Moisture Control in Exterior Building Envelopes
Craig K. Haney, FCSI, Specifications Director
Moisture problems in buildings are one of the hottest topics in the construction industry and account for most of the construction litigation. Understanding moisture control in exterior building envelopes and the difference between a weather barrier, moisture vapor retarder, and air barrier, where and when each is used, and what materials should be used for each is essential for a successful project.
Why are Moisture Problems More Prevalent Now?
Historically, buildings did not suffer from moisture infiltration problems nearly as much as newer buildings. Why is this?
The free movement of air minimizes the possibility of moisture issues.
Newer buildings are constructed to be substantially more airtight than older ones. Many new buildings are constructed without operable windows and with minimal outside air intake, preventing equalization from exterior to interior.
Mold is not new; it has been around for millions of years. It is just more prevalent because of new construction materials and techniques.
An Example of Moisture Issues:
I was once involved in investigations into moisture issues on a historic county courthouse in East Texas, where summers are hot and humidity is typically high. The building, constructed in the 1890’s with solid masonry exterior walls, never had a history of moisture problems. Then, during restoration, new wood windows were installed with weatherstripping (they never had it before) and modern joint sealers applied to seal up every crack and crevice in the exterior walls, and a non-breathable semigloss paint was applied to the interior face of the walls.
Within a year the paint on the interior face of the exterior walls began to peel, and the gypsum plaster began to show mold. Why was this occurring after 100 years with no issues? Simple; the windows, joint sealers, and paint had sealed up the walls as never before, drastically reducing air infiltration. Where previously any moisture entering the exterior wall had simply migrated to the interior face and evaporated, now it was being trapped in the walls.
The solution? We removed the interior plaster, applied a moisture vapor resistant coating to the interior face of the walls, reapplied the plaster, and painted it with flat latex paint.
Moisture Barrier, Vapor Retarder, or Air Barrier; What is the Difference?
Weather Barrier: Prevents the passage of water in liquid form (“bulk” water).
Moisture Vapor Retarder: Prevents the passage of water suspended in air.
Air Barrier: Prevents the passage of air, regardless of moisture content.
Can’t I Just Use One Product and be Done?
If you confuse their functions disaster will follow.
Choose the right product for the desired results and the conditions present.
Just because a material is good at one function, it may not be good at another.
Some materials may serve multiple purposes.
Condensation: Water from humid air collecting as droplets on a cold surface.
Dew Point: The temperature at which air with a given amount of moisture is fully saturated, so that condensation occurs.
As temperature drops, the relative humidity increases for air holding the same amount of moisture:
So, warm air has a higher capacity to hold moisture than cool air.
And, as the temperature drops, humidity increases.
If the temperature drops within a wall or roof assembly, condensation could occur in the assembly.
Just like nature hates a vacuum, nature always tries to reach an equilibrium on humidity.
The Effects of Condensation in a Wall or Roof Assembly:
Deterioration of Assembly Components: Particularly unprotected metal and gypsum board.
Mold Growth: Mold requires both food and water to thrive. Condensation provides the water and materials such as the paper on gypsum board and insulation provide the food.
Loss of Insulation Value: The best insulator is dead air. Insulation products work by trapping air in the material.
Northern Climates: Moisture vapor moves from the warm, humid interior to cold, dry exterior. Condensation occurs within the assembly unless a moisture vapor retarder is placed on the interior side.
Southern Climates: Moisture vapor moves from the warm, humid exterior to the cooler, dry interior. Condensation occurs within the assembly unless a moisture vapor retarder is placed on the exterior side.
The direction of vapor movement changes based on the time of year and atmospheric conditions.
Most sources place the line of demarcation for warm, humid climates on a line running several hundred miles north of the Gulf coast, and extending from Texas to the Atlantic coast.
Locate vapor retarders to prevent moist air from entering the assembly.
Ensure that the dew point occurs on the warm side of the vapor retarder.
Hot, Humid (tropical) Locations:
While an interior vapor retarder is on the cold side of the assembly in summer, the dew point temperature of hot, humid air is almost always lower than the interior temperature, so condensation will occur only rarely and will be of short duration.
Do not cool building below 76 degrees F on humid days.
If an exterior vapor retarder is used, do not humidify the building.
Types of Weather Barriers:
Building Papers: Kraft papers and asphalt impregnated papers.
Asphalt Impregnated Felts: No. 15 and No. 30 felts.
Proprietary Products: Plastic sheet (mechanically attached), rubberized asphalt sheet (self-adhering), or fluid-applied.
Details of Installation:
Locate the weather barrier on the exterior side of an exterior assembly.
May be used as either primary or secondary line of defense.
Single vs. Dual Weather Barrier System:
Single: Examples include a standard EIFS system and single wythe masonry. Moisture that penetrates the barrier enters the structure.
Dual: Examples include brick veneer over sheathing and drained EIFS systems. Moisture penetrating the first barrier must also penetrate the second barrier to enter the structure.
For obvious reasons a dual moisture barrier system is preferable.
Continuity is Critical: Weatherlap sheet products by beginning the installation at the low point of the assembly, and seal lapped ends and edges. Seal the weather barrier to adjacent materials and around penetrations.
Moisture Vapor Retarders:
Causes of Moisture Vapor Movement:
Diffusion: The movement of vapor from air with higher moisture content to air of lower content.
Air Movement: Via leaks through the assembly.
Heat Transfer: Assemblies with insufficient thermal resistance will experience vapor movement.
What Constitutes a Vapor Retarder?
The generally accepted definition is a product having a moisture vapor permeance (perm) rating of 1.0 or less, tested to ASTM E96/E96M.
Perm: The number of grains of water vapor passing through a square foot of material in 1 hour at 73.4 degrees F and with a pressure differential of 1 inch of mercury.
Types of Vapor Retarders:
Usually thin, flexible materials, but can include thicker products.
Reinforced and non-reinforced products available.
Materials such as rigid insulation and reinforced plastics are resistant to water vapor movement.
Coatings: Many opaque paint and coating products inhibit vapor movement to some extent.
Details of Installation:
Locate the vapor retarder on the interior side of exterior assembly in cool, dry climates and on the exterior side in warm, humid climates.
Continuity is Critical: Seal joints in sheet materials, and seal the vapor retarder to adjacent materials and around penetrations.
Never install two vapor retarders on opposite sides of an assembly.
Why use an Air Barrier?
Minimizing air movement across an assembly greatly reduces moisture vapor transmission.
The Canadian National Building Code requires an air barrier in all new construction.
Types of Air Barriers:
Many materials will serve as an air barrier if continuous and properly sealed:. Examples that you may not think of include painted gypsum board, metal decking, and metal wall or roof panels.
Must be a “structural” material or applied to a structural substrate to prevent deflection and air loss.
Details of Installation:
Locate on the exterior side of an exterior assembly.
Continuity is Critical: Seal joints in sheet materials, and seal the air barrier to adjacent materials and around penetrations.
Poorly constructed barriers allow air to pass freely when air pressure is present.
Use materials with the proven ability to produce the desired results.
When applicable, choose materials that serve multiple purposes, but ensure that their location in the exterior envelope assembly is appropriate.
SimpleSpecs™, the master guide specification system, contains sections covering all three of these critical building systems. For information on the SimpleSpecs™ Master Guide Specification System visit www.zerodocs.com and Section 07 28 00 Weather Barriers.
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