Introduction

Water can exist in three different states; solid, liquid, and gas. Solid and liquid are the two most common when thinking about preventing water intrusion with a building, however, water as a gas (or vapor) is just as much of a concern. Water vapor can enter a building by air movement as well as diffusion through materials and assemblies.

• Note: ‘vapor’/’gas’ for this article purpose are going to be used interchangeably and as the same definition. 

Relative Humidity & Moisture In Air

Water vapor is always present in air everywhere across the globe; the concept of 0% humidity is an impossibility. The amount of water that can be ‘held’ in air depends on the air’s temperature. Warm air is able to hold more water than cold air.

Relative humidity is the ratio of the water vapor of air to its saturation vapor pressure. Represented as a percentage, it’s the amount of water vapor in the air.

• Water vapor can enter a building by air movement and through materials.
• 25-30% humidity is generally considered the best for humans and buildings. It provides a little bit of moisture, but not too much.
• 60%+ will feel uncomfortable
• 100% Humidity means that it has reached the saturation point/dew point. The air is no longer able to hold any more water.

Dew Point

Dew Point is the ‘point’ at which water will condense in air. Typically everyone has experienced dew point through condensation that forms on a cold glass in summer. The condensation on the glass is an example of dew point.

Because warm air is able to hold more water than cold air, if air gets colder while the amount of water stays constant, the relative humidity will rise until it reaches 100%. Once reached 100%, the water will condense in the air, this is the dew point.

• In the example above, the dew/water on the outside of the glass is from the ambient temperature in the room. The temperature gets colder as it touches the glass, and condenses. This is why you will not typically get dew on thermally insulated glasses as it keeps the exterior surface of the glass warmer, not letting it ever get cold enough to condensate.

Building Cavity Issues

In buildings, if the warm moist air is cooled and reaches the dew point, it can cause major problems if this process occurs within a building cavity. This can introduce the moisture needed for mold growth, promote rust, and generally degrate building materials and make insulation less effective over time along with causing extreme long term health problems.

Vapor Retarders

A Vapor Retarder is a material that is used to slow the transmission/diffusion of water vapor between spaces. Vapor retarders themselves are not insulation, but they help the effectiveness of insulation. Typically these are thin sheet materials, generally plastic. Vapor retarders generally are intended to stop the warm air vapor from inside the building, traveling out (towards the cold) and condensing at a cold temperature.

• Vapor retarders should have a permanence rating of .04 perms or lower to be considered a ‘barrier’. And generally be at least 10 mils thick.
• Made of plastic sheeting, aluminum foil, self-adhering sheet membranes or fluid-applied membranes.
• Vapor retarders can also function as air barriers if the wall assembly is designed for them to be located in the same place.
• Vapor Barrier is an older term and not generally used anymore – learn more here. Instead, vapor retarders are now rated based on their permeability (below).
  • Air Barrier is still used as a term.
  • A Vapor Barrier is always a Vapor Retarder, but a Vapor Retarder is not always a Vapor Barrier.

Vapor Diffusion

Vapor diffusion is the movement of water molecules through vapor-permeable materials. Warm air migrates towards colder air, in the same way that heat always moves from hot to cold. 

• Vapor retarders can prevent or slow vapor diffusion.
• Differences in temperature and relative humidity between spaces leads to a difference in vapor pressure, and vapor pressure is what causes vapor diffusion.
• Vapor diffusion can be caused by diffusion through materials – however, vapor diffusion through air is a larger issue and should be dealt with. (See Air Barriers)

Permeability Definitions

Impermeable: Not able to pass

• Sometimes Impermeable 

Permeable: Able to pass

Retarder:  A substance/material added to slow down the rate of change

Vapor Retarder Classes (Perms – Based on IRC)

Permeance: Measure of how readily a material or membrane allows water vapor to pass through it. The unit of permeance is the Perm. Essentially, it’s a material’s resistance to water-vapor transmission. 

• Perm: The passage of one grain of water moisture per hour, through one square foot of material at a pressure differential of one inch of mercury in vapor pressure
• 1g/hr-ft2-in Hg
• If an air-barrier is vapor permeable, then it should have a Perm of 5 or greater.

Perms	Term	Considered	Meaning	IBC Rating	Examples
x < 0.1	Vapor Impermeable	Vapor Barrier	Does not allow liquids or gasses to pass through it	Class I	Aluminum Foil Glass Sheet MetalRubber Membranes
0.1 to 1.0	Semi Impermeable	Vapor Retarder	Semi-Does not allow liquid or gasses to pass through it (slows them significantly)	Class II	Unfaced Polystyrene Plywood Kraft Paper
1.0 to 10.0	Semi Permeable	Vapor Retarder	Semi-Allows liquid or gasses to pass through it (Somewhat slows)	Class III	Gypsum Board Fiberglass Insulation Brick CMU House Wrap
10.0 < x	Permeable	n/a	Allows all liquids or gasses to pass through it	–	–

Vapor Retarder Locations – Rules of Thumb

The location of the vapor retarder within a wall assembly varies based on the climatic region, design goals, and the construction/design of the wall assembly itself. 

Generally Speaking

In general, for more extreme environments, the vapor retarder should be impermeable and located on one side of the assembly.

• The IBC states where vapor retarders of each class can be located within a wall assembly based on the climate region.
• Vapor retarders are typically placed on the warm side of the insulation. They can be a standalone component or combined with sheathing or insulation. When warm air vapor connects with cold air it condenses. This prevents vapor in the air from traveling and hitting the cold air within the wall cavaity.

Cold Areas (Regions 5,6,7,8)

• Vapor Impermeable Retarders should be placed on the warmer side of the insulation.
  • During winter, warm moist interior air will migrate toward the cooler, drier outdoor air. This will lead to vapor condensing in the wall cavity at the insulation or elsewhere.
• Vapor Permeable Air Barrier should be placed outside the insulation to prevent air infiltration while allowing any accumulated moisture to dry out. In most cases, the air barrier is placed outside the sheathing for support, protection of the sheathing, and ease of construction.

Air Conditioned Hot-Humid Climates (Regions 1,2,3A below warm-humid line, 3C)

Impermeable Vapor Retarders should be placed on the warmer outside (exterior) face of insulation to prevent the warm, moist, air outside from migrating to the cooler, dehumidified interior spaces. The Vapor retarder should also serve as an air barrier.

Mixed Climate Regions (Region 4, some parts of 3A,3B,3C,4B and 5)

In mixed climates, the suggestions are guidelines only. Specific materials and conditions should be analyzed. 

Permeable vapor retarders generally should be placed outside the insulation, and no vapor retarders should be used. This allows any vapor or condensation to pass through the wall in either direction.

Air Barriers & Airtight Construction

Air Barrier

Air Barrier: Part of the exterior envelope that controls infiltration and exfiltration.

• It consists of materials, components, and assemblies on all vertical and horizontal surfaces exposed to the exterior.
• Help prevent infiltration of outside air, pollutants, and the migration of moisture.

In order for an air barrier to function properly…

• The air barrier, assemblies, and building must meet minimum permanence ratings (listed by ANSI/ASHRAE/IESNA or by local building code)
• Air barrier must be continuous (around all walls, roofs, foundations, etc)
• All joints between components and assemblies must be sealed.
• Air barrier must be sealed properly to all other assemblies and other air barriers
• All penetrations must be sealed
• Air barriers should be engineered to withstand applicable structural, wind, etc loads and securely attached to not tear or break away.
• At joints, barrier must be designed to allow expected movement (slack in the material)
• Must be durable and designed to last the entire length of the building
• If an air barrier and vapor retarder are used (two layers in the same assembly), the air barrier should be 10-20 times more permeable to water vapor than the vapor retarder, in order to not trap water vapor in between the two layers.
  • For example, a vapor retarder with a perm of .2 used on the inside of the building, paired with an air barrier located on the outside. The air barrier should have a perm rating of 2.0 or more.

Note: The location of an air barrier within a wall or roof assembly is not important for its effectiveness. Generally, the air barrier is located behind the exterior cladding and outside the sheathing (house wrap). This provides some protection to the sheathing as well in case water gets behind the cladding.

• If they are different materials, they should be located as best for the climate region and design of the envelope system.

Air Leakage: Infiltration and Exfiltration

A lot of energy is lost from Infiltration and Exfiltration. These are air leakage going into and out of a building by natural means other than the (intended) mechanical ventilation. This is caused by differences in air pressure in and out of the building. The differences in the air pressure can be caused by a combination of wind, stack effect (see below), mechanical system and other factors.

• Unless a building is well sealed, air leakage can account for more heat loss than transmission through the walls and roof. However, no matter what, no building can be perfectly sealed, there is always some level of leakage to be expected.
  • Typically 25-40% of the heating and cooling energy used by a building is lost due to leakage.
• Estimated that the amount of water vapor carried by air leakage is 10 to 200 times greater than that carried by diffusion through materials.
• Air leakage occurs through open doors, cracks around windows, other openings, flues and vents, and other gaps in the exterior construction.

• Infiltration: Air leakage going into the building
• Exfiltration: Air leakage going out of the building

Air leakage is normally unwanted for several reasons:

• Air that enters through infiltration
  • Must still be conditioned (which adds to heating/cooling loads/cost).
  • Not conditioned/filtered, so it may have undesirable dust and pollutants.
  • Infiltration carries water vapor into the building.
• Already conditioned air can be lost due to exfiltration (you are paying for conditioned air that is lost)

The numerous issues associated with air leakage in buildings, have led to buildings being constructed ‘airtight’. Many codes also now require verification through blower tests.

Airtight Construction

Airtight buildings are constructed with all exterior wall construction pieces of the building thoroughly sealed extremely tight, as much as possible. This can involve spraying foam in gaps, using sealants, and stuffing cavities. While this creates an airtight building which theoretically will perform better from an energy standpoint, it also causes some potential other issues that should be engineered.

• Airtight buildings can cause pressure differences, especially when running a kitchen hood or vent, there needs to be a way to introduce enough air to make up for the exhausted air.
• Pollutants, CO2 and other factors are not regularly ‘diffused’ throughout the building, and therefore the MEP system should be designed with enough fresh air considerations throughout.
• Current codes and standards require the use of air barriers and other construction methods to minimize air leakage.

Stack Effect

AKA Chimney Effect

Differences in pressure between the top and bottom of a building (or space) due to temperature differential. The effect is most pronounced in high-rise buildings.

Example

In a very cold climate, the air within the building will be warmer. Because hot air rises, warm air will make its way through the building and towards the top. This air will try to exit the building at the upper parts through a variety of openings. This creates a lower pressure at the bottom of a building which cold air from outside tries to enter. This causes exfiltration at the top and infiltration at the bottom, which can replace the entire building’s conditioned air within minutes if not addressed. 

Barrier and Retarder Recap

Vapor Impermeable Air Barrier: Acts as both an air barrier and a vapor retarder in the same material.

• Example: Rubber Membranes, Aluminum Foil

Vapor Permeable Air Barrier: Acts as an air barrier while allowing vapor to diffuse/dry out. Placed exterior.

• Example: House Wrap

Moisture Migration Through Slabs

Moisture migration through concrete slabs on grade is a large issue to contend with if designing this assembly. Moisture can migrate through slabs by capillary action or by movement of water vapor.

• Water vapor will move areas of high pressure to areas of low pressure – it does this through the process of diffusion (see above). 
• A vapor retarder must be placed under the slab to prevent the migration of moisture through the slab. Moisture can damage the flooring of the space, and can create problems for indoor air quality by supplying the moisture necessary for mold and mildew growth
• Water problems inside the building can be reduced by specifying low water cementitious materials.
  • If possible, construction should be scheduled to allow the slabs on grade as much possible time to cure and dry before sealants and flooring is installed. Ideally, the concrete should be allowed to cure for 6 weeks before any flooring is installed. Many times it is best practice to test the moisture content in the building to verify it is below acceptable thresholds.

Vapor Retarders for Slab on Grade

The best way to prevent moisture from navigating through the concrete slab is by placing a vapor retarder directly below the concrete on top of any sand cushion or rock subbase.

Note: Previously, it was recommended to place the sand on top of the barrier (below the slab) however, the sand has been noted to act like a sponge, and it will soak up all the moisture and hold it there, and then the vapor retarder traps the water in between the sand and the concrete floor.