Double-Pane Window Condensation Issues
Condensation is the accumulation of liquid water on relatively cold surfaces.
Almost all air contains water vapor, the gas phase of water composed of tiny water droplets. The molecules in warm air are far apart from one another and allow the containment of a relatively large quantity of water vapor. As air cools, its molecules get closer together and squeeze the tiny vapor droplets closer together, as well. A critical temperature, known as the dew point, exists where these water droplets will be forced so close together that they merge into visible liquid in a process called condensation.
Double-pane windows have a layer of gas (usually argon or air) trapped between two panes of glass that acts as insulation to reduce heat loss through the window. Other types of gas used in this space have various effects on heat gain or loss through the window. Some double-pane windows also have a thin film installed between panes that separates the space between the panes into two spaces, further reducing heat loss and heat gain through the double-pane window. If multiple-pane windows appear misty or foggy, it means that the seal protecting the double-pane window assembly has failed.
Silica Desiccant Use In Double-Pane Windows
A desiccant is an absorbent material designed to maintain dryness in the space it protects. In a double-pane window, silica pellets inside the aluminum perimeter strip absorb moisture from any incoming air that enters the space between the panes. If not for the silica desiccant, any moisture in the space between the panes would condense on the glass as the glass cools below the dew point temperature.
Silica gel has an immense surface area, approximately 7,200 square feet per gram, which allows it to absorb large amounts of water vapor. As the sealant protecting this space fails over time, increasing amounts of moisture-containing air will enter the space between the panes, and the silica pellets will eventually become saturated and will no longer be able to prevent condensation from forming. A double-pane window that appears foggy or that has visible condensation has failed and needs to be repaired or replaced.
Why Double-Pane Windows Fail: Solar (Thermal) Pumping
Although double-pane windows appear to be stable, they actually experience a daily cycle of expansion and contraction caused by thermal pumping. Sunlight heats the airspace between the panes and causes the gas there to heat up and expand, pressurizing the space between the panes. At night, the double-pane window cools and the space between the panes contracts. This motion acts like the bellows of a forge and is called thermal pumping.
Over time, the constant pressure fluctuations caused by thermal pumping will stress the seal. Eventually, the seal will develop small fractures that will slowly grow in size, allowing increasing amounts of infiltration and ex filtration of air from the space between the panes.
Failure Factors in Double-Pane Windows
Double-pane windows on the sunny side of a home will experience larger temperature swings, resulting in greater amounts of thermal pumping, seal stress and failure rates.
Vinyl double-pane window frames have a higher coefficient of expansion resulting in greater long-term stress on the double-pane assembly, and a higher failure rate. Double-pane windows also experience batch failure, which describes production runs of windows, especially vinyl windows, that are defective, meaning that the double-pane assemblies have been manufactured with seals that have small defects that will cause the window to fail prematurely.
The Nature of Damage to Double-Pane Windows
If it’s allowed to continue, double-pane window condensation will inevitably lead to irreversible physical window damage. This damage can appear in the following two ways:
- riverbedding. Condensed vapor between the glass panes will form droplets that run down the length of the window. Water that descends in this fashion has the tendency to follow narrow paths and carve grooves into the glass surface. These grooves are formed in a process similar to canyon formation.
- silica haze. Once the silica gel has been saturated, it will be eroded by passing air currents and accumulate as white “snowflakes” on the window surface. It is believed that if this damage is present, the window must be replaced.
Detecting Failure in Double-Pane Windows
Double-pane window condensation is not always visible. If the failure is recent, a failed double-pane window may not be obvious, since condensation doesn’t usually form until the window is heated by direct sunlight. Double-pane windows in the shade may show no evidence of failure, so inspectors should disclaim responsibility for discovering failed double-pane windows.
Thermal Imaging as a Detection Tool
Under the right conditions, it’s possible to use an infrared (IR) camera to detect failed double-pane windows. IR cameras are designed to record differences in temperature.
Recommendations for Failed Double-Pane Windows
According to industry experts, the glazing assembly can be replaced approximately 75% of the time. Occasionally, the sashes must be replaced, and only about 5% of those cases require that the entire window be replaced.
There are companies that claim to be able to repair misty double-pane windows through a process known as “defogging.”
This repair method proceeds in the following order:
- A hole is drilled into the window, usually from the outside, and a cleaning solution is sprayed into the air chamber.
- The solution and any other moisture are sucked out through a vacuum.
- A defogger device is permanently inserted into the hole that will allow the release of moisture during thermal pumping.
There is currently a debate as to whether this process is a suitable repair for windows that have failed, or if it merely removes the symptom of this failure. Condensation appears between double-paned windows when the window is compromised, and removal of this water will not fix the seal itself. A window “repaired” in this manner, although absent of condensation, might not provide any additional insulation. This method is still fairly new and opinions about its effectiveness range widely. Regardless, “defogging” certainly allows for cosmetic improvement, which is of some value to homeowners. It may also reduce the potential for damage caused by condensation in the form of mold or rot. Some skepticism exists about the effectiveness and cost effectiveness of this method of repair.
Condensation in double-pane windows indicates that the glazing assembly has failed and needs repair or replacement. Visible condensation can damage glazing and is the main indication of sealant failure.
Other Double-Pane Window Information
Security Window Bars
Window bars (also called safety bars and security bars) are metal bars that are installed to prevent intruders from entering a building. As an unintended consequence, window bars can slow or prevent egress during an emergency.
- Roughly 25 people die or are injured annually in fires where escape is hindered by window bars.
- According to the National Fire Protection Agency, the number of deaths caused by fire related to security bars is on the rise.
- The fear of burglary, theft and/or physical attack presents a greater perceived risk than the threat of fire.
- Seventy people died in a hotel fire on August 18, 2001 in the Philippines. The victims were trapped inside the six-story hotel by window bars.
- They are a deterrent to potential burglars. They are mostly used in ground-floor windows, which are most vulnerable to intrusion.
- They provide a sense of security to building occupants.
- They can prevent children from falling out of the window.
- They can block the exit for occupants during an emergency, such as a fire. The occupants may feel secure from burglary, but they have severely limited their avenues of egress. Ironically, it is possible for occupants to become trapped behind window bars while trying to escape from an intruder who has managed to enter the building.
- They can potentially block the entry point for firefighters.
- Houses equipped with window bars can potentially decrease the home’s property value. Window bars can make a neighborhood appear insecure to potential home buyers.
According to the 2006 International Residential Code (IRC), basements and sleeping rooms should have at least one operable emergency escape and rescue opening. Windows that are equipped with bars and which are intended for emergency egress should have a quick-release mechanism installed. If a room’s egress requirements are already satisfied by another window or door, it is still helpful for window bars to be equipped with a quick-release mechanism. Where window bars are installed in windows that are part of a building’s means of egress, the IRC requires that they be equipped with a quick-release mechanism that complies with the following requirements:
- It should be accessible from the inside of the house. Although not addressed by the IRC, the device should not be accessible from outside the house if the window were to be broken.
- It should not require a key or combination. Likely reasons for this requirement are as follows:
- During an emergency, occupants may become too panicked or confused to remember the combination or where they put the key.
- Fire and smoke may prevent access to the key or obscure view of the lock.
- Occupants may not know the combination or know where the key was placed.
- It should not require any special tools, such as a screwdriver.
- The mechanism should be able to be operated with relatively little force. Children and the elderly should be strong enough to operate the release mechanism.
- Operation of the mechanism should not require special knowledge.
Although beyond the scope of InterNACHI’s Standards of Practice, inspectors may want to test release mechanisms to make sure that they comply with the IRC’s requirements. Even if the mechanism appears functional, it is possible that its ability to operate has become compromised by rust, paint, or some other factor. Inspectors should call out any hindrances to the release mechanism’s functionality as a safety defect.
Window bars are valuable anti-burglary features in residences, but they should be able to be easily disengaged so occupants are not trapped during an emergency.
Window Films
Window Films
Homeowners and business owners can employ a number of methods to “green up” their indoor environments to make the interior more comfortable and to lower heating and cooling costs. Installing window films is one method.
Window films are represented in three general categories. Most familiar to consumers is the tinted cellophane-type material that can be applied temporarily or used as needed, as in glare-reducing retractable shades. Window films can also be semi-permanent, such as window tint that is applied directly to windows and remains in place. Window films can also be permanent, such as the product Heat Mirror™, which is different from typical window films and tints. Window films prices range from fairly inexpensive to very expensive. We can learn more about the different types of window films and the technology available, along with the performance characteristics of window films, which can be useful when it comes time to following an energy audit.
Where Can You Buy Window Films
Window films or tint are available for purchase from hardware stores in long sheet rolls.
Window films are relatively inexpensive and may be applied directly onto windows by the homeowner or business owner to provide shading without blocking all sunlight. This is the same type of tint found on vehicle windows. Heavy-gauge window films are sometimes applied to windows to keep them from shattering into dangerous shards in case of an impact. Most window films can be cut to fit any size or shape and can be removed with steam or a razor blade and alcohol.
Heat Mirror™ and low-E Window Films
These window films are factory-installed and permanent. Heat Mirror™ is a clear, three-layer polyester film that transmits light through insulated glass units (IGUs) while reflecting long-wave infrared energy. It was developed by Southwall Technologies in 1980 to reduce the amount of solar heat transferred into a home or commercial building, increasing the occupants’ comfort and reducing energy costs.
Heat Mirror™ can be mounted inside an insulated glass unit in a variety of configurations (one, two or three coated films, uncoated, or low-E coated glass) to provide energy conservation performance ranging from R-6 to R-20 in order to meet the unique requirements of commercial and residential new construction and renovation. Designations include a two-digit number that represents the amount of light transmitted through the coated film. For example, Heat Mirror™ 22 is designed to block more infrared light than Heat Mirror™ 88.
The polyester film bisects a layer of argon or krypton gas that fills the gap between two glass panes, creating an extra air space that significantly improves the window’s R-value and associated heating and cooling costs. 
Homeowners may become confused regarding the properties of Heat Mirror™ and other green products commonly applied to windows, such as low-E coatings, which also use a reflective layer to reflect infrared energy. Low-E film, however, is made from metal or metal oxide instead of polyester and is physically applied to the glass surface. Unlike Heat Mirror™, a low-E coating does not increase the number of air spaces in an IGU. Debate has persisted for many years concerning which is the better design, with proponents on each side pointing out defects and inefficiencies of the other product. The two designs are sometimes incorporated in the same IGU for additional protection against heat or infrared energy loss.
Amory Lovins is a Colorado resident who pays $0 in energy bills for what may be one of the greenest houses on the planet, according to MSN Money. Heat Mirror™ and other energy-saving features (many of which are covered in InterNACHI’s robust Inspectopedia article database) have allowed Lovins to harvest 28 banana crops in his indoor banana jungle without the aid of central heating, despite the fact that his Rocky Mountain estate experiences sub-zero blizzards every winter. There are also plans to install Heat Mirror™ in the Empire State Building’s 6,500 windows as part of the building’s energy retrofit project. According to BusinessWire, the installation of Heat Mirror™ in the windows of New York’s tallest building will decrease energy costs by $400,000, cut solar heat gain in half, and improve the windows’ R-values from R-2 to R-8. Skyscrapers and alpine banana jungles aside, most Heat Mirror™ window film is found in ordinary residential and commercial structures, although it is among the more expensive options available.
Be on the lookout for older types of Heat Mirror™ that are prone to discoloration, warping, becoming brittle, and seal failure. One such defect is yellowing, caused by impurities that make their way onto the film before it is sealed within the glass panes. Wrinkling is also a problem in older Heat Mirror™, as you can see in the accompanying photo. These problems have largely been resolved in recent years, as Southwall has corrected manufacturing errors and more closely scrutinized their licensed distributors.
Heat Mirror™ and other types of window films are designed to reflect solar heat, increase a window’s R-value, and/or provide shading while reducing the building’s energy costs for both heating and cooling, sometimes significantly, depending on the product. Inspectors who perform energy audits can help with recommendations on the most appropriate choices for the application and budget for window films.