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Most windows installed in the last several decades use insulated glass units, commonly called IGUs, consisting of two or more panes of glass separated by a spacer and sealed around the perimeter with a primary and secondary sealant. The space between the panes is typically filled with air or an inert gas such as argon, and a desiccant inside the spacer absorbs residual moisture to keep the interior cavity clear. This construction improves thermal performance, but it also introduces a failure mode that has nothing to do with storms: seal failure. The spacer bar itself may be aluminum, a hybrid material, or a warm edge composite designed to reduce heat transfer at the perimeter, and the specific spacer type can influence how quickly a unit shows condensation or frost at its edges even before any seal failure develops.
When the perimeter seal on an IGU breaks down, whether from age, manufacturing defect, or thermal cycling over many winters and summers, moisture and outside air can migrate into the sealed space. The visible result is a fogging or hazy appearance between the panes, sometimes with mineral deposits or streaking, that does not clear when the glass is cleaned because the moisture is trapped inside the sealed cavity. This condition develops gradually and can appear on windows that have never been struck by hail, wind-driven debris, or any other storm event. In a Minnesota climate, repeated winter to summer cycling accelerates the stress placed on a perimeter seal, so seal failure often appears earlier in a unit’s service life here than it might in a milder climate, independent of any specific weather event.
Impact damage looks different. A hailstone or wind-borne object striking glass typically produces a crack, chip, or in more severe cases a full fracture, often with a identifiable point of origin and radiating stress lines. Distinguishing a genuine impact fracture from a spontaneous glass failure, which can occur in tempered glass due to nickel sulfide inclusions or thermal stress, requires attention to the fracture pattern, the presence or absence of an impact point, and whether comparable damage exists on other glazing surfaces exposed to the same storm. Annealed glass tends to crack in long, relatively straight lines radiating from an identifiable origin point, while tempered glass, once its surface compression layer is breached, typically fractures across its entire area into small fragments, a difference that itself carries information about which pane type was involved and how the damage likely occurred.
Confusing seal failure with storm damage is a common source of disagreement in claims involving multiple windows, since a home may have some units failing on their own timeline while others sustain genuine storm-related damage during the same inspection period. A careful evaluator examines each unit individually, notes whether fogging is present, checks for impact marks on the glass surface or adjacent frame, and considers the age and manufacturer of the glass package where that information is available from a manufacturer date stamp on the spacer.
An appraiser working a dispute over window damage focuses on the physical condition of each unit and reaches an independent judgment about what is consistent with the reported event and accepted glazing industry knowledge. Whether seal failure itself is covered under a given policy is a separate question that belongs to the policy language and the parties, not to the appraisal process.
This article is general education about how the appraisal process commonly works. It is not legal advice, and specific procedures can vary by state and policy.
Russ Lis is a working property insurance appraiser and umpire based in Minnesota, serving clients nationwide. Contact Appraisal Resolution.