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Home / Condensation Critique

Condensation Critique

A discussion about condensation, its physical phenomena and influence are in order when considering moisture issues with the building envelope and especially the roof system.  The NIA has provided a handy white paper about condensation that is worth reading and submitted below.

“What is Condensation?

Condensation occurs when air is cooled to a temperature at which it is 100% saturated. Simply, air has a given capacity to hold water vapor. Warmer air has a greater capacity to hold water vapor than cooler air. Consequently, the total amount of water vapor that the air can hold is primarily a function of its temperature. As air is cooled, its capacity to hold water vapor is diminished. When cooled to a temperature at which it can no longer hold the water vapor it contains, the air is said to be saturated and "condensation" (liquid water) will become visible on surfaces.

What is Relative Humidity?

Relative humidity is a ratio of how much moisture vapor is in the air, compared to the air's total moisture vapor capacity at a given temperature. It is a measure of the moisture loading in the air.  If the relative humidity is 50% at 73°F, the air contains 50% of its water vapor capacity at that temperature. If this same air is then cooled, the relative humidity of the air will increase. This is because the air has less total capacity to hold that same quantity of water vapor when its temperature is reduced. When the air is cooled to a temperature at which its relative humidity increases to 100%, it is saturated and condensation will form.

How are Dew Point Temperature, Condensation and Relative Humidity Related?

The dew point temperature is the temperature at which air cools to its saturation point and condensation begins to form. Dew point temperatures can be determined, in Table 1, if the temperature and relative humidity of the air are known.

Troubleshooting Condensation Problems

High Interior Relative Humidity:

Excessively high interior relative humidity equates to high interior vapor pressure and is often the cause of condensation problems. High interior relative humidity can be a function of the building's end use, HVAC system, or improper construction practices. Human respiration and perspiration can also contribute to a significant amount of moisture in the building's environment. Other common sources of indoor moisture include:

  •  Combustion
    Oil and gas fired heaters give off a significant amount of moisture to the air. Water vapor introduced to the air through combustion is sometimes very difficult to detect because hot exhaust gasses can hold large quantities of moisture. As exhaust gasses cool, condensation can form on cold exterior surfaces. To avoid this situation, flue gases should always be vented outside.
  • Excavated Earth
    Significant amounts of moisture can be introduced into a building before it is completed. As damp soil within the building is excavated, moisture will likely evaporate. If the building is enclosed, this moisture will become trapped within the structure and elevate the relative humidity within the structure. As soon as the temperature drops below the dew point of that air/water mixture, condensation will form. NOTE: The higher the relative humidity, the higher the dew point temperature.
  • Concrete
    Fresh concrete is another large source of moisture within a building. A 100,000 ft² freshly poured 4" concrete slab has the potential to give off approximately 24,000 gallons of water into the building. When concrete is poured after the building has been enclosed, proper ventilation is critical to remove the excess moisture vapor. Ventilation should always be considered as a preventive measure during the construction schedule.

Types of Condensation and How to Control Condensation

  • Visible Condensation
    Visible condensation is evident when water appears on cool surfaces; it can be clearly seen. This type of condensation can form on windows, purlins or exposed exterior fasteners. While a nuisance, this type of condensation is far less damaging than hidden condensation. It can be controlled by adding insulation, proper ventilation and by reducing relative humidity levels.
  • Concealed Condensation / Moisture
    Concealed condensation is the most difficult to deal with and can be the most damaging. It occurs when moisture vapor condenses within the insulation system behind the vapor retarder or in other concealed building cavities.

Potential causes for concealed condensation are as follows:

  • Improper vapor retarder selection
  • Improper placement of the vapor retarder
  • Improper installation of the insulation system
  • Roof leaks
  • Water Infiltration
  • No structural air barrier
  • High humidity levels
  • Inadequate ventilation
  • Incorrect pressure differential between the inside and outside of the building. This is critical factor that must not be overlooked.

What are the Effects of Moisture?

Moisture from rain, ground water, and concrete all increase the relative humidity in a building and increase the probability of condensation. Liquid moisture has the potential to create severe problems in metal buildings.

  • Metal and Condensation
    Most of the surfaces in a metal building are protected against corrosion/rust under normal operating conditions.  However, when metal parts are exposed to moisture for extended periods of time they can oxidize, become weaker, and shorten the life expectancy of the building. Even though many surface treatments are applied to ferrous and nonferrous metals to prevent oxidation, the best protection is to eliminate a key component - moisture. 
  • Fiberglass Insulation and Condensation
    Fiberglass is the most commonly used thermal insulation in metal buildings. The presence of condensed water in fiberglass insulation will significantly increase its thermal conductivity and reduce its insulating value. Fiberglass insulation however, is durable. If allowed to fully dry and return to its design thickness, fiberglass will revert back to its original insulating value. (Note: This statement pertains only to thermal performance. No claims are being made with respect to mold related issues associated with condensation or roof leaks)

The Importance of Vapor Retarders

Vapor retarders inhibit the passage of moisture into the insulation and reduce the potential for condensation on the cold side of the insulation. Vapor retarders do not totally stop water vapor transmission; they significantly reduce its movement.

Vapor Retarder Ratings

Vapor retarders, commonly referred to as "Facings" are rated by the amount of moisture that can pass through them. Facings with lower perm ratings are more effective vapor retarders. Flexible vapor retarders come in a wide variety of materials and perm ratings. The most common are membrane vapor retarders, which are typically laminated to fiberglass metal building insulation. Materials with ratings of greater than 1.0 perm are not classified as vapor retarders. In building construction where a high humidity environment is probable, a facing option with 0.02 or 0.09 perm is recommended. Perm ratings for the facings most commonly used on metal building insulation are shown in the table below:

Type Perm Rating

  • PSK Polypropylene/Scrim/Kraft 0.02 - 0.09
  • FSK Foil/Scrim/Kraft 0.02
  • PSKP Polypropylene/Scrim/Kraft/Metallized Polyester 0.02
  • PSP Polypropylene/Scrim/Metallized Polyester 0.02
  • PSF Polypropylene/Scrim/Foil 0.02
  • VRP Vinyl/Scrim/Metallized Polyester 0.02
  • VRV Vinyl/Scrim/Vinyl 0.90
  • Vinyl Plain Vinyl 1.0

For applications where a permeable insulation covering is required, most membrane vapor retarders can be perforated to increase the permeability and allow moisture migration.”

Sponsored by the National Insulation Association