Window Condensation: Double Glazing Failure, Diagnosis & Replacement Criteria

Summary

Window condensation complaints are one of the most common calls a joiner or glazier receives. Most customers report "steamed-up windows" without distinguishing between the three very different causes: failed sealed unit (requires glazing work), condensation on the room-facing surface (ventilation problem), or external condensation (entirely normal and actually a sign of a well-insulated unit).

Correctly diagnosing which type of condensation is present before starting any work prevents unnecessary unit replacements and sets the right customer expectations. This article provides the diagnostic framework and guidance on when and how to replace failed sealed units.

Key Facts

• IGU (Insulating Glass Unit) — a sealed double-glazed unit; two panes of glass with a spacer bar around the edge, hermetically sealed with a primary and secondary sealant; the cavity is filled with air or argon gas
• Seal failure — when the primary seal (butyl) and/or secondary seal (polysulphide or silicone) degrade or are physically damaged, moisture enters the cavity; the desiccant in the spacer bar is exhausted; condensation forms between the panes
• Visual signs of seal failure — moisture streaks or misting between panes; white mineral deposits (calcium carbonate from evaporated water) between panes; permanent haze that moves when temperature changes
• Cannot be repaired — once the desiccant is saturated and the seal has failed, the unit must be replaced; attempts to drill and inject silica are short-term and not effective
• Causes of seal failure — UV degradation of the secondary sealant (typically 10–25 years lifespan); physical damage to the edge seal; pressure cycles (large temperature swings cause the glass to bow and stress the seal); water sitting in the rebate
• Room-side condensation (single or double glaze) — forms when warm humid air meets a cold glass surface; the glass surface temperature is below the dew point of the room air; not a glazing defect; resolved by reducing indoor humidity, improving ventilation, or improving glazing U-value
• External condensation — forms on the outer pane when the glass surface is colder than the outdoor dew point (typically cold early mornings after a clear night); this only happens with well-insulated units (the outer pane doesn't receive heat from the room, so it gets very cold); it is a sign of a good unit, not a defect
• U-value impact — standard double glazing with air cavity: approximately 2.8 W/m²K; argon-filled double glazing: 2.4–2.8 W/m²K; low-E coating + argon: 1.6–1.8 W/m²K; triple glazing: 1.0–1.4 W/m²K
• Low-E coating — thin metallic coating on one glass face that reflects long-wave infrared radiation back into the room; reduces U-value significantly; coating is typically on the room-facing surface of the inner pane (surface 3 of 4); check orientation when replacing
• Spacer bar — aluminium (traditional, thermally bridging) vs warm edge (TGI, Thermix, Superspacer — polymer or composite; reduces cold edge and frame condensation risk); warm edge reduces condensation risk at the glass edge
• Glass types — standard clear float; tempered/toughened (required in critical locations per Approved Document K); laminated (required for overhead glazing); self-cleaning (TiO2 coating on outer pane)

Quick Reference Table

Detailed Guidance

Diagnosing the Type of Condensation

Step 1: Touch the glass

• Condensation on the outer surface and the glass feels cold: this may be external condensation (normal) or a very cold outer pane with the room humid — check for causes
• Condensation on the inner surface (room side) with moisture wiping off with your hand: room humidity problem, not glazing defect
• Condensation between the panes that cannot be touched: sealed unit failure — the moisture is inside the cavity

Step 2: Check the dew point For room-side condensation, the surface temperature of the glass is below the dew point of the room. Check:

• Is the room unusually damp? (cooking, drying laundry, shower without extraction)
• Is there adequate trickle ventilation? (check trickle vent position on the window frame — many are inadvertently closed)
• Is the heating operating correctly? (cold rooms condensate faster)

Step 3: Check the timing

• External condensation typically appears early morning after a cold clear night and clears as the sun warms the glass — this is normal
• Sealed unit failure: condensation is present at varied temperatures and does not clear; may have visible streaks or deposits

Step 4: Document Take a photograph (with the light behind you) showing the condensation location clearly. This is important for customer communication and potential warranty claims on younger windows.

Replacing a Failed Sealed Unit (IGU)

Measuring the unit:

1. Measure the overall width and height of the existing unit (pane-to-pane; not frame-to-frame)
2. Measure the unit thickness (from outer glass face to inner glass face)
3. Note any visible Low-E coating (iridescent surface on one face; check by holding a lighter to each glass face — the reflected flame appears differently on coated glass)
4. Note the spacer bar colour and type (aluminium or warm edge)

Standard IGU specification for Building Regulations compliance (Part L 2021): When replacing an IGU in an existing property, the new unit must meet the minimum U-value of ≤1.6 W/m²K for windows (Part L, existing buildings). For new build or refurbishment to higher standard: ≤1.4 W/m²K. Standard specification: 4mm glass, 16mm argon-filled cavity with warm edge spacer, 4mm glass with soft-coat Low-E (4-16-4 Ar Low-E); achieves approximately 1.6 W/m²K.

Ordering the replacement: Give the glazing supplier: overall width × height × thickness (e.g., 600 × 800 × 24mm); Low-E on surface 3 (room side inner pane facing surface); soft coat or hard coat (specify — most modern domestic units are soft coat); toughened if the unit is in a critical location.

Removal:

1. Remove the bead from the inside — use a stiff putty knife inserted at the corner; beads are typically push-fit and clip into the frame; note the order for replacement
2. Lift the unit out (with assistance — even a small IGU is heavier than it looks; 4-16-4 IGU is approximately 20–25 kg/m²)
3. Clean the rebate — remove old glazing blocks, dirt, and any failed setting block rubber

Installation:

1. Position new glazing blocks — typically two 6mm PVC blocks per pane, at the quarter points on the sill; blocks centre the IGU and ensure drainage; do not use timber or wedges
2. Offer the new unit into the frame — tilt bottom in first
3. Check the unit is level and centred in the frame
4. Refit the beads — clip back in; ensure a consistent gap on all four sides (typically 3–5mm rebate coverage)
5. Check for rocking (indicates uneven glazing blocks) and adjust
6. Seal: in timber frames, a continuous bead of compatible sealant (e.g., low-modulus silicone) between the glass and the bead provides a weather seal; in PVCu frames, a rubber gasket within the bead profile provides the seal — ensure it is correctly seated

Preventing Future Seal Failure

The lifespan of an IGU seal is 10–25 years depending on exposure and installation quality. Key factors affecting lifespan:

1. Water sitting in the rebate — the most common cause of premature seal failure; the frame must be sealed and drained to prevent water pooling against the unit edge; drainage slots in the bottom rail of PVCu frames must be clear
2. Sealant compatibility — silicone is incompatible with some IGU edge sealants; use an IGU-compatible sealant in the rebate; most manufacturers specify compatible sealants
3. Thermal stress — units facing south in exposed locations experience greater thermal cycling; aluminium spacers transfer more stress to the sealant than warm edge spacers

Frequently Asked Questions

My customer says their windows were replaced 5 years ago but the seal has already failed. Is that normal?

No — a quality IGU should last 10–20+ years with correct installation. Premature failure at 5 years suggests: incorrect installation (water in rebate; wrong sealant; frame incompatibility); manufacture defect (very occasionally); unusually severe exposure conditions. Check with the original installer — there may be a manufacturer's warranty on the unit.

Can I just drill the unit to let the moisture out?

Some glaziers offer this service — drill a small hole into the cavity and inject desiccant. It provides a temporary improvement (the cavity dries out), but the seal has failed and moisture will re-enter through the same route. The improvement typically lasts 6–24 months. This is not a permanent fix and is generally not recommended for an occupied property; replacement is the correct solution.

Do I need Building Control notification to replace a sealed unit?

No — replacing a like-for-like sealed unit within an existing frame is not notifiable to Building Control. However, replacing an entire window (frame and unit) is notifiable (and must be done by a FENSA or CERTASS-registered installer, or submitted to Building Control). Sealed unit replacement within an existing frame is maintenance work, not installation of a new window for regulatory purposes.

Regulations & Standards

• Approved Document L:2021 — Energy conservation; replacement glazing U-value requirement ≤1.6 W/m²K

• Approved Document K — Safety glazing; critical locations requiring toughened or laminated glass

• BS 6262-4:2005 — Glazing for buildings: safety relating to human impact

• FENSA — Competent person scheme for window and door installation; not required for sealed unit replacement within an existing frame

• Glass and Glazing Federation (GGF) — Technical guidance on IGU specification and condensation

• Planning Portal: Windows — When replacement requires planning permission

• FENSA — Registered installer scheme; notification requirements

• security locks — Window lock requirements when replacing units

• draught proofing — Sealing around windows and doors

• condensation — Room condensation causes and ventilation solutions

• conservatory planning — Glazing specifications for conservatories