Condensation Between Panes Decision Tree

Why this matters

A homeowner photo of moisture or haze between the two panes of glass is a warranty triage moment, not a diagnostic mystery. Moisture between the panes can only mean the insulating glass unit (IGU) seal has failed - the desiccant in the spacer has reached saturation and the IGU is now drawing humidity from outside air. The question is not what happened; it is who pays, how fast can the replacement IGU ship, and is there a related field defect that also needs to be corrected. This decision tree handles intake, warranty path, replacement logistics, and the related-failure scan that prevents the same SKU coming back in 18 months.

Step 1 - confirm the moisture is between the panes

Customers routinely report fogging that turns out to be interior-surface condensation from indoor humidity or exterior-surface condensation from radiative cooling at night. Both are normal physics, not glass defects. Field test: wipe the interior surface with a dry microfiber and wait 60 seconds. If the fog returns on the interior surface, it is interior condensation - the cause is high indoor humidity (cooking, bathing, humidifier, gas combustion appliance, unvented dryer) combined with a cold glass surface, and it is a humidity-management conversation with the homeowner, not a glass warranty claim. If the fog wipes clear on the interior surface and persists in the same pattern between the panes (or shows visible mineral haze, "ghost" outline, or rivulet streaks that move with temperature change but cannot be reached from either side), the IGU seal has failed.

Step 2 - identify the unit serial and warranty status

Every IGU manufactured for the major residential lines carries a spacer-bar etched code: manufacturer code, sash size or order code, manufacturing date in week-and-year format. Open the sash and photograph the spacer at the bottom edge with raking light. Cross-reference the date against the warranty terms for the line: Andersen 400 Series glass warranty is 20 years, 100 Series is 10 years; Pella Architect Series IGU is 20 years; Marvin Essential and Elevate IGU is 20 years per Marvin's published Limited Warranty; Milgard Full Lifetime applies to original homeowner across Tuscany and Trinsic. If the spacer is illegible, contact the dealer with the original purchase invoice; the dealer can pull the order from the manufacturer's order history by address and date.

Step 3 - file the warranty claim correctly

Manufacturer warranty claims for IGU failure require: clear photo of the failure showing the haze or moisture pattern between the panes, photo of the spacer code etched on the unit, photo of the exterior of the window showing the sash configuration, copy of original sales order or purchase invoice. Andersen routes claims through the dealer where the unit was purchased; Pella and Marvin route through their dealer network; Milgard accepts direct homeowner claims through its service portal. The replacement IGU - not the whole sash - is the standard remedy. Field labor coverage varies: Andersen 400 includes labor under the original-homeowner glass warranty in defined service territories; Pella Architect labor depends on service contract; Marvin labor is typically dealer-billed unless under a dealer service agreement; Milgard SunCoast and standard programs include labor for original homeowner in defined territories.

Step 4 - lead-time triage and homeowner expectations

Replacement IGU production typically runs 4 to 8 weeks from claim acceptance to dealer delivery. Coastal-impact glazing and triple-pane configurations push past 10 weeks. The dealer cannot accelerate manufacturer production; the only acceleration path is escalation through a regional dealer-rep contact, which is reserved for systemic claims (multiple units in one house, or a multi-unit project). Set the homeowner expectation explicitly in writing at intake: claim filing date, expected production window, expected install date, and the fact that the window remains functional and weathertight during the wait - the fogging is cosmetic, not a structural defect. The single most common service complaint on IGU warranty claims is not the failure itself; it is the perception that the dealer is slow-walking the claim because the homeowner was not given a written timeline upfront.

Step 5 - scan for the related defect

A single IGU failure on a 1- to 5-year-old install is a unit defect. A second IGU failure on the same project within 18 months of the first, especially on the same elevation or in the same sash configuration, is a pattern that points at a related field condition. Scan the project for: south or west elevations with no overhang (thermal cycling above the IGU rating), sashes installed without a proper sill pan and back-dam (water pooling at the bottom seal accelerates desiccant exhaustion), sashes that were stored or shipped flat instead of vertical (transit-induced seal stress), and units that were field-modified during install (a sash that was field-routed for a lock retrofit voids the IGU warranty). Document any related condition in the file and surface it to the dealer rep when filing the second claim.

Step 6 - the field-installed insulating storm exception

In a small number of cases, a homeowner installs an aftermarket interior storm window or a thermal cellular shade tight to the sash and reports IGU fogging six months later. The mechanism is real: the interior add-on traps a colder boundary layer at the original sash and shifts the dew point onto the IGU surface, accelerating any marginal seal leak into full failure. The warranty claim still proceeds, but the dealer rep will sometimes ask whether an interior storm or tight-sealed shade has been installed; document the answer truthfully because the manufacturer may decline labor coverage if interior modifications altered the operating environment outside the warranty assumptions.

When to recommend whole-sash replacement instead

If the unit is out of warranty and the IGU replacement cost approaches the cost of a new sash (or the sash itself shows secondary defects - cracked vinyl welded corner, rotted wood at the bottom rail, failed sash balance system), recommend whole-sash replacement instead of IGU re-glazing. Do not field-reglaze a wood-clad sash that is delaminating at the corners; the new IGU will outlast the surrounding sash and the homeowner will be back in 2 years for the same conversation about a different failure mode. Document the recommendation in writing with photos of the secondary defects so the homeowner has a clear decision record.

References

• ASTM E2188 Standard Test Method for Insulating Glass Unit Performance
• ASTM E2190 Standard Specification for Insulating Glass Unit Performance and Evaluation
• AAMA/WDMA/CSA 101/I.S.2/A440 North American Fenestration Standard (NAFS-22), insulating glass durability provisions
• Insulating Glass Manufacturers Alliance (IGMA) certification program documentation
• NFRC 100 Procedure for Determining Fenestration Product U-factors