An insulated glass unit (IGU) is the sealed multi‑pane unit (two or more glass lites) with a spacer and a sealed air or gas cavity; it’s what fails when you see fog, haze, or condensation trapped between panes[1][2][10].
Choose IGU‑only replacement when the IGU seal has failed but the sash, frame, flashing, and hardware are sound and you want to preserve the existing frame or lower cost[1][2][4]. Choose whole‑window replacement when frames or sashes are rotted, warped, leaking, or hardware is failing, or when you need a performance upgrade that the existing frame can’t accept[1][3][5].
How to tell if the IGU has failed (not just dirty glass)
Visible condensation, fogging, or a milky haze between the panes indicates the IGU seal has failed and moisture has entered the cavity — this is not surface dirt and cannot be wiped away from the inside of the room[1][4][2].
Other signs that point to a seal failure: streaking or permanent residue between panes, or a clear change in optical clarity over time consistent with trapped moisture[1][10].
Repair options and what they really fix
Temporary/low‑cost defogging (drill, clean, dry)
Defogging or restoration services commonly involve drilling small holes, drying and cleaning the cavity, and sealing. These are lower‑cost, typically cosmetic/temporary options (consumer guides commonly show about $100–$250 per window) but they do not reliably restore factory gas fill or the original factory seal performance[4][11].
IGU (glass‑only) replacement
Replacing just the IGU — ordering a new sealed unit and installing it into the existing sash — can restore insulating performance when the replacement matches the desired specifications (low‑E coatings, argon fill, spacer type) and is produced to recognized standards and certifications such as ASTM E2190 and FGIA product certification[6][7][2].
Glass‑only replacement is the common recommendation when the frame and sash are in good condition and the homeowner wants a lower‑cost option or to preserve an existing frame (for example, with historic sashes)[1][2][4].
Full window replacement
Replace the entire window when frames or sashes are rotted, warped, leaking, hardware is failing, or when you want a performance upgrade (different frame materials, improved U‑factor, or triple glazing) that the existing frame cannot accommodate[1][3][5].
Costs: what homeowners can reasonably expect
Typical installed glass‑only IGU replacement for standard sizes commonly ranges roughly $200–$800 per unit; specialty, oversized, low‑E, argon‑filled, or triple units can cost substantially more (examples and industry guides note $200–$1,500+ for custom or high‑performance units)[3][2][4][11].
Defogging services are commonly cheaper but temporary and often listed in the ~$100–$250 per window range by consumer guides and glazing shops[4][11]. Full window replacement will typically be several hundred to $1,000+ per window depending on brand, frame, and features; compare that against the glass‑only option when making a decision[3][2][11].
Warranties, standards, and durability
ASTM E2190 is the standard specification used as the baseline for IGU performance and evaluation — buyers should look for units produced to that standard as an assurance of manufacturing test practices and expected durability[6].
Industry product certification (FGIA) and technical guidance (IGMA technical bulletins) document recommended materials, sealant compatibility, and testing practices manufacturers use to support IGU durability; these resources explain why material compatibility and proper sealant systems matter for long service life[7][8].
Warranties and durability differ between manufacturer/brand‑installed full‑window replacements and aftermarket glass‑only replacements; homeowners should check warranty terms before they proceed[10].
When to pick IGU‑only vs. whole‑window: a short checklist
Choose IGU‑only replacement if: sash and frame are solid, weatherstripping and flashing are intact, and the owner wants lower cost or to preserve original frames (e.g., historic sashes)[1][2][4].
Choose whole‑window replacement if: frames or sashes are rotted, warped, leaking, hardware is failing, or desired thermal/air‑tightness improvements require a new frame system[1][3][5].
Consider defogging only as a temporary or cosmetic fix; it may be appropriate when budget or timing rules out replacement in the short term, but plan for eventual IGU replacement if insulating performance is a priority[4][11].
Step‑by‑step: IGU replacement inside an existing sash (overview)
Note: this is an overview of common industry steps. Local practices and sash constructions vary; use a qualified glazing contractor or follow manufacturer guidance for your sash type. Ensure safety glazing requirements are met if the unit is in a code‑required hazardous location (safety glazing rules like the CPSC final rule/ANSI methods apply)[9].
Confirm diagnosis: verify condensation/fog between panes and confirm the sash and frame are structurally sound and dry[1][4].
Measure the existing IGU accurately (visible sightlines and rabbet depths) and document desired performance (low‑E, argon, spacer type) so the replacement unit can be ordered to ASTM E2190/FGIA expectations[6][7][2].
Remove stops or glazing beads carefully to access the IGU, preserving paint and sealant where possible; keep track of clips and hardware so reinstallation matches original sealing details[2].
Take out the failed IGU and clean the rabbet and sash channel; inspect for rot, corrosion, or water intrusion that would argue for whole‑window replacement[1][2].
Install the new IGU with proper setting blocks, perimeter sealant as recommended for the sash material, and replace glazing beads/stops. Ensure the replacement IGU’s certification and materials match the specified performance[6][7][8].
Check operation, sightlines, and seals; test for water intrusion and proper glazing bite. Review warranty paperwork from the IGU producer and the installer[10][2].
Practical tips for homeowners
Ask whether the replacement IGU is produced to ASTM E2190 and whether the maker or installer carries FGIA or other product certification — that helps ensure consistent manufacturing and testing practices[6][7].
If your unit is in a location where code requires safety glazing (near doors, low glazing adjacent to walking surfaces), replacement glass must meet the same safety glazing requirements and referenced standards[9].
Get quotes for both IGU‑only and full‑window replacement so you can compare installed costs, expected U‑factor benefits, and warranty differences before committing[3][2][10].
Consider that defogging services can restore clarity short term but typically won’t restore original gas fill or full factory seal performance — plan accordingly if thermal performance matters[4][11].
Further reading and resources
ASTM E2190 — Standard specification for insulating glass unit performance and evaluation[6]
FGIA product certification for insulating glass and IGMA technical bulletins on sealant compatibility for IGU longevity[7][8]
Abodivo Tool
Should you replace the IGU or the whole window?
Quick check
A short checklist to decide between IGU‑only replacement, whole‑window replacement, or defogging.
The article does not provide a separate instruction for when none of the checklist items apply; consult a professional or follow the 'When to call a pro' guidance.
Confirm your decision with a qualified glazing contractor and check warranty and code requirements.