Foggy Double-Pane Windows: Replace Just the Glass or the Whole Window?

Fog or a milky haze between the two panes of a double-pane window usually means the seal around the glass has failed, and no amount of cleaning will clear it. If the frame and sash (the part of the window that holds the glass) are still in good shape, you can usually replace just the sealed glass unit, which is often faster and less expensive than a new window. Under California's 2025 Energy Code, building departments such as Elk Grove's count new glass in an existing sash and frame as a repair, and Phoenix's energy code lists it as a repair too, so the energy limits for replacement windows do not apply. Replace the whole window when the frame is damaged, the window no longer works well, or you want a new size or style.

What Fog Between the Panes Means

A double-pane window is a sealed package. Two sheets of glass are held apart by a spacer bar around the edge, and the spacer holds a drying material called desiccant that soaks up stray moisture. The glass industry calls this package an insulated glass unit, or IGU. When the edge seal weakens, moist outdoor air gets inside. Once there is more moisture than the desiccant can absorb, it condenses on the inside faces of the glass, right where you can't reach it.

The industry has a clear definition for this. A technical bulletin known as IGMA TB-1205-89 (IGMA is the Insulating Glass Manufacturers Alliance) treats a unit as failed when moisture, dust or film on the inside surfaces of the glass permanently blocks your view. An engineer writing in The Construction Specifier adds that once the seal is breached, the fogging is permanent and will not reverse.

A failed seal costs you more than the view. ENERGY STAR says failed seals reduce a window's insulating power and can leave the glass cloudy and impossible to clean. Seal failure can also let out the insulating gas some units are filled with.

A quick check: if you can wipe the moisture off either side of the glass, it is surface condensation. On the room side, that usually comes from humid indoor air. Fog you cannot touch is between the panes.

Your Three Options at a Glance

Most foggy windows come down to one of three fixes, and how much you replace decides which rules apply.

Option	What gets replaced	How codes treat it	Usually makes sense when
New glass unit	Only the sealed glass. The frame and sash stay.	A repair on Elk Grove's 2025 Energy Code form and in Phoenix's energy code	The frame is solid and the window opens, closes and locks well
New sash	The moving panel and its glass. The frame stays.	A repair on Elk Grove's 2025 Energy Code form	The sash is worn or damaged but the frame is sound
Whole new window	Glass, sash and frame	A replacement: permit, energy limits and bedroom escape rules can apply	The frame is damaged, the window no longer works well, or you want a new size or style

Elk Grove's January 2026 window form, written for the 2025 California Energy Code, spells this out: glass replaced in an existing sash and frame, or a new sash in an existing frame, counts as a repair. The California Energy Commission's 2025 compliance manual explains that the Energy Code applies to additions and alterations, but not to repairs. On that same Elk Grove form, replacement windows must have a maximum U-factor of 0.27 and a maximum solar heat gain coefficient (SHGC) of 0.23, unless an exception applies. ENERGY STAR explains the two numbers simply. The lower the U-factor, the better the window insulates. The lower the SHGC, the less of the sun's heat comes through.

In Arizona, building codes are adopted and enforced locally rather than by the state. Phoenix put its 2024 codes, including its energy code, into effect on August 1, 2025, and that energy code lists glass-only replacement in an existing sash and frame as a repair (Section R504.2). Elsewhere in Arizona, your city or county building department can tell you which rules it follows.

When Replacing Just the Glass Makes Sense

The glass-only fix fits when the frame is solid, with no rot, warping, cracks or corrosion, and the window still opens, closes and locks the way it should. If the frame is sound, re-glazing is often faster and less expensive than replacing the whole window. With our glass repair and replacement service, we replace the failed IGU inside your existing frame and match the glass thickness, tint and coating so the repair blends in with the windows around it.

A new unit is also a chance to choose better glass. A U.S. Department of Energy weatherization guide says replacement glass should meet or exceed the specifications of the glass it replaces. Through our energy efficient glass upgrades, we recommend a Low-E coating, gas fill and spacer based on your climate zone and the direction the window faces. For a window that takes hard afternoon sun in Phoenix or an inland California valley, a lower SHGC means less heat coming through.

Safety rules still apply to glass-only work. Culver City's window handout, for example, says glass-only work needs no building permit there, and it adds that the glass must still meet code. Between them, the Culver City and San Bernardino County handouts list the spots that need safety glass, such as tempered glass. They include glass within 24 inches of the edge of a door, and glass in a wall that encloses or faces a tub, shower, spa or pool when its bottom edge is less than 60 inches above the floor or walking surface. They also include a large pane (over 9 square feet) when its bottom edge is less than 18 inches off the floor, its top edge is more than 36 inches up, and a walkway is within 36 inches of it. Culver City adds that tempered glass must carry a permanent maker's mark, such as a sandblasted or etched stamp. A sticker does not count.

When the Whole Window Should Go

ENERGY STAR lists the signs that point to a new window: single-pane glass, air leaks and drafts, worn-out sills and sashes, and windows that will not open or are painted shut. A bent or corroded frame, or a wish for a bigger opening or a different style, points the same way. If many units in the house are fogging at once, it can be worth pricing new windows alongside new glass. When a whole new window is the right call, our window installation team can replace it, and most single window replacements take two to four hours.

A whole new window brings more rules with it:

• Permit. Culver City and San Bernardino County both require a building permit to replace a whole window, even a same-size swap or a retrofit window that leaves the old frame in place.
• Energy numbers. New windows must meet your area's U-factor and SHGC limits. Under California's 2025 Energy Code, the U-factor limit is 0.27 or 0.30 depending on your climate zone, so local forms differ. Elk Grove's form lists 0.27 and 0.23 with some exceptions, and San Bernardino County's handout lists 0.30 and 0.23, so check your own city's form.
• Bedroom escape openings. Each bedroom needs at least one emergency escape opening, often called an egress window. A window that serves as one must have a clear opening of at least 5.7 square feet (5.0 at grade level), at least 24 inches high and 20 inches wide, with the bottom of the opening no more than 44 inches above the floor. San Bernardino County and Culver City both exempt a replacement that is the maker's largest standard-size window that fits the existing frame or rough opening.
• Fire areas. In San Bernardino County's Fire Safety Overlay area, new and replacement windows need multipane glass with at least one tempered pane.
• Older homes. In homes built before 1978, old window paint may contain lead. EPA's lead renovation rule requires firms that disturb lead-based paint in those homes to be certified and to use certified renovators, so ask about lead-safe work before the old windows come out.

ENERGY STAR notes that many local utilities offer rebates for high-performance windows, so check with yours before you order.

Check the Warranty Before Anyone Touches the Glass

If your windows are fairly new, find the paperwork before you book any repair. Some window warranties cover seal failure. One vinyl window warranty effective May 25, 2025, for example, covers seal failure and moisture or dust between the panes for as long as the original buyer owns and lives in the home, and for 20 years from the date of manufacture for the next owner. It covers labor for only one year, and it does not cover improper repairs or changes made by anyone other than the maker or its agents. So call the window maker first. If coverage has ended, a new sealed unit in your existing frame is still a good option.

Keep the New Seal Healthy

Water is hard on the seal. The Construction Specifier article notes that water touching the edges of the glass unit breaks down the sealant and loosens its grip over time. The required maintenance list in that same window warranty gives homeowners an easy routine:

• Keep the sill track clean so water can drain outside.
• Keep the weep holes, the small drain slots at the bottom of the frame, clear.
• Check the outside caulking regularly and replace it when needed.
• Keep indoor humidity at moderate levels.

These habits help any double-pane window last, whether it sees coastal fog in California or summer storms in Arizona.

Frequently Asked Questions

Not sure whether your window needs new glass or a whole new window? Send us a photo or the opening size and we will recommend the best option. We serve homeowners across California and Arizona, with crews dispatched from Los Angeles and Phoenix, under California C-17 license 1034872 and Arizona ROC license 343677. Our in-home estimates are free, and we email a written quote within one to two business days. Request a Free Consultation