Vapor Barriers - Where and when do you need them?

The single most common building science question I get from home builders, engineers, code officials and architects is, "Do I need a vapor barrier?" The queries come from all corners of the continent, and from people involved in all kinds of buildings. The answer is usually simple, but first one has to know more about the question and the specific situation.

Remarkably, the decision of where and what kind of a vapor retarder to use depends on scientific principles. In my work as a structural engineer, I have never been asked, "Do I need an 8-inch deep I-beam?" Everyone in the building business accepts that such a question is naïve, and impossible to answer without more information. The same is true about vapor barriers.

The most important fact to recognize is that an air barrier and a vapor barrier perform different functions. Both control the movement of water vapor into enclosures, and hence both aim to control excessive (i.e., damaging) condensation from occurring. Vapor barriers reduce (they do not stop) moisture transport driven by diffusion. Diffusion is simply the movement of water vapor from regions of more to less vapor. Air barriers control the moisture that is transported along with airflow. Airflow is driven by air pressure differences, and it moves air and its water vapor in the direction that the air pressure drives it. Air barrier systems are needed in just about every type of building, whereas specific vapor retarding layers are not needed in many situations. Air barriers must be sealed, continuous, strong, air impermeable, and stiff. Vapor barriers need only be vapor impermeable, and sealing joints, small punctures, and cracks is not usually necessary.

The widespread confusion between air barriers and vapor barriers has perhaps arisen because some materials, glass, and sheet metal can be used as part of an air barrier system while at the same time acting as a vapor barrier. An example does not prove the rule of course, and materials like unpainted gypsum board can be excellent air barrier materials while providing little vapor diffusion control. Add to this mix materials like unsealed or torn polyethylene, which act as a vapor barrier but not as part of an effective air barrier system, and the confusion becomes understandable. Building codes add to the mess by requiring vapor barriers (arbitrarily defined as materials with a permeance of the conveniently round number of 1 US perm) while remaining mute on the much greater need for air barrier systems. This stance is simply technically incorrect.

Manufacturers and others often use the term air-vapor retarders to effectively limit the whole discussion to only one limited choice-that of a system which combines the material and location of both airflow and vapor diffusion control functions.

Why not just place a vapor barrier on both sides of an enclosure all the time and stop worrying? Because this is the surest way to rot, corrode, and otherwise destroy your building in record time. Vapor barriers stop drying, and so we must be sure that we do not use a vapor barrier on the wrong side or, even worse, on both sides, since this will practically stop any moisture that might (will!) get in from drying out. To avoid creating vapor barriers on both sides we sometimes use ventilated cladding systems that would otherwise be vapor barriers (e.g., metal and vinyl siding).

To decide how to control vapor diffusion properly you must have information about three different aspects of your specific situation: the exterior climate, interior conditions, and the properties and arrangements of the enclosure assembly. Lets consider each.

Exterior Climate

As mentioned above, vapor diffusion moves from more to less. For a hot, humid climate like Miami, Florida, where the vapor outdoors is higher than indoors almost all the time, it stands to reason that you should place a vapor barrier on the exterior side of the wall assembly. Not all codes recognize this yet, but it is a fact. Similarly, for a climate with less moisture outside all the time (e.g., northern Alaska) a vapor barrier should usually be placed near the interior. For all other situations, we need to know more before we decide.

It must also be remembered that "outside" could also mean the conditions created behind rain-wetted, absorbent cladding (like brick, cedar shakes, stucco, wood, cement board) exposed to sunshine. This creates a "climate" outside of the wall or roof similar to a sauna, and hence drives moisture inward. For enclosures with absorbent claddings in rainy temperate climates, this effect can become quite important.

Interior Conditions

If you are building an indoor swimming pool, you can be quite sure that it will be very humid and warm inside all year long. Thus, a vapor barrier on the inside is practically mandatory in all but the hottest and most humid climates. On the other hand, if the enclosure is around a deep-freeze storage facility, there will be more moisture outside most of the time, and the vapor barrier goes on the outside, even in a climate like Pittsburgh, PA. Houses should typically be maintained at a moderate interior humidity level by using ventilation or dehumidification.

Wall Assembly

Obviously, the wall assembly plays a very significant role in deciding on your vapor diffusion control needs. Although designers tend to be fixated on the need to label vapor barriers, the fact is that many materials in an assembly may control vapor diffusion. While batt insulation (permeable: 20 perms) has practically no vapor resistance, 8 inches of concrete is a pretty good barrier (impermeable: 0.5 perms) and latex paint on gypsum board is semi-permeable (about 3 perms). Thus, a wall with painted gypsum already has some pretty good vapor control, and would not need an additional layer if used to separate a moderate exterior climate (e.g., Boston, MA) from a moderate interior climate (say a house with good ventilation). For a colder climate (say Minneapolis, MN), an 8-inch structural concrete wall or 6 inches of expanded polystyrene insulation (about 0.75 perms) would be sufficient for all but very humid interior conditions. The order in which layers of different permeance materials are arranged in an enclosure is also important. For example, using an unventilated low-permeance layer (such as a roofing membrane, precast concrete, etc.) on the exterior in a cold climate will prevent water vapor from escaping to the exterior (this slows drying to the outside). The permeance of the interior layers must be considerably less than the permeance of outer layers (various rules place the ratio at 3:1 to as much as 10:1). Using insulating sheathing also changes the behavior drastically.

The rules are reversed for hot climates. Increasing the temperature inboard of the insulated sheathing essentially transports the wall to a warmer and more temperate climate zone, thereby also reducing the need for low-permeance vapor barriers. For example, an R-12 wood-frame house wall with R-7.5 insulated sheathing in Nebraska would not require a sheet vapor barrier, but would require a normal latex paint layer.

Figure it out

So how do you answer the question, "Do I need a vapor barrier?" Well, figure it out. Given the information I've shared above, it's reasonably easy to decide if, where, and what kind of vapor barrier you need. Keep in mind that air barriers are important and necessary components of almost all building enclosures in all climates, whereas vapor barriers are typically less important components that may or may not be needed. As you do your figuring, remember that you must include the exterior climate, interior conditions, the properties of materials (e.g., permeance, capacity for wetting) and the arrangements of the enclosure assembly. A useful tool, which describes the process in detail, is Chpt 22 of the Handbook of Fundamentals, published by the American Society of Heating, Air Conditioning, and Refrigerating Engineers. More sophisticated users should investigate these aspects using a dynamic computer model, such as WUFI, available for free at:
WUFI/ORNL Free version
WUFI Professional - follow the "Software" link