What causes windows to fog?

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 the window, on the inside or roomside surfaces, or in between the panes of glass.

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 interior moisture to help alleviate interior glass condensation.

See Pella’s Understanding Condensation fact sheet at:


Moisture and condensation between panes of insulating glass can occur when the seal between the panes of glass fails. This typically occurs in older windows or doors where the seal has failed and allows moisture to leak in. 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 prevalent on the sunny sides because the additional heat from the sun tends to 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 - generating infrared rays from the sun. Using low-E glass is generally considered the minimum necessary for comfort and energy savings during heating and cooling seasons.

Eventually, fogging will 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.”

A clear choice

“Choosing and investing in a glass system that meets your needs is an important decision in buying new or replacing old windows,” said Pella glass expert Dave Modtland. “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
In state-of-the-art product-testing facilities, Pella tests the components of its 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. And Pella tests for air and water infiltration during the manufacturing process. These stringent measures help ensure that Pella windows and patio doors will perform for years.

Look for verification

Check for third-party validation that the window meets the high standards of outside sources like the recognized gold standard 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 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.

For more information, visit: www.pella.com/news or call 888-847-3552.



Tags: window fogging, fogging windows, glass fogging, foggy windows, foggy glass, glass seal failure, fog on windows, fog between windows, condensation