How To Change A Double Glazed Unit: The Complete Professional Replacement Guide

Replacing a failed double glazed unit—commonly known as a sealed unit or double glazed glass pane—restores your window's thermal efficiency, eliminates internal condensation, and eliminates drafts. This comprehensive guide walks you through the exact measurements, safe bead removal, glass extraction, and reglazing techniques required to complete the job to professional fenestration standards.

Pre-Operation & Planning for Double Glazed Unit Replacement

Successfully changing a double glazed unit requires a methodical approach, precise measurements, and specialized handling equipment. Unlike single-pane glass, double glazed units are heavy, factory-sealed insulating glass units (IGUs) composed of two panes of glass separated by a spacer bar and filled with dehydrated air or inert gas such as argon.

• Essential Gear, Tools, and Materials:
  
  
  • Heavy-duty glass suction cups (double pad lifters)
  • Soft-headed mallet or nylon glazing mallet
  • Rigid putty knife or a specialized glazing chisel/bead removal tool
  • Flathead screwdriver
  • Glazing shovels or setting blocks (in various thicknesses: 1mm, 2mm, 3mm, 5mm)
  • Silicone sealant gun and neutral cure low-modulus silicone or glazing tape
  • Spirit level
  • Protective cut-resistant gloves, safety glasses, and sturdy work boots
• Mandatory Prerequisite Knowledge and Standards:
  
  
  • Verify whether your windows feature internal or external glazing beads. Most modern uPVC and aluminum windows feature internal beads, meaning removal happens from the inside. Timber windows often feature external putty or wooden beads.
  • Understand the safety requirements of handling tempered or toughened safety glass, which must not be stressed or struck at its corners.
  • Comply with local building regulations regarding safety glass in critical locations (such as low window sills and doors).
• Project Scope Benchmarks:
  
  
  • Estimated duration: 45 to 90 minutes per window unit.
  • Estimated budget: Cost of the replacement IGU plus basic tooling (if not already owned).

Step-by-Step Execution of Double Glazed Unit Replacement

Step 1: Accurately Measuring the Existing Sealed Unit

Before ordering a replacement unit, you must measure the width, height, and overall thickness of the existing glass. Never assume squareness; measure both diagonals and width/height at multiple points. Remove one glazing bead to measure the exact thickness of the unit (typically 24mm, 28mm, or 32mm) using a digital caliper. Deduct 5mm to 6mm from the overall width and height measurements to allow for thermal expansion and proper placement on setting blocks.

Warning: Ordering an IGU that is even a fraction of a millimeter too thick will prevent the glazing beads from snapping back into their locking channels, rendering the window unsecure.

Step 2: Removing the Internal or External Glazing Beads

Locate the seam of the glazing beads on the frame. Insert the flat blade of your glazing chisel or a stiff putty knife into the center join of the longest vertical bead. Tap the tool gently with a mallet away from the glass until the bead pops out of its retention groove. Remove the opposite vertical bead next, followed by the top and bottom beads. Mark each bead with a piece of tape indicating its original position (e.g., top, left, bottom, right), as they must be reinstalled in their exact locations.

Pro-Tip: Always remove the longest beads first and reinstall them last to prevent warping or snapping the PVC-u or aluminum extrusions.

Step 3: Extracting the Failed Double Glazed Unit

With the beads removed, the unit is held in place only by double-sided security tape or old silicone. Slide your glazing shovel carefully between the glass and the frame to break any adhesive bonds. Attach your double suction cup lifters securely to the center of the outer glass pane. Carefully tilt the heavy unit toward you, lift it off the plastic setting blocks at the bottom of the frame, and walk it away to a secure, upright storage area on wooden blocks.

Step 4: Inspecting and Preparing the Window Rebate

Examine the window rebate and drainage channels. Clear out any dirt, old silicone, or perished glazing tape. Check that the plastic setting blocks (bridge packers) are clean and correctly positioned at the quarter points of the bottom frame member. These blocks support the entire weight of the new glass and ensure the window remains square while allowing adequate drainage beneath the IGU.

Step 5: Installing the New Double Glazed Unit

Using your suction cups, lift the new double glazed unit and lower it evenly onto the setting blocks inside the rebate. Ensure the unit is centered and does not make direct contact with the aluminum or uPVC frame at any point. Use a spirit level across the frame to check that the window is plumb and square. If the window opener is sagging, adjust the position of the setting blocks using thinner or thicker packers on the diagonal corners to toe and heel the sash properly.

Step 6: Re-securing the Glazing Beads

Take your previously marked glazing beads and snap them back into place. Start with the shorter horizontal beads first, followed by the longer vertical beads. Align one end of the bead into the channel and strike it firmly with a nylon glazing mallet toward the other end until it locks securely with an audible snap. Ensure the rubber gasket compressed against the glass creates a watertight, uniform seal around the entire perimeter of the window.

Technical Specifications of Insulating Glass Units (IGUs)

Common Site Failures and Field Fixes

• Root Cause: The replacement glass unit is slightly too thick, preventing the final glazing bead from locking into the extrusion channel.
  
  
  • Actionable Fix: Remove the unit and check the spacer bar thickness or edge tape width. Re-measure the rebate depth and order a replacement unit manufactured to the correct millimeter tolerance.
• Root Cause: The window sash drops or rubs against the frame after installing the new, heavier glass unit.
  
  
  • Actionable Fix: Re-open the sash and adjust the "toe and heeling" setting blocks. Insert a packer under the bottom corner opposite the hinge side and raise the top corner to square the operational sash back into alignment.
• Root Cause: Internal condensation reappears inside the new unit within weeks of installation.
  
  
  • Actionable Fix: The IGU seal was compromised during transport or manufacturing. Contact your supplier immediately under warranty for a replacement unit, as field repairs on broken hermetic seals are impossible.

Frequently Asked Questions

Can I replace just the glass without buying a whole new window frame?

Yes, replacing the double glazed unit (the glass pane and spacer assembly) is a standard maintenance procedure. As long as your existing window frames (uPVC, aluminum, or timber) remain structurally sound and free from rot or distortion, you only need to order a custom-sized replacement sealed unit.

How do I know if my double glazed unit has failed?

A failed unit is typically identified by internal condensation, fogging, or a milky white film trapped between the two glass panes. This occurs when the primary and secondary butyl sealants break down, allowing moisture-laden air to enter the cavity and saturate the internal desiccant.

Is it safe to replace a double glazed unit by myself?

Replacing a double glazed unit is well within the capabilities of a competent DIYer, provided you use proper safety gear, cut-resistant gloves, and glass suction cups. Because standard IGUs can weigh anywhere from 15kg to over 40kg, working with a second person is strongly recommended for safety and control.

Do I need to apply silicone sealant around the edge of the new glass?

Most modern window systems rely on internal rubber gaskets integrated into the frame and glazing beads to maintain weather tightness. However, if you are working with older timber frames or specialized systems that require external face-putty or bead-sealing, apply a continuous bead of low-modulus neutral cure silicone to seal the exterior joints.

How long should a new double glazed unit last?

A high-quality insulating glass unit manufactured to industry standards typically lasts between 15 and 25 years. Longevity depends heavily on proper installation, adequate drainage within the window rebate, and exposure to extreme thermal fluctuations.

Upgrade Your Home's Energy Efficiency Today

Restore optimal thermal performance, eliminate unsightly condensation, and secure your property by replacing your failed glazing units with modern, high-efficiency IGUs. Browse our premium selection of custom-measured replacement double glazed units or connect with our fenestration specialists to get started on your project today.