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We have 10 y.o. "Low-E" windows. Last autumn we noticed one opening panel "hazing" up inside: enter image description here

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What is it? Does that indicate loss of hermitization? How to fix it? Can it be done DIY-style?

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    Do not think it can be fixed DIY. The only diy solution I know of is to vent the space between window panes to reduce the fog/haze.
    – crip659
    Commented Mar 24, 2021 at 22:37
  • Remove the sash and take it to a glass shop, or order a replacement.
    – Hot Licks
    Commented Mar 25, 2021 at 11:38

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"Hazing" within a dual-glazed window or door is a symptom of failure in the sealing that's supposed to maintain unchanged whatever (air, other gas, vacuum) is within the space between the two panes of glass.

Windows and sliding doors in my house were replaced with dual-glazed panels in 2008; several panels have recently developed a similar appearance. The manufacturer (Milgard) provided a lifetime warranty to the original purchaser, and Milgard's immediate reaction to my warranty claim was to schedule a date for replacement of the defective panels.

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  • Still air is a good thermal insulator. In fact trapped air is the principle behind numerous insulating materials such as foam, blown fiberglass, blown cellulose and bedding materials.
    – Katie Epps
    Commented Mar 25, 2021 at 12:32
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    @MicahEpps: Air is only a good thermal insulator if it doesn't move between regions with different temperatures. If a vertical space separates two sheets of material at different temperatures, air in contact with the hotter side will be less dense than air that's in contact with the bottom side, resulting in convection currents. Other gases may reduce heat transfer compared with air
    – supercat
    Commented Mar 25, 2021 at 18:09
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The seal between the two panes of glass has failed, allowing water vapor to diffuse into that space. When temperatures are right the vapor condenses as a haze/mist/fog on the colder piece of glass.

You can't "fix" the glass unit DIY, but replacement is within the realm of DIY (assuming there isn't a warranty that will take care of this for you!). It's particularly easy in this case because it's the moving (ie removable!) panel that has failed.

Remove the slider from the window frame. Bring it into the room so you can easily see the exterior side of it. You'll find some kind of plastic trim piece bordering the glass. I'm sure it varies among manufacturers, but the one I did had trim strips about 1/2" wide and snapped into place. I removed them with the help of a putty knife.

With that trim piece removed you can measure the dimension of the glass unit itself. Make note of the width and height, and also the thickness of the glass panes individually and the unit as a whole.

Call one or more local window suppliers until you find one who'll make a new glass unit for you. You'll need to give them those measurements, specify the color and size of the grid, make choices about coatings on the glass and gas fill of the unit, and so on.

When the replacement unit arrives installation is relatively easy. The glass unit is likely secured in the plastic frame with foam mounting tape. Use the putty knife or other tool to cut the tape all the way around, pop the old glass unit out, and install the new unit. Make sure it is oriented correctly (the correct side faces outward) if appropriate.

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