Information Brief - Condensation Resistance, Window

High indoor humidity and cold outdoor temperatures can result in condensation on cold interior surfaces, caused by thermal bridging to the exterior. This is particularly common on windows because frames and glass can lose significant amounts of heat and windows are the least insulated part of a wall system. High levels of indoor humidity also increase the likelihood of condensation.

Recently, standard measurements have been introduced to gauge the susceptibility of a window to condensation. The new standards join the well-known measurements of window performance attributes, such as thermal conductivity, air leakage, and center of glass performance. None of these specifically address the likelihood of condensation on a particular window assembly, which is caused by a combination of factors. Standards that measure condensation resistance take into account thermal conductivity, geometry, thermal variation, and airflow resistance. In the United States, there are two competing standards: the Condensation Resistance Rating (CR), created by the National Fenestration Rating Council (NFRC); and the Condensation Resistance Factor (CRF), created by the American Architectural Manufacturer's Association (AAMA).

Condensation Resistance Rating (CR)
Windows rated on the Condensation Resistance Rating scale from the NFRC are scored from 1 to 100. A window with a higher value is more resistant to condensation. The rating is based on a series of tests or simulations that assess the minimum performance of several parts of the window assembly (center-of-glass, edge-of-glass, and frame) across a series of temperatures and humidity levels.

The CR is not intended to determine whether a window will cause condensation at a specific relative humidity and outdoor temperature, but instead provide a relative assessment of a window's condensation resistance. Condensation resistance is an optional rating on the NFRC Performance label, which provides a set of standardized window performance metrics, including U-value, Solar Heat Gain Coefficient, and Visible Transmittance.

The CR incorporates procedures for a test-based or a simulation-based window rating. The simulation-based rating is intended to be the primary rating method and is based on NFRC-approved software models.

Users are cautioned that windows selected according to results from AAMA's ocomponentnline calculator may not always prevent condensation, since the CRF is based on a weighting of indoor temperatures. Some parts of a window may be cold enough to allow condensation, though the weighted average of the whole window would satisfy AAMA's requirements.

Considerations
The differences between the CR and CRF ratings are significant, though their goals are the same. The primary method of determining the CR rating is through simulation, while the CRF is based on measured data. Both should be used primarily as comparative evaluations between windows. Since there is no current data available to compare CR and CRF ratings, determining whether a CRF rated window performs better than a CR rated window, or visa versa, is difficult.

Neither the CR nor the CRF take into account factors, other than the window, that can contribute to indoor condensation. Window treatments are a good example. Shades, curtains, and blinds often reduce the amount of airflow across the window, allowing the window to cool and reducing evaporation. This increases the likelihood of condensation. Window detailing and mounting can also affect window condensation. For example, windows mounted towards the outside of the wall plane will receive less interior airflow. In addition, the window frame will remain colder because it is in line with the coldest elements of the wall, such as the sheathing and cladding. Together, these conditions result in a window that is more prone to condensation.

General Recommendations
Select a window with an NFRC Condensation Resistance (CR) rating greater than 50. If using AAMA's condensation resistance factor (CRF), some environmental conditions need to be determined first. If the typical indoor temperature in the winter is 70°F with a relative humidity of 40%, a CRF of 64 will prevent most condensation down to an outdoor temperature of 0°F. An A CRF rating of 70 will prevent most condensation down to -15°, and a CRF of 72 will do the same down to -20°F. According to ASHRAE, the winter design temperature at 97.5% varies from -12°F to -26°F for various cities across Minnesota. As mentioned above, the CRF is a weighted average of window performance across different areas of the window, so some condensation may still occur at specific points on the window.

Environmental Context
Window condensation resistance is primarily focused on improving indoor environmental quality and limiting the amount of moisture that accumulates on adjacent to windows due to indoor humidity. Moisture accumulation on windows can easily damage window frame finishes. Prolonged condensation, and even ice build-up, can damage the frames and drain into the wall cavity, damaging sheet rock and possibly leading to mold and structural decay. Drainage into the wall can be particularly detrimental if the wall is detailed so that water is trapped behind a vapor barrier, preventing it from drying to the interior. In general, windows are the weak point in exterior walls, commonly providing infiltration points for exterior moisture and cold air. Proper air sealing and flashing of windows is critical to avoid these problems.

High indoor humidity levels not only create window condensation issues, but can also lead to mold and mildew growth throughout the home. During the winter, indoor relative humidity should be kept below 50% to reduce these issues. Properly designed and installed ventilation equipment is required to ensure reasonable seasonal humidity levels are maintained. This includes whole house systems such as HRVs and ERVs, as well as point source exhaust for bathroom, kitchen, and laundry areas. The EPA refers to the Energy Efficient Building Association (EEBA) Builder's Guide as a resource on further moisture reduction strategies.

Mold, mildew, and bacterial growth can result from moisture intrusion in walls. The resulting health impacts range from mild respiratory and eye irritation to more severe asthmatic reactions. People with more severe asthma and those more susceptible to environmental irritants, including children, the elderly, and the immune-compromised, should take steps to ensure that moisture is controlled in their homes to avoid unnecessary exposure to irritants.

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