Taken from the January 2002 issue of  *Energy Design Update*.

by John Straube

The most common building science question I get from home builders, engineers, code officials, and architects is, "Do I need a vapor barrier?"  The answer is usually simple, but first one has to know more about the question and the specific situation.

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 wall assembly. Let's consider each.

Exterior Climate

Vapor diffusion moves from areas of 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, which 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, Pennsylvania. 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. Although 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 semipermeable (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, Massachusetts) from a moderate interior climate (say a house with good ventilation). For a colder climate (e.g., Minneapolis, Minnesota), 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

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 in almost all building enclosures in all climates, whereas vapor barriers are typically less important components that may or may not be needed.

As you decide, 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 Chapter 22 of the *Handbook of Fundamentals*, published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers.  More sophisticated users should investigate these aspects using a dynamic computer model, such as WUFI, available for free at http://www.ornl.gov/ORNL/BTC/moisture .



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