08:11 AM

What Causes Windows to Fog?

What is it, what causes it and what to keep in mind when purchasing windows.

What causes windows to fog?

Many factors may contribute to a foggy window. For starters, it’s essential to first understand where the fog is occurring: on the outside or exterior of your home and window, on the inside/room side interior of your home and window, or in-between the panes of glass within the window.

Exterior condensation on windows occurs when the temperature inside the home falls well below that outside, like on a hot humid day. When dramatic differences in interior and exterior temperatures and humidity levels occur, moisture condensation can build up on the coldest surface—the glass. It’s the same phenomenon that causes a cold glass of your favorite beverage to sweat on a sizzling summer day.

Likewise, when interior humidity is high, as temperatures drop outdoors, condensation can occur on interior glass surfaces. To protect your investment in your home and home furnishings, reduce the level of interior moisture inside your home. The use of a dehumidifier may help alleviate the interior glass condensation.

Moisture and condensation between panes of insulating glass can occur when the seal between the panes of glass fails and allows moisture to leak in. This typically occurs in older windows or doors. It’s not uncommon to walk into older homes and see windows on the sunny side of the home with a glass seal failure that allows condensation and fogging between the panes. This same fogging can occur on any side of the home but tends to be more visible on the sunny sides because the additional heat from the sun can accelerate the seal failure. Also, with the sun shining through the glass, the fogging can be more visible and readily apparent.

Here’s what’s happening:

A technical explanation of moisture between the panes of glass is well put by Pella engineer, Joe Hayden:

“Today’s insulating glass windows are sealed with improved materials and technology, and also contain inert gasses like argon between the panes. Further, an extremely thin silver oxide coating is applied to the inside face of the outside pane. This low emissivity, or “low-E”, coating acts as a reflector for the infrared rays of the sun. Using low-E glass is generally considered the minimum necessary for comfort and energy savings during heating and cooling seasons.

Eventually, fogging may develop between the panes of any insulating glass product. Given enough time, the seals between the panes will break down and allow moisture to enter. The desiccant material will attract and hold as much moisture as it is capable of, but will eventually become saturated. When that happens, fogging will occur. Exactly how long this will take is dependent on the actual environmental conditions, the sealing and desiccant materials used, the design of the insulating glass, the overall design of the window, and a host of other factors too numerous to mention here. But simply stated, nothing lasts forever.”

The glass

“Choosing and investing in a glass system that meets your needs is an important decision in buying new or replacing old windows,” said Pella engineer Bruce Baier. “Quality windows and doors that include insulating glass can lower energy bills, reduce fading damage to interior fabrics and provide a more comfortable home.”


  • Choose window and door manufacturers that test products for moisture infiltration

Pella tests the components of our windows and patio doors under the harshest conditions—the burning rays of the sun, salt air, acid rain, excessive humidity, extreme heat and cold, high wind and driving rain, many times beyond industry standards. And Pella tests for air and water infiltration during the manufacturing process. These stringent measures help ensure your Pella windows and patio doors will perform for years.

  • Look for verification

Check for third-party validation that the window meets the strict standards of outside sources like the ENERGY STAR label or certification from the Window and Door Manufacturers Association (WDMA) which sponsors a Hallmark Certification Program. Considered a mark of excellence, WDMA Hallmark indicates that the manufacturer participates in periodic in-plant inspections by a third-party administrator. The inspections include auditing manufacturer quality control processes, and a review to check that products are manufactured in accordance with the appropriate WDMA and other performance standards.

  • Choose the right glass for your region/climate

The better your window glass insulates, the more comfortable your interior room will feel. Climate affects which glass you should choose for a window or door, and exposure to the sun, noise, window placement, altitude, and size should also be considered.

  • Consider all the glass options

Pella offers a variety of glass innovations. Four popular choices include:

  • Insulating glass with argon or other inert gas between the panes, and a low emissivity (low-E) coating as described above.
  • Insulating glass with Integral Light Technology® grilles which create the look of true divided light while providing superior energy efficiency.
  • Double-pane glass with between-glass blinds and removable grilles
  • Triple-pane glass with between glass cellular shades, blinds, decorative panels and removable grilles.
  • Seal the deal

Check the manufacturer’s warranty to thoroughly understand what it does and does not cover. Most Pella product warranties provide at least 20 years of coverage on glass.