Fw: Demystifying the Use of Vapor Barriers

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: Welcome to CutterEdge Buildings, the weekly e-mail service

: from Cutter Information Corp. The following is excerpted

: from an article in the January 2002 issue of

: *Energy Design Update*.

:

: DEMYSTIFYING THE USE OF VAPOR BARRIERS

: by 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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