You can replace just the glass, not the whole window frame

Double pane glass replacement means removing the sealed glass unit from your window frame and installing a new one. The frame itself stays in place. This is cheaper and faster than replacing the entire window, and you can do it yourself if you have basic tools and patience—though the job takes several hours and requires careful measurement.

The sealed unit (called an insulated glass unit or IGU) is what fails over time. When the seal breaks, moisture gets trapped between the panes, and you see condensation or fogging that won't wipe away. You cannot repair this—the unit must be replaced as a whole.

Key Takeaways

  • Measure the glass opening inside the frame (the sight line) in three places—top, middle, and bottom—and use the smallest measurement to order your replacement unit.
  • Remove the old glass by prying out the rubber splines or vinyl stops that hold it in place, then lift the unit straight out and set it on a padded surface.
  • Clean the frame channel thoroughly before installing the new unit, because dirt or old sealant will prevent a tight fit.
  • Press the new unit into the frame by hand first, then tap the splines or stops back in with a rubber mallet to create an airtight seal.
  • The entire job—measuring, removal, installation, and cleanup—typically takes three to five hours for a single window.

Measure the glass opening three times before ordering

The opening where the glass sits is called the sight line. Measure the width at the top, middle, and bottom of the opening. Measure the height on the left, center, and right. Write down all six measurements. Order a replacement unit that is 1/8 inch smaller than your smallest measurement in each direction—this leaves room for the rubber seal and prevents binding.

If your measurements vary by more than 1/4 inch from top to bottom or left to right, your frame may be out of square. Take a photo and bring it to the glass supplier; they can advise whether the unit will still fit or whether the frame needs shimming. Do not guess on this step—an ill-fitting unit will not seal properly and will leak or rattle.

Order the replacement unit from a glass supplier, not a big-box store. Suppliers can match the thickness, tint, and coating of your original glass and will cut the unit to your exact measurements. Expect to wait one to two weeks. The cost ranges widely depending on size and glass type, but a standard double pane unit for a 3-by-4-foot window typically costs $150 to $300.

Remove the old glass by prying out the splines or stops

The glass is held in place by either rubber splines (cord-like strips that fit into a groove) or vinyl stops (plastic strips screwed or glued to the frame). Look at your window to see which type you have. Lay a drop cloth underneath to catch glass and debris.

For splines: Use a flathead screwdriver or a spline removal tool to pry one end of the spline out of its groove. Once you have a grip, pull the spline straight out—it should come free in one piece. Do this on all four sides. The glass will now be loose.

For stops: Unscrew any visible fasteners, or carefully pry the stop away from the frame with a flathead screwdriver. Work slowly so you do not crack the plastic or wood. Once all stops are removed, the glass will lift free.

Lift the old glass unit straight up and out of the frame. Set it on a padded surface (an old blanket works) so it does not crack further. If the glass is broken, wear heavy gloves and safety glasses. Dispose of the old unit at a glass recycler or hazardous waste facility—do not put it in the trash.

Clean the frame channel and check for damage

The channel where the glass sits will have old sealant, dust, and debris. Use a putty knife or old credit card to scrape away dried caulk or glazing compound. Vacuum out the loose material. Wipe the channel with a damp cloth and let it dry completely—moisture trapped under the new unit will cause fogging.

Inspect the frame for rot, especially in wood frames. Press a screwdriver into the wood at the corners and along the bottom. If the wood is soft or spongy, it has rotted and must be repaired or the frame replaced. If the frame is sound, you can proceed.

Check that the frame is still square by measuring the diagonals (corner to opposite corner). Both diagonal measurements should be equal. If they differ by more than 1/4 inch, the frame is racked and may need shimming. Ask your glass supplier whether your replacement unit will still fit, or whether you need to adjust the frame before installation.

Install the new glass and press the splines or stops back in place

Carefully lower the new glass unit into the frame. Center it so there is equal space on all sides. Press it down by hand until it sits flat and snug in the channel. Do not force it—if it binds, lift it out and check for debris or misalignment.

For splines: Roll the spline back into its groove, starting at one corner and working around the frame. Use a spline roller tool (a plastic wheel on a handle) or a rubber mallet and a block of wood to tap the spline in evenly. The spline should sit flush in the groove without gaps. Work slowly and check often that the glass is not shifting.

For stops: Press the stop back against the frame and reinstall any fasteners. Tighten screws snugly but not so hard that you crack the plastic or crush the glass seal. If the stops were glued, explore new adhesive (silicone or construction adhesive) before pressing them back in place.

Seal the perimeter and let the sealant cure

Once the splines or stops are in place, run a bead of silicone caulk around the outside perimeter where the frame meets the wall or trim. This seals out air and water. Use a caulk gun and smooth the bead with a wet finger or caulk tool. Let the caulk cure for 24 hours before opening or closing the window.

Do not use the window for at least 24 hours. The sealant needs time to set, and the glass unit needs time to stabilize in the frame. After 24 hours, open and close the window gently a few times to make sure it moves freely.

Common mistakes that cause leaks or fogging

Measuring only once instead of three times is the most common error. Frames are often slightly out of square, and a unit that is too large will bind and crack the seal. Always take six measurements and use the smallest one.

Leaving dirt or old sealant in the channel prevents the new unit from sitting flush. Spend time cleaning—it is the difference between a tight seal and a leaky window. Moisture trapped under the glass will cause fogging within weeks.

Forcing the spline or stop back in too hard can crack the glass or compress the seal unevenly. Work slowly and check alignment often. If you feel strong resistance, stop and look for the cause instead of pushing harder.

Caulking the inside perimeter instead of the outside is another mistake. The caulk should be on the exterior where water runs off. Interior caulk traps moisture and can cause rot.

Frequently Asked Questions

Can I replace just one pane if only one is fogged?

No. Double pane units are sealed as a single assembly. If one pane is fogged, the entire unit has failed and must be replaced. You cannot separate the panes or reseal them yourself.

What if my window has a wood frame and the putty is hard and cracked?

Scrape out the old putty with a putty knife. If it is very hard, soften it first with a heat gun (hold it 6 inches away and move it constantly so you do not crack the glass). Once the frame is clean and dry, install the new unit and explore fresh silicone caulk around the perimeter instead of putty—it lasts longer and is easier to remove if you need to replace the glass again.

How do I know if my frame is damaged enough that I should replace the whole window instead?

If the frame is rotted, severely warped, or the sash does not open and close smoothly, replacement may be more cost-effective than repair. If the frame is solid and the window operates normally, glass replacement alone is the right choice. A glass supplier can assess this during your measurement visit.

Do I need to remove the window from the wall to replace the glass?

No. You can replace the glass while the window is in the frame. The entire job happens from inside or outside the house. Removing the window makes some steps easier, but it is not necessary and adds time and complexity.

What if I crack the new glass while installing it?

Stop when ready and contact your glass supplier. Most suppliers offer a one-time replacement may provide if the unit is damaged during installation. Do not try to seal or use a cracked unit—it will not insulate properly and may fail suddenly.