Windows: Preventing Condensation

window condensation
As part of a larger renovation, the Center For Equal Justice building in Toledo, OH features fiberglass replacement windows that contribute to the high insulating value of exterior walls. (Photo: Comfort Line)

By Dean Lewis
From the May/June 2016 Issue

When considering condensation in facilities, the problem is moisture; a compound essential to life on a planet that is covered more than 70% with it, but an enemy of most materials used in building construction. Improved methods, materials, and components of construction, along with updated HVAC systems, have done much to alleviate the problem, but these same improvements can also complicate the situation.

It is an unavoidable consequence of thermal physics: Condensation forms on a surface when its temperature falls below the dew point (the temperature at which water vapor in the air turns to a liquid). The amount of airborne water vapor is expressed as relative humidity (the percentage of the actual amount of water vapor present compared to the maximum it could hold at a given temperature). So relative humidity is temperature dependent; the warmer the air, the more moisture it can hold. As the relative humidity increases, the dew point also increases, and condensation will occur at warmer surface temperatures.

Indoors, when water vapor in the air comes into contact with the surface of glass or framing with a temperature below the dew point, condensation will form. Restrooms, shower facilities, and kitchen areas are familiar high humidity environments, so condensation on mirrors and windows is rather common when these rooms are in use.

To prevent condensation from occurring, either the temperature of the condensing surface needs to be raised, or the dew point of the air needs to be reduced (lower relative humidity).

The Consequences

At one time, condensation on windows was accepted as little more than a nuisance and an inevitable consequence of single-pane glazing and solid metal frames. But given the stakes of today’s focus on moisture prevention, facility managers cannot afford to consider condensation as a mere nuisance. Obvious reasons are occupant dissatisfaction and even lawsuits over anything from repair of stain damaged interior walls to health problems caused by mold.

window condensation
Moisture prevention, including condensation on windows, is important not only for building integrity but also for occupant comfort and health. (Photo: Comfort Line)

Some commercial environments require special attention to reduce the likelihood of condensation, such as museums—where moisture can damage priceless artifacts and medical facilities—where moisture provides fertile ground for growth of molds and bacteria that could be dangerous to patients with compromised immune systems.

Less obvious are the positive improvements that can be obtained in the indoor environment by reducing the likelihood of condensation. These include less maintenance, higher allowable relative humidity indoors (for greater comfort at a lower heat setting and less, annoying static electricity in winter), and better clarity of view through the fenestration system glass.

Fenestration refers to openings in a building—such as windows, doors, skylights, sloped glazing, curtain walls, and storefronts—that are designed to permit the passage of air, light, or people.

There are times when the solution to one problem serves to exacerbate another. Building conversion, renovation, remodeling, or upgrading often includes replacement of windows or glazing; tightening of the building envelope—such as with tuckpointing; caulking; replacement of weather seals, window flashing, and seals; and replacement of HVAC system equipment or components. Reduced air infiltration, whether due to a tighter building envelope in general, or improved sealing of fenestration openings alone, can lead to fewer air changes and a subsequent increase in indoor relative humidity. Adding or increasing humidification for comfort control purposes will have the same effect. It should be noted that both warm air and humid air rise. And, interior remodeling that opens lower floors to those on higher levels can create both temperature and humidity differentials, resulting in condensation not seen prior to the conversion.

The Solutions

As stated earlier, to prevent condensation either the temperature of the condensing surface needs to be raised, or the dew point of the air needs to be reduced. The latter solution involves controlling the indoor relative humidity through a well-designed HVAC system that includes exhaust fans, ceiling fans, dehumidifiers, chillers, etc. The fenestration industry has provided a number of significant improvements to address the former, ensuring that the inside surface temperature of windows and architectural glazing stays above the dew point temperature. These same features contribute to energy conservation in general, and are well proven, keeping the inside surface temperature elevated when outdoor temperatures are frigid.

Controlling Condensation In Fenestration

Insulating Glass. The alternatives to drive down window thermal conductivity (U-factor) have evolved well beyond simple two-lite glass with a “dead air” space between. Inert gases, such as argon or krypton infill, and low-emissivity (low-e) glass have driven U-factors down significantly. In cold climates where condensation is a greater risk, a low-e coating on the internal surface of the interior pane of an insulating glass unit reflects heat back into the room, reducing heat loss through the window. The surface of the glazing is also warmer when low-e glazing is used, helping to reduce condensation.

Thermally Improved Framing. For commercial applications, aluminum has long been the framing material of choice due to its inherent structural strength and relatively light weight. Its natural property of high thermal conductivity has been largely overcome by thermal barrier technology, including “poured and de-bridged” barriers, thermoplastic inserts, neoprene gaskets, or structural silicone separations.

Warm Edge Technology. The traditional spacer that separates and retains the two (or three) lites of glass around the edges of an insulating glass unit is made of metal for reasons of strength, durability, and light weight. However, metal spacers conduct heat, so the edges of the glass tend to lose more heat than the center of the glass. To overcome this drawback, “warm edge technology” evolved. Warm edge refers to the type of spacer material whose greater insulating properties increase the inside edge temperature by 10°F or more under the same set of conditions, thus reducing the likelihood of condensation around the window perimeter.

Comparing Performance

Tools are available to facility professionals who are comparing fenestration products’ resistance to condensation.

The American Architectural Manufacturers Association (AAMA) Condensation Resistance Factor (CRF) is a numerical index that measures the ability of a product to resist the formation of condensation on its interior surface. The higher the CRF rating is, the greater the product’s resistance to condensation (i.e., the less condensation build-up the window allows). The index generally falls in the range of 35 to 80, with 35 recommended as the minimum acceptable rating for a thermally improved product. This allows the specifier to designate CRF values for fenestration systems based on anticipated inside relative humidity and outside design temperatures.

For general guidance on suggesting a minimum CRF based on a project-specific set of environmental conditions, the tool can be found at the AAMA website.

The AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, provides the means to determine the CRF of all types of fenestration products, including large units and glazed wall sections as often used in commercial applications.

Another tool is the AAMA 507, Standard Practice for Determining the Thermal Performance Characteristics of Fenestration Systems Installed in Commercial Buildings. This provides specifiers with a means to verify the energy performance of glazed fenestration systems used in commercial buildings.

The National Fenestration Rating Council (NFRC) also sponsors rating and labeling programs to help consumers compare the thermal performance features of residential windows, doors, and skylights, as well as a Component Modeling Approach for non-residential products.

It should be noted that while not absolute values, the condensation ratings of either the AAMA or NFRC scale allow a credible comparison of the performance of different products.

window condensationLewis is the technical manager of training and education for the American Architectural Manufacturers Association, helping to advance the group’s professional certification programs and other educational initiatives. He has served on committees of ASTM, ANSI, ASHRAE, and numerous standards and certification committees. Lewis can be reached at

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