Fogged Insulated Glass Unit Replacement Technique

Why this matters

By the time a fogged insulating glass unit lands on a work order, the diagnosis is already done and the warranty path is already decided; what is left is putting the correct replacement unit into the frame without creating a second problem on top of the first. This is where a reglaze goes wrong: the low-E coating faces the wrong direction and the customer's summer cooling bill goes up instead of down, the wrong glazing bead is used on a frame system that needed a different one, or a hazard-location sash gets reglazed with annealed glass because nobody checked the tempered requirement before ordering. This HowTo owns the on-site swap: measuring for the replacement unit, removing the failed IGU, setting the new one correctly, and verifying the seal before the crew leaves. The diagnosis of what kind of fogging this is and the warranty claim itself are covered elsewhere; see the References for both.

Confirm you are ready to reglaze, not diagnose

This procedure assumes the IGU has already been confirmed as a between-the-panes seal failure, not surface condensation, and that a claim has been filed or the replacement is being paid outside warranty. If either of those has not happened, stop here; the condensation between panes decision tree owns the wipe-test diagnosis and the warranty routing by manufacturer, and reglazing a unit before the claim paperwork exists means you cannot document the failed unit's spacer code once it is out of the frame.

Measure for the replacement unit, not the opening

The order dimension is not the sight-opening dimension. It is the sight opening minus the sightline coverage of the stop or glazing bead on all four sides, minus a small edge-clearance allowance (commonly 1/8 to 3/16 in per side, per the frame manufacturer's glazing detail) so the unit is never wedged tight against the rabbet. Measure the existing failed unit directly where possible, since a unit that was custom-cut to a slightly out-of-square opening will not match a nominal size calculated from the frame dimension. Record the following before ordering: overall size, glass makeup (thickness and number of lites), low-E coating position by surface number (counted 1 through 4 from the exterior face inward, with the coated surface stated explicitly rather than just "low-E," since a coating can be specified on surface 2 or surface 3 depending on climate strategy and getting this backward measurably changes the unit's performance), spacer color and warm-edge type if the customer's other units carry a matched spacer, gas fill if specified, and whether the location requires safety glazing.

Check the hazard-location requirement before you order, not after the new unit arrives. A window within roughly 24 in of a door, within 18 in of the floor, adjacent to a walking surface, or in specific other locations IRC R308.4 (or the equivalent building-code hazard-location table your jurisdiction has adopted) enumerates requires tempered or otherwise safety-rated glazing. If the failed unit's remaining sticker or etch mark shows a tempered stamp, the replacement is ordered tempered; if the original unit predates consistent tempering practice and the location now qualifies as a hazard location under the code your jurisdiction has adopted, order tempered regardless of what is coming out, because you are not required to reproduce a code violation.

Remove the failed unit

1. Confirm the sash or fixed frame is fully supported and, on an operable unit, remove the sash to a bench rather than working it in the opening; this controls the work at a stable height and keeps broken glazing compound off the customer's floor. See the glass handling and two-person lift SOP for crew size and carrying method before moving anything larger than a single tech can control at arm's length.
2. Identify the glazing method before cutting anything. Wood sash commonly uses wet glazing compound over points or clips; vinyl and most aluminum systems use a snap-in glazing bead with a dry gasket; some aluminum storefront systems use a structural sealant that requires a different removal approach entirely and is outside this HowTo's scope. Pulling a snap bead like it is a wet-glazed stop cracks the vinyl leg it rides in.
3. On wet-glazed wood: score the cured compound at the glass line with a glazier's knife, working in short strokes rather than one long pull, then pry the compound free in sections. Remove the glazing points beneath it with pliers, saving intact ones for reuse only if the new unit's edge thickness matches; a thicker replacement unit needs new points set at the correct depth, not the old ones bent to fit.
4. On dry-glazed snap bead: work a flat trim tool under one corner of the bead and pull it free along its length rather than levering at the middle, which is what cracks the vinyl retaining leg. Keep the bead intact for reuse; a bead broken during removal needs to be matched and reordered, which stops the job at the bench.
5. Once the retaining method is off, lift the failed unit straight out of the rabbet, never levering it out at an angle. A fogged IGU has already lost its seal and can have edge cracks that are not visible until the unit is under load; angled prying concentrates load at one corner and that is where it lets go. Two hands under the unit's weight at all times, cut-resistant gloves on before the retaining method comes off, not after.
6. Photograph the failed unit's spacer-bar etch code before it leaves the shop or the job site, even though the claim was filed with an earlier photo; this is the record that ties the physical unit to the claim if a warranty desk later disputes the failure.

Set the new unit

1. Clean the rabbet of old compound, dust, and any glazing point remnants down to bare material; a rabbet with old compound residue prevents the new unit from seating flush and telegraphs a visible gap at the sightline.
2. Place setting blocks at the quarter points of the sill (roughly one-quarter of the width in from each bottom corner), never at the corners and never centered alone. Setting blocks carry the glass weight and keep the unit off the bottom rabbet so water in the weep path can drain under it rather than pooling against the edge seal. A unit resting directly on the rabbet with no setting block sits in standing water at the first rain and the edge seal it just received starts degrading from day one.
3. Confirm the low-E coating orientation against the order record before the unit goes in, not after; once the unit is glazed and sealed, a backward coating is a full second reglaze, not an adjustment. A simple field check where the order record is unclear: a coated surface shows a faint color shift or a slightly different reflection when viewed at a low angle against a plain background compared to an uncoated surface, but do not rely on eye alone when the stakes are a full reglaze; the order paperwork from the glass fabricator is the authoritative source.
4. Set the unit into the rabbet on its blocks, centered with roughly equal edge clearance on all four sides, confirmed by eye against the gap and corrected before the retaining method goes back on; an off-center unit reads as crooked to the customer even when the edge clearance is technically within tolerance.
5. Reinstall the retaining method: fresh glazing points at the correct spacing (commonly every 10 to 12 in around the perimeter, tighter on larger lites) and a full bead of glazing compound tooled to a consistent angle on wet-glazed wood, or the snap bead pressed home along its full length with hand pressure, checking for full seating at the corners where a bead most often pops back out on the first cold night if it was not fully seated.
6. On wet-glazed compound, prime bare wood in the rabbet before the compound goes on if the manufacturer's instructions call for it; compound applied to unprimed wood dries unevenly and can crack along the glass line within a season, which reopens a water path directly at the new seal.

Worked example: wood casement, wet-glazed, low-E surface 3

A ten-year-old wood casement, single unit fogged, warranty claim already approved and the replacement unit received. Order record: overall size, clear low-E on surface 3 (the interior face of the exterior lite, standard for the climate zone this shop works in), clear spacer, no tempering required at this location per a hazard-location check that confirmed the sill sits well above 18 in and is not adjacent to a door or walking surface.

Sash removed to the bench. Compound scored and removed in sections; six glazing points recovered intact and reusable since the new unit is the same nominal thickness as the failed one. Failed unit lifted straight out with two hands, spacer code photographed for the file, and set aside for the warranty desk's own disposal procedure rather than being discarded on site.

Rabbet cleaned to bare wood. Setting blocks placed at the quarter points, roughly 3 in in from each bottom corner on a 12 in wide sash. Coating orientation checked against the order paperwork before setting: surface 3 confirmed, unit set with the coated lite facing correctly. Unit centered with edge clearance checked by eye at all four sides and found even.

Points reset at roughly 11 in spacing around the perimeter, six points total matching the six removed, with two additional points added at the corners where the original spacing had left a longer unsupported run than the new manufacturer's instructions specify for a unit this size. That is the acceptance check on point spacing: the instructions state a maximum run between points, and a straight count-and-measure against the actual spacing found the original six points did not meet it even though the old unit had held for a decade on that count. Compound tooled to a consistent 45-degree angle, primed rabbet confirmed dry before application.

Cure time allowed per the compound manufacturer's instructions before the sash is rehung and operated; the sash sits on the bench overnight rather than being reinstalled and cycled the same day, because operating a casement immediately loads the fresh compound at the hinge-side edge before it has set.

How to verify you got this right

Confirm no gap at the sightline on any of the four sides by running a finger along the glazing bead or the tooled compound line; a gap you can catch a fingernail on is a leak path. Confirm the unit sits centered with roughly even edge clearance, since an off-center unit under load from the frame can develop point-loading at one edge. Cycle an operable sash through full travel once cure time has passed and confirm no binding introduced by the reglaze. Photograph the finished glazing line for the file alongside the failed unit's spacer code photo, so the job record shows both what came out and what went in.

References

• See related: the condensation between panes decision tree, which owns the interior-versus-seal-failure diagnosis and the manufacturer warranty routing.
• See related: the glass unit failure warranty claim standard SOP, which owns the intake-through-closeout claim workflow this reglaze sits inside.
• See related: the glass handling and two-person lift SOP, for crew size and carrying method.
• IRC R308.4 (or the equivalent hazard-location glazing table in the edition your jurisdiction has adopted), for safety-glazing requirements at the reglaze location.
• ASTM E2190, Standard Specification for Insulating Glass Unit Performance and Evaluation.