What Causes Double Pane Windows to Fog?

Exterior Fog

• Condensation and fog occur on the exterior of the window when indoor temperatures fall significantly below outdoor temperatures. This can occur on hot, humid summer days when your air conditioner is running. The dramatic difference in temperature and humidity level causes moisture to build on the coldest surface available to the humid outdoor air, which is the glass window.

Room-Side Fog

• In the winter, when the temperature and humidity level drop outside, you employ the use of a furnace and humidifier to keep the interior comfortable. Then, fog forms on the interior side of the glass as the air immediately next to the window condensates as a reaction to the colder air outside. To prevent damage to your window frames and home furnishings, keep the humidity level indoors under control to lessen the amount of fog that forms.

Fog Between the Panes

• In newer windows, there should never be fog between the panes. As windows age, the seal between the panes of glass can begin to fail, enabling moisture to seep in between them. Windows continually exposed to the sun are more likely to experience seal failure sooner than other windows in a home. The amount of time it takes for a window’s seal to fail depends on the building materials used, the design of the glass and the window, and many other factors.

Preventing Fog

• Investing in a quality window is the best way to prevent premature seal failure that results in moisture and fog between double pane windows. Choose windows that are designed to succeed in your climate, keeping factors like sun exposure, altitude and general weather conditions in mind. Check for warranties before you purchase new windows and understand what they cover. Look for a warranty of at least 20 years to ensure you won’t have to pay for window replacements any time soon.