Condensation Study of Windows

1.1 Condensation Overview and Brief Literature Review

Condensation potential in glazed facade systems is an important design consideration. Heat, from inside the building, escapes through the most thermally conductive components of the envelope, and condensation forms first at the surface locations with coldest surface temperatures, which, in building enclosures, are typically the components of window systems in cold seasons (NFRC, 2020). Specifically, condensation occurs when the interior surface temperature of window components falls below the dew point temperature of the interior environment. This is the basis for determining the likelihood of window condensation. Even in high-performing window systems, condensation might first develop at a location along the interior perimeter of the glazing or the window frame, especially in utilization of frames with smaller thermal barriers non-insulating metal spacers in insulating glass units (IGUs) (NFRC, 2020). Condensation in window systems is of concern, as its continued presence can drain into the exterior wall assembly and degrade facade components, including structural members, insulation, window frames, window ledges, seals, and interior wall surfaces and finishes (NFRC, 2020). In very cold climates, condensation can also cause a potential build-up of ice. Additionally, continued presence of moisture can also contribute toward development of mold in the building skin components, interior finishes and furnishings, resulting in a harmful environment to occupants’ health (NFRC, 2020). Thus, preventing condensation has become an increasingly important fenestration product selection criteria in predominantly cold climates (NFRC, 2020).

Condensation potential increases as the difference between the interior and exterior temperature rises, and as the interior relative humidity (RH) increases. This is because the dew point temperature of the interior environment linearly rises with increased levels of interior RH. Comfortable interior RH levels are between 30 and 60% (ANSI/ASHRAE, 2013); yet, measured interior RH levels in buildings often reach up to 70% during cold seasons (NFRC, 2020). Although thermally comfortable, these high levels of interior RH can cause condensation to occur at commonly observed exterior temperatures in cold climates. Aside from temperature and humidity levels, additional environmental and physical conditions which may, singularly or in combination, affect interior surface temperatures include the following: wind velocity, solar radiation and orientation, interior and exterior air pressure, interior air movement, night-time radiation temperatures of facade components, and the rate and amount of water vapor permeating through the facade system to the interior (NFRC, 2020). some of the additional physical conditions include location and presence of air and vapor barrier in the wall assembly, humidification controls and method of heat distribution along the facade perimeter, presence and function (closed vs. open) of window treatments and shading devices, presence and material properties of interior finishes and furnishings, presence, size and quantity of plants, room function (ex: bathrooms, kitchens, swimming pools, conservatories), and presence, number and activity of occupants (NFRC, 2020). This sheer multitude of factors makes it challenging for window manufacturers to warrant their products against condensation, as all the potential combinations of factors are difficult to predict or test, both by physical and computational methods.

One method to reduce the potential of condensation is to implement high-performing window systems with improved thermal performance, measured by the heat transfer coefficient (U-factor or U-Value). As the U-factor averages the thermal performance of all system components, utilizing high-performance framing systems (such as frames fabricated from materials with low thermal conductivity and aluminum frames which integrate low-conductivity thermal barriers) and high-performance glazing types (such as IGUs that implement two or more panes of glass, low-emissivity coatings, noble gasses such as argon or krypton, and low-conductivity warm edge spacers) will result in a lower U-factor of the overall glazing system (NFRC, 2020). A critical component of the window system is the glass spacer. Thermally efficient spacer will reduce potential for heat transfer between the glazing and the window frame, specifically the junction points between interior edge of glass and the interior edge of frame (its interior gasket). Additional ways to decrease the potential for condensation include raising the indoor temperature, lowering the interior humidity, and increasing air circulation near fenestration. These methods raise the interior dew point temperature and/or promote evaporation (NFRC, 2020). However, closed and partially open window treatments, which are common practice during cold seasons to prevent heat loss, have an adverse effect on condensation formation as they contribute toward lowering of surface temperatures of window components (NFRC, 2020). Additionally, while there may be some benefits to increasing condensation resistance with the integration of low-e coatings in cold climates (Zhang et al., 2021), more research needs to be done in this area. There is concern that the application of low-e coatings on surface #4 (room side) to IGUs may inadvertently increase the risk of condensation.

Few publications exist on the topic of condensation resistance in fenestration systems. However, several studies were published that either compare accuracy of software used to analyze heat transfer through window systems against actual tested data, or studies which evaluate the components of window systems (specifically the glass spacer) relative to condensation resistance. A case study was conducted by Ordner (2011) to examine simulation procedures, specifically the THERM and WINDOW software programs, and to compare simulated performance data with actual performance data of a high-rise office building curtain wall system. Results demonstrated that THERM simulations provide a fairly accurate representation of actual surface temperatures for the representative case study. Moreover, the study emphasizes importance of understanding how actual installation details affect the system’s thermal performance. Another study was conducted to evaluate thermal and condensation performance of windows, considering different spacer components (Elmahdy, 2006). This study investigated 10 different spacer designs and indicated that warm-edge spacers can reduce potential of condensation and lower the U-factor. The study also showed that it is important to consider the performance of a combination of individualized components to prevent condensation (the frame, glazing and the spacer), in addition to specific climate conditions where the window system will be used. Similar conclusions regarding the importance of spacers were determined by another experimental study (Hong et al., 2016). This study concluded that the edge of glass was highly susceptible to temperature variations and that the lowest temperature at this location was caused by the thermal bridging through the spacer.

1.2 NFRC Condensation Resistance (CR) Rating Method

National Fenestration Rating Council (NFRC) developed a standardized method for calculating condensation resistance rating (NFRC, 2020). Condensation Resistance (CR) value is defined as a relative indicator of how well a window resists the formation of condensation on the inside surface of its components, where the higher CR value (whole integer from 1-100) implies greater resistance to the formation of condensation. Two-dimensional heat transfer software, THERM, is used to calculate the CR value for the frame (CR_f) and the CR value for the edge-of-glazing (CR_e), while one-dimensional heat transfer software, WINDOW, is used to calculate the CR value for the center-of-glazing (CR_c) and the overall CR value of the fenestration system (NFRC, 2017b). The individual component CR values are area weighted for relative comparison and normalization, and the lowest value among the three resulting CRs (of the frame, edge of glass, and center of glass) is taken as the CR value of the whole fenestration system. Resulting CR value, thus, is a standardized value of an averaged, weighted, and lowest performing window component.

NFRC standardizes CR calculations with simulations for the individual window components under the following environmental conditions: interior dry-bulb temperature 70ºF (21ºC); exterior dry-bulb temperature 0ºF (-18ºC); 12.3 mph (5.5 m/s) wind conditions; and three interior relative humidity values (30% RH, 50% RH, and 70% RH) with their corresponding interior dew point temperatures of 37ºF (3ºC), 51ºF (11ºC), and 60ºF (16ºC); and maintaining the mean radiant temperature input equal to the exterior ambient temperature (NFRC, 2017a).

WINDOW and THERM software can be used to concurrently calculate the overall U-factor, solar heat gain coefficient (SHGC) and visual transmittance (VT) of fenestration systems using the same window system cross section details, which are mandatory ratings for NRFC certification. They are supplemented by voluntary ratings for Air Leakage (AL), and Condensation Resistance (CR) (NFRC, 2017a). Although not required, CR values are increasingly common on the NFRC product labels. However, CR values do not indicate whether condensation will occur, under which exterior and interior conditions, and to what extent. This leaves designers, owners, and consumers unsure as to what exactly they are comparing and why the CR values are relevant if they do not a guarantee specific performance. Similar misunderstanding exists with all currently available condensation resistance methods.

Depending on specific environmental conditions, condensation either occurs or does not occur, and its extent and severity increase with lower exterior temperatures and higher interior RH. A method which could inform the user whether condensation will occur and under which specific conditions would be more applicable and useful than an non-guaranteed probability of an averaged ratio of implemented methods. The goal of this study was to quantify and graphically analyze CR performance of an array of different window systems to illustrate relationships between condensation and thermal performance relative to these window systems’ range of CR and U-factor values. The concept was to provide a more visual understanding and quantification of the extent of condensation rather than an abstract number. The study has utilized the NFRC’s CR rating system since simulations tools can be used for calculations.

ANSI/ ASHRAE. 2013. “ANSI/ ASHRAE Standard 55-2013: Thermal Environmental Conditions for Human Occupancy”. American National Standards Institute (ANSI) and American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE), Inc. Atlanta, GA.

ANSI/ NFRC. 2020. “ANSI/ NFRC 500-2020[E0A0]: Procedure for Determining Fenestration Product Condensation Index Ratings. “NFRC (National Fenestration Rating Council), Inc. Greenbelt, MD. ­

Elmahdy, Hakim. 2006. “Assessment of Spacer Bar Design and Frame Material on the Thermal Performance of Windows,” ASHRAE Transactions, Volume 112, Part 2:30-43.

Hong, Goopyo, Daeung Kim, and Byungseon Kim. 2016. “Experimental Investigation of Thermal Behaviors in Window Systems by Monitoring of Surface Condensation Using Full-Scale Measurements and Simulation Tools.” Energies 9 (11). https://doi.org/10.3390/en9110979.

NFRC. 2020. “NFRC 501-2020[E0A0] - User Guide to the Procedure for Determining Fenestration Product Condensation Index Rating.” NFRC (National Fenestration Rating Council), Inc. Greenbelt, MD.

NFRC. 2017a. “NFRC 500-2017[E0A0] – Procedure for Determining Fenestration Product Condensation Resistance Rating.” NFRC (National Fenestration Rating Council), Inc. Greenbelt, MD.

NFRC. 2017b. “THERM 7 / WINDOW 7 NFRC Simulation Manual.” NFRC (National Fenestration Rating Council), Inc. https://windows.lbl.gov/sites/default/files/Downloads/NFRCSim7-July2017.pdf

Ordner, Elizabeth. 2011. “Fenestration Condensation Resistance: Computer Simulation and In Situ Performance,” Condensation in Exterior Building Wall Systems 2011, 2011 (1498): 269–85. https://doi-org.silk.library.umass.edu/10.1520/STP49392S.

Zhang, Chong, Jinbo Wang, Liao Li, and Wenjie Gang. 2021. “Condensation Risk of Exhaust Air Heat Recovery Window System: Assessment, Key Parameters, and Prevention Measure.” Case Studies in Thermal Engineering 24 (April). https://doi.org/10.1016/j.csite.2020.100830.