A dual-pane window is an Insulated Glass Unit (IGU), consisting of two panes of glass separated by a sealed spacer bar, creating an air or gas-filled cavity. This sealed space provides significant thermal insulation. Over time, the seal around the perimeter can fail due to temperature fluctuations and movement, allowing moist air to enter the cavity. This seal failure leads to the characteristic fogging or condensation between the panes. When this occurs, it is often possible to replace just the glass unit itself without needing to replace the entire window frame or sash.
Preliminary Assessment and Required Tools
Safety involves wearing thick work gloves and protective eyewear to guard against sharp edges. The most important initial assessment is determining whether the window is internally or externally glazed, as this dictates the entire repair approach. An internally glazed window means the glass is secured from inside the building, usually by removable vinyl or wood glazing beads visible from the interior. Conversely, an externally glazed window is secured from the outside, often by exterior glazing compound, specialized screw-in stops, or exterior trim pieces.
A specific set of tools is necessary to manipulate the frame and handle the glass unit. A stiff putty knife and a utility knife are needed to score and remove old caulk or glazing sealant around the perimeter. For safely handling the heavy glass, a pair of glass suction cups is recommended, especially for larger units, to maintain a secure grip. Re-sealing materials include a high-quality silicone sealant or caulk gun, along with the specialized driver bits or tools needed to remove existing frame screws or trim pieces.
Precise Measurement and Ordering the Insulated Glass Unit
Accurately measuring the size of the replacement IGU, to the nearest sixteenth of an inch, is essential for a successful replacement. The first measurement is the width, taken horizontally from the interior edges of the frame pocket where the glass rests, known as the sight line. The height is then measured vertically along the sight lines, ensuring measurement in two or three different spots for both dimensions to account for any slight variations. These measurements represent the size of the glass unit itself, not the exterior sash opening.
After determining the width and height, the thickness of the old unit must be precisely measured, as it directly affects how the new unit fits into the frame’s rabbet. Thicknesses commonly fall into ranges like 5/8 inch, 3/4 inch, or 1 inch, and this dimension must include both panes of glass and the separating air space. Use a caliper or precise measuring tape to measure the thickness by exposing a small section of the glass edge. Incorrect thickness will prevent the glazing beads from being reinstalled correctly, compromising the seal.
Ordering the new unit requires specifying any special features present in the existing glass, such as Low-E coatings. These are microscopically thin metallic layers designed to reflect infrared light and improve thermal performance. To confirm the presence of a Low-E coating, hold a lit match or a penlight near the glass and observe the reflected light. If four reflections are visible, and one has a distinct, often purple or blue tint, a Low-E coating is present. If the window is located in a safety-regulated area, such as near a door or within 18 inches of the floor, tempered glass may also be required by local building codes.
Once all specifications are gathered—including precise dimensions, thickness, and any special coatings or safety requirements—the information is submitted to a local glass supplier. Suppliers fabricate the custom IGU to the exact specifications. They often use argon gas instead of air in the cavity to enhance insulation properties. Confirming the frame pocket measurements ensures the new custom unit will be manufactured to fit perfectly.
Step-by-Step Removal and Installation
The physical removal process begins by carefully accessing the IGU’s edge, which differs depending on the glazing method identified earlier. For internally glazed windows, the interior glazing beads—often secured by small nails, staples, or friction—must be gently pried away using a stiff putty knife to expose the glass perimeter. For externally glazed windows, the exterior sealant or glazing compound must be deeply scored and cut away with a utility knife, or any exterior trim stops must be unscrewed and removed. All securing components must be cleared to allow the glass unit to move freely.
With the securing mechanism removed, the old IGU is ready for extraction, which requires two people and the use of glass suction cups to maintain control. The utility knife is used to slice through any remaining adhesive or silicone holding the unit in the frame, working carefully around all four edges. Once the sealant is cut, the old unit can be lifted straight out of the frame pocket, being mindful of its weight and potential for sharp edges. The old frame pocket must then be thoroughly cleaned, scraping away all residues of old caulk, sealant, and debris to ensure a smooth surface for the new unit.
The installation of the new IGU starts with the proper placement of setting blocks, which are small rubber or plastic shims placed at the bottom of the frame pocket. These blocks support the weight of the glass and elevate the unit slightly, allowing for proper drainage and preventing the glass from sitting directly on the frame. The placement of these blocks is dictated by the window type, usually placed 2 to 3 inches from the corners, ensuring the frame’s pressure equalization system functions correctly.
The new IGU is carefully lowered into the clean frame pocket and onto the setting blocks, ensuring it is centered horizontally and vertically within the opening. Once the unit is correctly positioned, the glazing stops or trim pieces are reinstalled to secure the glass in place. For internally glazed units, the interior beads are snapped or lightly tapped back into their original position, ensuring a tight fit against the glass surface. For externally glazed units, the exterior stops are screwed back into place or the exterior perimeter is prepared for final sealing.
The final step is to create a weather-tight perimeter seal around the newly installed IGU. A bead of high-quality, exterior-grade silicone sealant is applied along the entire edge where the glass meets the frame or the reinstalled stops. This sealant prevents moisture intrusion into the frame and protects the new IGU’s perimeter seal from direct exposure to the elements, minimizing the risk of premature seal failure. After the sealant cures according to the manufacturer’s directions, the window operation is tested to ensure smooth movement and a successful, weather-tight installation.