Predicting condensation performance of custom-designed curtain wall

by sadia_badhon | April 6, 2021 1:24 am

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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.

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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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Given the possible consequences, designers and specifiers are advised to verify the performance of the design and installation method of fenestration systems. This is especially important for customized project-specific designs, before the surfacing of post-installation problems that will be time-consuming and expensive to rectify.

There are several ways to assess potential and actual condensation performance, such as:

• comparison of standardized similar products;
• before design (predictive); and
• design verification via pre-installation testing.

Several published methods serve as aids to implement these approaches.

Comparison of standardized or similar products

Three different condensation rating systems are available: the Fenestration and Glazing Industry Alliance’s (FGIA’s) condensation resistance factor (CRF), the National Fenestration Rating Council’s (NFRC) condensation rating (CR), and the Canadian Standards Association’s (CSA’s) temperature index (I). All three allow comparison of the condensation resistance performance of similar types of complete fenestration products and serve as a basis of specifications for non-condensation-critical applications (schools, offices, and some retail buildings). All use a numerical index between 1 and 100 with higher values indicating better performance. While similar—all are based on standardized conditions of –17.8 C (0 F) exterior ambient air temperature and 21 C (70 F) interior room temperature—they are determined by different methods.

Condensation resistance factor

CRF is derived from actual interior surface temperature readings obtained from laboratory testing of samples or design mockups per the American Architectural Manufacturers Association (AAMA) 1503-09, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, an FGIA document; or from NFRC 100-20, Procedure for Determining Fenestration Product U-factors.

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CRF is a dimensionless ratio of laboratory-measured temperature differentials, derived as defined in AAMA 1503 that serves as a rating number. It is obtained under specified test conditions in order to allow a relative comparison of the condensation performance of a product. CRF provides a comparative rating and permits the determination of the conditions beyond which an objectionable amount of condensation may occur.

The most direct application of the CRF value is its use in the prediction of what set of exterior temperatures, interior temperatures, and interior humidity conditions will initiate condensation. It may also be used for comparative analysis of similar products of the same general configuration, although some interpretation may be needed in comparing dissimilar products (e.g. wall sections versus operating windows or fixed glazing).

Specifically, CRF is the numerical value determined by the lower of either the weighted frame temperature (FT) or the average glazing temperature (GT) in relation to the difference between the cold side air temperature and the warm side air temperature.

During the guarded hot box test described in AAMA 1503, interior surface temperatures at 24 locations are monitored using thermocouples, after the test specimen reaches steady state conditions. The minimum of glass surface temperature (GT) or coldpoint-weighted FT is used to calculate a unitless ratio, which is multiplied by 100 to yield a whole number CRF.

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where:

T_int = average temperature of warm side air in degrees F;

T_ext = average temperature of cold side air in degrees F;

100 = a multiplier to make CRF a whole number;

FT = weighted average frame temperature in degrees F, as defined in AAMA 1503; and

GT = average glazing temperature in degrees F, as defined in AAMA 1503.

CRF is thus essentially the ratio of the difference between the lower of a product’s average interior frame and glass temperature (as separately determined) and the exterior ambient air temperature, divided by the interior/exterior ambient air temperature differential.

Note that the lower performing glass or frame determines the value of the overall product CRF.

As per AAMA 1504, Voluntary Specification for Thermal Performance of Windows, Doors and Glazed Wall Sections, an FGIA document, the product CRF should not be less than the value shown for the corresponding “CRF Class” in Figure 1.

The higher the number, the stronger the product’s tendency to be able to resist condensation.

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FGIA also offers a CRF tool[7] to assist specifiers in determining a target minimum CRF based on a project-specific set of environmental conditions.

The CRF tool is a quick online calculator to determine the dewpoint temperature, as well as the CRF, by entering winter outdoor and indoor design temperature and the RH for the project location. For example, given an indoor air temperature of 21 C (70 F) and an indoor RH of 40 percent, the recommended CRF of the same type of product for use in Atlanta is 50, while it is 70 for use in Minneapolis.

Specifiers can obtain CRF figures for specific products or designs under consideration directly from various manufacturers, thereby determining the best-performing existing or proposed product for the chosen application.

Condensation resistance measurement

CR measurement is similar to FGIA’s CRF. The principal difference is that CR is derived from simulations using software tools (Lawrence Berkeley National Laboratory’s THERM and WINDOW programs) to model and calculate a CR rating, while the AAMA CRF is derived from actual test data. The CR method is described in NFRC 500-2020, Procedure for Determining Fenestration Product Condensation Index Ratings.

Temperature index

The I rating, like CRF, is obtained through laboratory testing at standard test conditions, albeit with different thermocouple locations, as prescribed in CSA A440.2, Fenestration energy performance. Generally, a given product’s temperature index will be lower than its CRF.

It should be emphasized that CR, CRF, and I ratings differ significantly, are not interchangeable, and no method exists to convert between the different indices. A manufacturer will typically settle on only one of these ratings for all its products.

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It is important to also note the usefulness of such ratings or indexes in assessing and predicting condensation performance is limited for condensation-critical applications. They are particularly less reliable when used with curtain walls due to the heightened importance of several variables, such as the thermal conductivity of surrounding building construction, interior/exterior trim, humidification control, and the method of heat distribution on the interior plane of the assembly.

An FGIA publication, AAMA CRS-15, A Comparison of Condensation Rating Systems for Fenestration, describes the three methods in detail, including calculation formulas and applications.

Predictive analysis before design

During the design and development of curtain wall systems, it is recommended the design engineer determine the appropriate level of condensation performance for a particular occupancy and given a set of stakeholder expectations. For example, the following questions should be addressed to determine the degree of condensation prevention:

• is the effect of potential visible condensation on aesthetics very critical;
• are there window coverings or recessed window wells that could block airflow;
• are window coverings controlled by occupants;
• are there any health or comfort concerns with the effects
  of condensation;
• is the building occupied 24/7 so that nighttime condensation would be apparent;
• is the interior humidity mechanically controlled; and
• would condensation affect the product’s useful life, durability, or (if applicable) warranty?

During the design stage of a wall system, engineers often use two-dimensional finite element (FE) thermal modeling tools to predict interior surface temperatures for standard or project-specific conditions. In the case of custom-designed curtain wall, this process can be complicated by a variety of factors, such as conflicting project specifications, model accuracy for aluminum framing, repeatability/reproducibility of modeling and testing, effects of substrates, and end-user expectations. AAMA 515-19, Voluntary Procedure for Determination of Fenestration Surface Temperatures by THERM Modeling, an FGIA document, offers guidance in applying FE analysis to improve its applicability to curtain wall thermal performance including condensation.

The project-specific variables inherent in a custom-designed curtain wall system introduce an additional level of complication in the case of custom-designed curtain wall systems. For this reason, predictive methods may not be sufficiently reliable to verify accrual performance and avoid liability.

Design verification via testing

Large curtain wall mockups are often built to replicate an area of fenestration and the surrounding wall construction. These mockups can then be subjected to a battery of tests to check resistance to water penetration, seismic and wind-induced inter-story drift, and human impact.

A similar approach can be used to evaluate condensation. AAMA 501.9-19, Surface Temperature Assessment for Condensation Evaluation of Exterior Wall Systems, an FGIA document, prescribes a standardized means to evaluate the potential for formation of condensation on interior surfaces under wintertime temperature and humidity conditions using large, job-specific curtain wall mockups. This is accomplished by measuring interior surface temperatures and comparing them to anticipated dewpoint temperatures likely to be experienced at the actual project site. This method does not provide a rating or quantitative measure of condensation.

To apply AAMA 501.9, the specifier must first determine the exterior and interior winter design conditions for the project, including interior temperature and humidity levels and exterior temperature. If no conditions are specified, the default exterior temperature is determined per the latest the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook of Fundamentals, based on data from the closest weather station to the project site. Absent this information, the default interior temperature and humidity used to determine dewpoint is 21 C (70 F) and 25 percent RH unless otherwise specified.

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To conduct the test, a representative mockup of the specific curtain wall system must be prepared, measuring no larger than a single floor-to-floor height and 4.6 m (15 ft) wide. The mockup must contain wall components or details that are representative of the wall system and have been identified as possible condensation control concerns. All parts of the specimen must be full size, using the same materials, methods of construction, and anchorage as specified for the actual project. Portions of the interior wall representative of project conditions (e.g. drywall, insulation, etc.) should be included to at least 305 mm (12 in.) from the edges of the specimen to properly evaluate the perimeter conditions. Each operable element of the specimen must be securely closed and locked prior to testing. Any chamber fixture that is not included in the finished project (e.g. observation decks/floors and non-project support steel) and is located within 610 mm (24 in.) of a thermocouple location must be insulated. It may also be useful to include fenestration covering, such as blinds and curtains, if they could impact the eventual occurrence of condensation.

The mockup specimen is sealed into the opening of an insulated laboratory test chamber. The insulated outdoor side of the chamber is equipped with a means to lower its ambient temperature to the specified exterior winter design temperature and simulate exterior airflow. The latter is accomplished using wind generators to provide a flow of 5.5 ± 1.3 m/s (12.3 ± 3 mph), as calibrated using a wind speed measuring device (i.e. anemometer), applied so that the flow is generally parallel to the long face of the specimen test area being evaluated. The indoor side of the chamber is equipped with a means to both control temperature and limit RH to prevent the formation of condensation in the areas where surface temperature data is recorded. Indoor air circulation, used to minimize temperature stratification and fluctuation, is limited to 1.6 km/h (1 mph).

The interior surfaces of the test specimen are instrumented with a total of at least three sensors, such as Type T thermocouples, located near the top, middle, and bottom of the test area. Alternate temperature measuring equipment (e.g. non-contact pyrometers) may be used so long as the accuracy meets or exceeds that of ASTM C1363-19, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus. The thermocouples are taped tightly to the surface using aluminum foil tape and painted flat white or alternatively to meet the emissivity of the surface being analyzed. The exterior surface of the test sample may also be instrumented with thermocouples, but this is not a requirement for this method.

The test apparatus must control the average ambient temperature within ±2 C (±4 F) of the specified set points. Additional thermocouples are positioned at least 75 mm (3 in.) away from the wall specimen to measure interior and exterior ambient air temperatures. Individual ambient air thermocouples readings may vary more than the ±2 C range so long as the average of all thermocouples remains within these bounds.

Test procedure

The specimen is maintained at the specified interior and exterior conditions until the interior surface temperatures have reached steady-state conditions, the time for which varies with the material composition of the wall, such as masonry versus metal components. The interior and exterior air temperatures, surface temperatures, and RH are monitored and recorded for a period of not less than two hours after steady-state is achieved. All surface temperature data is compared to the specified dewpoint for the project, and all locations at which temperatures fall below the dewpoint are identified. All evidence of condensation is recorded, including quantity and location, and documented using photographs.

AAMA 501.9 enables building professionals to specify meaningful testing to ensure intended performance prior to occupancy. Since large curtain wall mockups require a lot of planning, coordination, and expense to build, it is often advantageous to add some of the other optional tests as well. After all, in the final analysis, testing is a lot less expensive than revisions after the fact.

[10]Steven Saffell oversees the standards, product certification, and codes and regulatory affairs aspects of the Fenestration & Glazing Industry Alliance (FGIA). His background is a tapestry of architectural firm work, modular design, as well as residential and commercial fenestration experience. Saffell also spent three years teaching as an adjunct professor. He is experienced in managing technical teams, including employee development, operational strategy, and financial management. He can be reached at ssaffell@fgiaonline.org[11].

Source URL: https://www.constructionspecifier.com/predicting-condensation-performance-of-custom-designed-curtain-wall/

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