Air Barriers The Latest Tool in Moisture Control

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From BDC eNews:

Air Barriers The Latest Tool in Moisture

Control

Often confused with vapor barriers and moisture

barriers, air barriers may be the key to controlling moisture at the

building envelope.

By Dave Barista, Managing Editor

January 1, 2007

Building Design and ConstructionWhen it comes to

controlling moisture at the building envelope, the construction industry

has traditionally focused on blocking water and vapor from entering and

getting trapped in the external wall assembly.

But recent theories suggest that there's a third culprit in the moisture

intrusion mystery­air.

If allowed to pass freely through a building's envelope, moisture-laden

air can cause condensation on internal wall system components, which can

lead to moisture problems like mold and rot on exterior components, and

can cause freeze-thaw damage, paint failure, staining, and oxidation.

This theory applies to the intrusion of outside air and air from within

the building.

“The flow of air can carry a lot of moisture with it,” says

Altenhofen, director of technology in the Philadelphia office of Hillier

Architecture, Princeton, N.J. He says the idea that air leakage can

contribute to moisture intrusion is fairly new and is often misunderstood

by AEC professionals when designing and constructing building

envelopes.

“It's well understood that within any exterior wall assembly there's a

need to control the transfer of heat through insulation; transfer of

moisture through water-resistant barriers like building felts, housewrap,

or dampproofing; and transfer of vapors with vapor barriers,” says

Altenhofen. “But the idea of the ability of air to carry moisture is

fairly new.”

To combat the effects of moisture damage as a result of air leakage,

designers like Altenhofen are specifying the installation of air barriers

in conjunction with vapor barriers and water-resistant barriers. Some AEC

professionals confuse air barriers with vapor and water-resistant

barriers because one material may function as one, two, or all three of

these barriers (see sidebar). Regardless, Altenhofen says the very

specific functions of all three systems must be included in the enclosure

assembly.

Air barriers create a near-airtight seal around all six sides of the

building (floor, walls, and roof) to prevent or restrict the passage of

air through the building enclosure system. This seamless air seal is

achieved by joining any number of building enclosure components (windows,

doors, walls, roof systems, etc.) that are rated for a minimum level of

air permeance­a measure of the amount of air that will work its way

through a material. Air barrier components range from common building

materials such as gypsum wallboard and cast-in-place concrete assemblies

to specially designed liquid-applied or self-adhered air barrier

membranes. A common air barrier wall assembly for the commercial sector,

for example, may involve a brick wall or metal-panel veneer backed up

with an air cavity, insulation, and a spray-applied or peel-and-stick air

barrier installed on the CMU or sheathing and metal stud backup. In this

case, the air barrier is also the vapor and water-resistant barrier.

BASF Corp., Carlisle Coatings and Waterproofing, Grace Construction

Products, and Henry Co. are among the companies that manufacture

specialty air barrier products. Most offer three-in-one products that

perform the function of the air barrier, water-resistant barrier, and

vapor barrier. Altenhofen cautions designers to take into account the

regional climate, interior conditions, and location of insulation when

incorporating these vapor-tight air barriers into a wall assembly

(vapor-permeable air barriers are also available).

“Vapor barriers must be installed on the warm side of the insulation, so

in Miami, Phoenix, and Los Angeles, that's the exterior face, while in

Chicago and Boston, it's the inside face,” says Altenhofen. The decision

isn't as cut and dried in mixed climates, such as Washington, D.C., and

the Carolinas, because the heating and cooling seasons are almost equal.

An exception is when the insulation is located within an air cavity where

all materials are selected for their ability to withstand

wetting.

Another key challenge, says Altenhofen, is ensuring a continuous air

barrier on all six sides of the structure. “If you take a cross-section

drawing of a building and put a pencil down on the line that represents

the air barrier, you should be able to trace around the perimeter of the

building without lifting the pencil,” he says. “The membrane itself may

not be continuous (for example, at the windows and doors), but the

barrier should be continuous.”

Getting the walls, roof, and floors airtight is not too difficult. Most

mishaps occur at the joints between the roof and the walls and between

the windows and the walls. Altenhofen says most architects and

contractors are so focused on making the outer veneer watertight, that

gaps in the air barrier often go undetected.

“It's a big problem for architects to understand the detailing and for

contractors to understand the construction, especially if the air barrier

also acts as the water-resistant barrier,” says Altenhofen. “It's more

important that the air barrier be continuous and not leaking than trying

to keep moisture from penetrating the veneer.” This requires increased

attention to the construction details like flashing around the windows

and doors and the sealant between curtain wall systems and

walls.

When Building Teams get the air barrier right, the building owner will

find that the benefits go beyond mitigation of moisture intrusion. Air

barriers can also improve thermal comfort, HVAC performance, indoor air

quality, and energy efficiency. A June 2005 report by the National

Institute of Standards and Technology shows that owners can achieve a

30-40% reduction in heating and cooling costs by making their building

envelope airtight.¹

References

¹“Investigation of the Impact of Commercial Building Envelope

Airtightness on HVAC Energy Use,” J. Emmerich, P.

McDowell, Wagdy Anis, June 2005.

www.fire.nist.gov/bfrlpubs/build05/PDF/b05007.pdf

Do you know the difference between an air barrier, vapor barrier, and

water-resistant barrier?Moisture protection at

the building envelope is one of the most misunderstood aspects of

building construction.

At the center of the confusion is distinguishing the difference between

air barriers, vapor barriers, and water-resistant barriers. These terms

are often used interchangeably when discussing building envelopes, but

each has unique characteristics and plays a distinct role in mitigating

moisture, says Altenhofen, director of technology in the

Philadelphia office of Hillier Architecture, Princeton, N.J.

“There are three sets of criteria,” says Altenhofen. “One material might

take care of all three of those functions or you might have to treat them

separately.”

Air barriers prevent or restrict the passage of air through the

building enclosure system. An air barrier can be achieved by applying any

number of building envelope components that are rated for a minimum level

of air permeance­a measure of the amount of air that will work its way

through a material. This includes everything from common drywall,

plywood, and concrete assemblies to specially designed liquid-applied and

self-adhered air barrier systems or spray-on foam. Air barriers must be

continuous and seamless, meaning that all six sides of a building must be

sealed, from the floors to the walls to the roof.

Vapor barriers , or vapor retarders, on the other hand, are not

airtight. They are designed to restrict the flow of water vapor through a

building assembly by controlling the molecular diffusion of water vapor

molecules through the actual material. Assuming a building has a

functional air barrier, a vapor barrier does not have to be perfectly

continuous; it can have small holes and gaps, depending on the specified

vapor diffusion control rate. In other words, a vapor barrier that covers

99% of the wall surface is 99% effective. Vapor barriers are typically

installed on the warm side of the insulation.

Water-resistive barriers, or moisture barriers, are designed to

keep liquid water from entering the building enclosure. Moisture barriers

are typically combined with flashing and other materials to create a

shingled assembly to expel liquid water that may have entered the wall

assembly. The barriers are installed directly below an overlying veneer

or siding. Barrier materials such as precast concrete are integrally

water-resistant.

Source: Air Barrier Association of America,

www.airbarrier.org