Condensation is the accumulation of liquid water on relatively cold surfaces.
Almost all aircontains 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 thecontainment of a relatively large quantity of water vapor. As air cools, itsmolecules get closer together and squeeze the tiny vapor droplets closertogether, as well. A critical temperature, known as the dew point, exists wherethese water droplets will be forced so close together that they merge intovisible liquid in a process called condensation.
Double-pane windowshave a layer of gas (usually argon or air) trapped between two panes of glassthat acts as insulation to reduce heat loss through the window. Other types ofgas used in this space have various effects on heat gain or loss through thewindow. Some windows also have a thin film installed between panes thatseparates the space between the panes into two spaces, further reducing heat lossand heat gain through the window. If multiple-pane windows appear misty orfoggy, it means that the seal protecting the window assembly has failed.
A desiccant is anabsorbent material designed to maintain dryness in the space it protects. In adouble-paned window, silica pellets inside the aluminum perimeter strip absorbmoisture from any incoming air that enters the space between the panes. If notfor the silica desiccant, any moisture in the space between the panes wouldcondense on the glass as the glass cools below the dew point temperature.
Silica gel has animmense surface area, approximately 7,200 square feet per gram, which allows itto absorb large amounts of water vapor. As the sealant protecting this spacefails over time, increasing amounts of moisture-containing air will enter thespace between the panes, and the silica pellets will eventually becomesaturated and will no longer be able to prevent condensation from forming. Adouble-paned window that appears foggy or that has visible condensation hasfailed and needs to be repaired or replaced.
Why Double-Paned Windows Fail: Solar (Thermal) Pumping
Although double-paned windows appear to be stable, they actuallyexperience a daily cycle of expansion and contraction caused by thermalpumping. Sunlight heats the airspace between the panes and causes the gas thereto heat up and expand, pressurizing the space between the panes. At night, thewindow cools and the space between the panes contracts. This motion acts likethe bellows of a forge and is called thermal pumping.
Over time, theconstant pressure fluctuations caused by thermal pumping will stress the seal.Eventually, the seal will develop small fractures that will slowly grow insize, allowing increasing amounts of infiltration and exfiltration of air fromthe space between the panes.
Windows on the sunnyside of a home will experience larger temperature swings, resulting in greateramounts of thermal pumping, seal stress and failure rates.
Vinyl window frameshave a higher coefficient of expansion resulting in greater long-term stress onthe double-pane assembly, and a higher failure rate. Windows also experiencebatch failure, which describes production runs of windows, especially vinylwindows, that are defective, meaning that the pane assemblies have beenmanufactured with seals that have small defects that will cause the window tofail prematurely.
The Nature of Damage
If it’s allowed tocontinue, window condensation will inevitably lead to irreversible physicalwindow damage. This damage can appear in the following two ways:
river bedding: Condensed vapor between the glass panes will formdroplets that run down the length of the window. Water that descends in thisfashion has the tendency to follow narrow paths and carve grooves into theglass surface. These grooves are formed in a process similar to canyon formation.
silica haze: Once the silica gel has been saturated, it will beeroded by passing air currents and accumulate as white “snowflakes” on thewindow surface. It is believed that if this damage is present, the window mustbe replaced.
Condensation is notalways visible. If the failure is recent, a failed window may not be obvious,since condensation doesn’t usually form until the window is heated by directsunlight. Windows in the shade may show no evidence of failure, so inspectorsshould disclaim responsibility for discovering failed double-paned windows.
In summary, condensation in double-paned windows indicates that the glazingassembly has failed and needs repair or replacement. Visible condensation candamage glazing and is the main indication of sealant failure.