act as a pretty effective vapor barrier. However, 15-pound asphalt-impregnated felt is not a vapor barrier. Kraft paper is a vapor retarder when relative humidity is low, but it absorbs moisture when the relative humidity is high, and it's permeability increases. So Kraft-faced batts will allow a wall cavity to dry to the indoors in winter when indoor relative humidity increases. OSB, which is not listed on this table, is around 2 perms. A wall sheathed with OSB will eventually dry to the outdoors if the sheathing is not continuously wetted by condensation.
A useful rule of thumb is to keep the outside of the wall five times more permeable than the inside. To calculate the added value of two or more layered materials, you need to add the inverse of their perm ratings. This inverse figure is called a rep, and is a measure of a material’s resistance to vapor transmission.
The bottom line: Keep the outside of the building significantly more permeable to moisture than the inside and you’ll avoid trouble.
Q. If the house is covered with a housewrap on the outside, do I still need a tight air/vapor barrier on the inside?
A. Yes. Housewrap on the outside, even if the seams are taped, will not keep air from leaking into the walls and ceiling. Housewrap functions primarily as a wind barrier, preventing air from blowing through the gaps in the sheathing and around the band joist. It also acts as a secondary rain barrier during construction, and after the building is complete, it protects against any water that leaks behind siding and trim. But because housewrap is typically installed only over the wall sheathing, it does not form a continuous envelope around the house. You still need to seal the most prominent air leaks in a house — around doors and windows, along the mudsill, behind tubs and cabinet soffits, and penetrations through the ceiling.
The bottom line: In a climates zones, install a continuous air barrier even if you use housewrap on the exterior.
Q. Where should the vapor barrier go in a finished basement?
A. Moisture control in the walls of a finished basements is tricky, since the vapor drive between the soil and the basement is small and could go either way depending on conditions. The most common practice is to place the vapor barrier below the wall finish as you would with an above-grade wall. This is a safe bet because it keeps heated basement air from contacting the cool basement walls where it might condense. Dampproofing or waterproofing the basement wall on the exterior is also helpful for keeping soil-borne moisture from migrating into the building.
The bottom line: Treat below-grade walls the same as above-grade walls.
Q. Are vapor barriers needed below slabs? What if the vapor barrier gets ripped when the concrete is placed?
A. In a new home, it’s imperative to install a vapor barrier beneath an enclosed slab since ground moisture evaporating up through a slab is a major source of indoor humidity. Here the plastic sheeting is functioning as a capillary break, keeping moisture from wicking into the slab.
Don’t worry about punctures or small tears since, as we said, a vapor barrier that covers 90% of the surface is 90% effective. For best results use a heavy-duty poly made for below-grade use.
The bottom line: Put a vapor barrier below slabs in new construction.
Q. Are vapor barriers needed in crawlspaces? Do you have to seal the edges to the foundation?
A. A vapor barrier is essential on the dirt floor of a crawlspace to keep moisture in the soil and out of the house. Even if the soil appears dry, a lot of moisture evaporates up into the house, particularly if the water table is high. Moreover, a wet crawlspace with no ground cover and inadequate ventilation can spawn all manner of fungi on the floor structure because you’ve got the two conditions necessary for rot: damp wood and warm temperatures. This can happen in any climate that has warm weather part of the year.
Unless you are creating a sealed crawlspace, and intend for the crawlspace to be inside the conditioned space of the home, you don’t have to seal the edges of the poly to the foundation. Remember the 90% rule stated above.
The bottom line: Always put a ground cover in a crawlspace.
Q. How does foam insulation on framed walls affect moisture problems?
A. In very cold climates, foam on the interior of a wall can work okay, since it serves as an interior vapor barrier. However, there is one important difference. Because of the high R-value of the foam, the wall cavity is now significantly colder, and moist air that does get past the foam is more likely to condense inside the wall. So it’s super important to seal the foam well against air leaks with construction tape and foam sealant.
When foam is used on the exterior, it usually contradicts the old rule of thumb — to keep the outside of the wall five times as permeable as the inside. Yet it usually doesn’t cause problems. Here’s why: Although the foam will tend to trap moisture in the wall, it also warms the wall cavity, making it less prone to condensation. Moisture that stays in its vapor state and does not condense is harmless to the building. Therefore, it’s best to use a relatively permeable foam such as extruded polystyrene rather than the foil-faced products. Also, the colder the climate, the higher the R-value you’ll want on the exterior foam to keep the wall cavity warm (see “Where Does the Dew Drop?”).
The bottom line: If you put foam on the interior, seal it well. If you’re going to put foam on the exterior, use an adequate thickness for the climate.
Q. Do you need to add a vapor barrier when you retrofit insulation into walls or ceilings?
A. An uninsulated leaky wall is generally not prone to moisture problems for two reasons:
1.) the wall cavity is relatively warm from leaking household heat, and
2.) the wind blows through and clears out any moisture that gets in. Once you insulate, however, the wall becomes colder at the outside sheathing and ventilation in the wall cavities is reduced. That means there may be more condensation on the sheathing, which can lead to possible exterior paint problems. There’s a fair amount of anecdotal evidence linking retrofit insulation with peeling paint. Therefore, it’s a good idea to add as good an air and vapor barrier as is practical.
If the wall is older, it probably has more than a couple of coats of oil-based paint on the plaster, so the vapor barrier is already there. As for the air barrier, caulk as many cracks as you can find at baseboards and other trim, and seal electrical outlets with foam and gaskets. When sealing an old house, it’s worth working with someone with a blower door. By pressurizing the house with a blower door, you can pinpoint leaks that might be quite significant, but are not immediately obvious.
The bottom line: You should try to seal the walls against moisture transport whenever you retrofit insulation. Pay particular attention to air sealing.
Q. Does it do any good to have a vapor barrier only in an addition? Only in the ceiling?
A. If a modern addition has modern insulation, it ought to have modern air and vapor barriers. As for ceiling-only vapor barriers, it’s not something we recommend for new construction. But since the ceiling — particularly cathedral ceilings — are one of the most likely places to have moisture problems, ceilings are the best place to concentrate your air (and in cold climates, vapor) sealing efforts. Ceilings and attics are more vulnerable to moisture damage than walls because convection currents carry lots of warm, moist air into those spaces.
The bottom line: When you add on, add an air barrier throughout — not just in the ceiling. In very cold climates this advice applies to the vapor barrier, as well.
Q. Do vapor barriers contribute to moisture problems or air quality problems inside a home?
A. No, but air barriers might, since a good air barrier will prevent moisture and indoor toxins from dissipating. Remember, however, that the best way to reduce moisture problems in the walls is to reduce excess moisture in the home or vent the moisture directly at its source. That means you should install good bathroom and kitchen exhaust fans; vent the dryer outside; eliminate uncovered crawlspaces; and don’t air-dry laundry or store firewood indoors. Similarly, the best way to reduce air quality problems is to keep strong toxins out of the home or ventilate them directly. For example, avoid products with a high formaldehyde content, such as cheap wood paneling, and ventilate hobby areas, such as a photo darkroom, directly outdoors.
With very tight construction, a whole-house ventilator, such as an air-to-air heat exchanger, might be required.
The bottom line: Build tight and ventilate right. The tighter the home, the more you’ll need to reduce moisture and pollution sources and add mechanical ventilation.
