Predicting condensation performance of custom-designed curtain wall

Photo courtesy Wasco
Photo courtesy Wasco

by Steven Saffell

Condensation on glazed areas of building envelopes may seem innocuous—at worst an annoying interference with outdoor views due to glass fogging. However, in addition to being unsightly, it can be unsanitary and over long periods of time, damage adjacent building materials through staining, mold growth, or rot that can lead to costly remediation and even lawsuits over alleged toxic reactions. It can be of major importance in condensation-critical applications, where high levels of humidity are regularly encountered or are particularly detrimental, such as hospitals, laboratories, and museums.

Condensation will occur in any interior surface that falls below the dewpoint—or, the temperature at which airborne moisture (water vapor) turns into a liquid—of the interior ambient air, which is also dependent on relative humidity (RH). Generally, the greater the indoor-outdoor temperature difference, the more likely condensation is to form, particularly when higher levels of moisture are present. If the dewpoint is below 0 C (32 F), the condensation can be in the form of frost or even ice.

The challenge to minimize condensation is thus to lower the indoor humidity and/or keep the inside surface temperature of the glass and frame above the dewpoint temperature, especially when outdoor temperatures are frigid. Most design features that improve thermal performance (i.e. decrease U-factor) also improve condensation resistance, although there can be inherent trade-offs with other design features. Some features that typically improve both U-factor and condensation resistance rating include:

  • multiple-cavity insulating glass (IG);
  • low-emissivity (low-e) coatings within the
    IG cavity;
  • warm-edge IG spacers;
  • insulating fill gas, such as argon; and
  • lower thermal conductivity frame materials
    or design.

This mission to reduce indoor condensation is complicated by many variables that may affect interior surface temperatures and thus affect the potential for condensation. These may include, for example, the following.

Figure 1: The minimum condensation resistance  factor (CRF) of  a product for the corresponding CRF class.  Image courtesy FGIA
Figure 1: The minimum condensation resistance factor (CRF) of a product for the corresponding CRF class.
Image courtesy FGIA

Type of wall construction and thermal mass of the material(s) used therein

Construction providing higher U-value and higher thermal mass reduces the likelihood of interior surfaces chilling below the dewpoint and forming condensation. Higher thermal mass means that the building envelope and interior construction can store more heat, providing ‘inertia’ against temperature fluctuations (i.e. when outside temperatures fluctuate throughout the day, a large thermal mass within the building can serve to ‘flatten out’ the fluctuations).

Component thermal performance

U-factor and condensation resistance of windows and doors.

Closed drapes and/or shades

Drapes and shades tend to cool the interior surfaces of fenestration.

Wind pressure

Positive exterior wind pressure or negative pressure within the building due to HVAC characteristics may increase infiltration of cold air. This can be a function of the height of product above grade, the location of surrounding buildings and type of terrain, and wind velocity.

Interior trim coverage

Interior trim coverage could reduce air infiltration and preserve warmer temperatures near the interior surface of the wall.

Solar radiation and orientation

Impinging solar rays increase warmth of the fenestration surfaces and interior, a desirable effect in winter.

Water vapor

Rate and amount of water vapor released to interior (expressed by interior RH). Warmer interior air can hold more moisture than cold air.

Air movement

Air movement over interior surfaces affects the formation of condensation. Location of diffusers or fin tubes, air stagnation in soffits or projections, and interior furnishings and traffic can change air movement patterns.

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