What actually fails in a fogged window
The glass is fine. What failed is the seal around its edge, and the assembly explains the rest.
An insulating glass unit — an IGU — is two lites of glass, sometimes three, separated by a spacer containing a desiccant that absorbs whatever moisture is sealed in at the factory. The spacer is bonded to the glass by a primary seal, typically polyisobutylene, which is the actual vapour barrier, and a secondary seal of silicone, polysulfide or polyurethane for structural bond. The cavity may hold air or a fill gas such as argon. It is the seal, not the glass, that carries the performance.
Failure runs in sequence. The unit breathes — barometric and temperature swings flex the lites and cycle the seal, and the daily swing on a west-facing window in August is exactly that loading. Solar heat, ultraviolet at the seal edge and standing water from a blocked sill weep degrade the sealant, until the seal admits moisture faster than the desiccant can hold it. Moisture then condenses between the panes whenever the glass drops below dew point: fogging, first at the bottom edge, most obvious on cool mornings, gone by afternoon. Over time, residues deposit permanently on the inner surfaces — the etching stage, at which point the glass itself is compromised. Any fill gas escapes, and the insulating value falls toward a plain double pane.
Three things follow, and they are the reassuring part of this page. A failed seal does not leak water into the house, does not spread to neighbouring units by any mechanism but shared age and exposure, and damages nothing else — not the frame, not the wall, not the finishes.
Why nobody can honestly tell you how long one lasts
Ask how long an insulated glass unit lasts and you will get a confident number. Ask where it comes from and the trail goes cold.
The standard that certifies IGU durability is ASTM E2190, Standard Specification for Insulating Glass Unit Performance and Evaluation, certified through the IGCC/IGMA program under the Fenestration and Glazing Industry Alliance. The testing:
- Weather cycling: "Expose Six IGU Specimens for 252 Cycles (63 Days). Each cycle: 6 h + 5 min."
- Frost point: "Final Dew Point Shall be −40 °F or colder."
That is an accelerated laboratory durability test: six specimens, 252 cycles over 63 days, with a dew-point pass or fail. It establishes that a design survives a defined punishment and still keeps moisture out. What the certification documents do not contain is any statement relating it to field service life.
So when a salesperson, a glazier or an inspector says a unit "should last twenty years," the figure is not coming from the certification standard, because the standard makes no such claim. It is an estimate — sometimes well informed, never a published service life, and this site will not print one as though it were. The same absence applies to failure rates and to the measured consequences of "defogging": every retrievable source traces back to a company selling the remedy. What is knowable is the mechanism, the observable evidence, and the fact that units of the same vintage on the same elevations share an exposure history.
What the Texas standards actually require
22 TAC §535.228(g), Exterior and Interior Glazing, is short and decisive. The inspector shall report as deficient:
- "insulated windows that are obviously fogged or display other evidence of broken seals";
- "deficiencies in glazing, weather stripping and glazing compound";
- "absence of safety glass in hazardous locations";
- "absence of fall protection at windows located less than 24 inches from finished floor and greater than 72 inches from finished grade."
Then come three limits that resolve most post-closing window arguments. The inspector is not required to "exhaustively inspect windows for broken seals," not required to exhaustively inspect glazing for identifying labels, and not required to "identify specific locations of damage."
"Obviously fogged" is a plain-language threshold, not a laboratory detection standard. "Not required to exhaustively inspect" means the report is not a warranty that every unit is sound. "Not required to identify specific locations of damage" means the inspector need not map which pane of a divided unit failed. Almost no consumer content quotes these lines, which is why buyers arrive at a dispute assuming a guarantee they never had. TREC also states plainly that "the standards of practice are not based on a specific requirement such as those promulgated by model building codes" — an inspection is not a code compliance review. The standards are published in full.
Why a failed unit gets missed, and why that is not negligence
This is the paragraph that prevents lawsuits, so here it is plainly: a failed insulated glass unit often looks perfectly clear.
Fogging is a condensation event, requiring the inner glass surface to fall below the dew point of the air trapped in the cavity. A unit that fogs visibly at seven on a November morning can be optically flawless at two on a July afternoon, once the sun has driven the moisture back into vapour. An inspection is a single visit, at one time of day, in whatever weather that day supplied — and direct sun, screens, blinds, reflections, tint and the faint colour cast of a low-emissivity coating obscure it further.
That is a physical limitation, not a failure of diligence, and it is why the standards say the inspector need not inspect exhaustively for broken seals. Two related conditions get misdiagnosed as seal failure: condensation on the room side is an indoor humidity and temperature issue, and condensation on the exterior surface of a high-performance window on a humid morning is normal — evidence the low-emissivity coating is keeping the outer lite cool. The test is simple: if you cannot wipe it off from either side, it is between the panes.
Repair, replace, or defog
Three genuine options. This is a decision, not a verdict.
Replace the sealed unit only, sometimes called "glass only." Where the sash and frame are sound and the unit is a standard removable IGU, a glazier swaps the sealed unit and keeps the existing sash — the usual answer for aluminium and many vinyl windows, and the least expensive path. Obsolete or proprietary sash may not be serviceable, and specialty shapes, tempered units and matching the original coating and fill all push the price up.
Replace the sash, or the whole window. Required where the sash has failed — broken balances, distorted vinyl, rot — where the glazing is not removable, or where you want an upgrade rather than a repair.
"Defogging" — drilling the unit, cleaning it, fitting breather vents. Marketed as the cheap alternative, and it deserves an honest description. It removes the visible fog. It also converts a sealed unit into a vented one: the fill gas is gone permanently, the rated U-factor and certification label no longer describe what is in the opening, and permanent etching is not reversed. It is a cosmetic remedy with a stated tradeoff, not a restoration of thermal performance.
Cost is structural, not a price. What drives the number: how many units failed; whether the sash accepts a replacement unit or the whole window must go; size, shape, thickness and tempering; whether the original coating and gas fill are matched; and glazier access. A glass-only swap and a whole-house replacement differ by an order of magnitude.
The window findings that are not about the glass
Some of the most consequential window items have nothing to do with fogging.
Safety glazing in hazardous locations. The standards require reporting the absence of safety glass where required, and IRC R308.4 defines where: in and adjacent to doors, in railings and guards, near tubs, showers and pools, and beside stairs, ramps and the bottom landing. For an ordinary window the test is four conditions met together: exposed area over 9 square feet, bottom edge under 18 inches from the floor, top edge over 36 inches above it, and a walking surface within 36 inches horizontally. R308.1 requires tempered panes to carry a permanently etched or embossed label — the small mark in the corner.
Fall protection and egress. The fall-protection trigger is specific and rarely published: a sill less than 24 inches from the finished floor and greater than 72 inches from finished grade. The absence of performing emergency escape and rescue openings in sleeping rooms must also be reported, and TREC's guidance is that the inspector applies reasonable judgment to whether the opening serves its purpose — "an inspector is not required to recommend 'fixes'."
Weeps and water. Aluminium and vinyl windows have small weeps at the sill to drain the frame track; blocked weeps hold water in the sill, stress the seal edge, and can rot a wood sill below. Homeowners caulk them shut, exactly as they do brick weeps. Interior staining at a window head or sill corner is a flashing finding, not a glass finding, and the fix is in the wall.
Frames in Texas heat. Vinyl softens as it nears its heat deflection temperature, and dark colours on south and west elevations run well above ambient. The reported failure modes are sash bowing, corner weld separation, loss of squareness so the sash will not latch, and racking of large operable units — which then stresses the glazing unit inside the frame. No threshold temperature appears here, because none could be sourced to a materials standard.
What to do about it
Get a count from a glazier, not from the inspection report. The standards require neither an exhaustive search for broken seals nor identification of which pane failed, so the inspector's list is a sample, not an inventory. A glazier walking the house on a cool morning, blinds up and screens off, produces a better count and a serviceability-based quote. If you are still inside an option period, go back yourself early on a cool morning — that is when units fog.
Ask about warranty before assuming there is none. Many manufacturers warrant sealed units; terms vary by product and year, and some are transferable. Ask the seller for documentation, then read it rather than what anyone says it covers.
Keep it in proportion. A fogged window looks alarming and photographs badly, but it is not structural, it is not a safety issue, it admits no water, and it gets worse only in appearance and insulating value. In a negotiation it belongs in the budget column, and treating it as a reason to walk is almost always an overreaction.
Escalate the ones that are not about glass. Missing safety glazing, a sleeping room with no performing escape opening, a sash that will not latch, and interior staining at a head or sill are different findings with different urgency. Those get evaluated before closing; the fog does not. A glazier assesses unit condition and count; a building envelope consultant is the right party where water is entering at the opening rather than sitting between the panes.
Common questions
How long is an insulated glass unit supposed to last?
There is no published, sourced answer, and anyone who gives you a confident number is estimating. The certification standard for insulating glass units, ASTM E2190, is an accelerated laboratory durability test — six specimens through 252 weather cycles over 63 days, with a final dew point of minus 40 degrees Fahrenheit or colder — and the certification documents make no statement relating that test to field service life. What is knowable is that units of the same vintage on the same elevations share the same exposure and tend to fail in the same period.
Can a fogged window be repaired without replacing the glass?
There is a treatment marketed as defogging, in which the unit is drilled, cleaned and fitted with breather vents. It removes the visible fog, but it permanently converts a sealed unit into a vented one: the fill gas is gone, the rated U-factor no longer describes the assembly, and permanent etching of the inner surfaces is not reversed. It is a cosmetic remedy with a real tradeoff, not a restoration of thermal performance. The alternative is replacing the sealed unit while keeping the existing sash and frame, which is usually the more durable answer.
My inspector missed a fogged window. Was that negligence?
Almost certainly not. A failed unit frequently looks clear: fogging depends on the glass falling below the dew point of the trapped air, so a window that fogs on a cool morning can be optically perfect on a warm dry afternoon. Sun, screens, blinds, tint and low-emissivity coatings obscure it further. The Texas standards recognise this directly by stating that the inspector is not required to exhaustively inspect windows for broken seals, and is not required to identify specific locations of damage.
Does a failed seal cause water damage or mould inside the wall?
No. The moisture that fogs the glass is inside the sealed cavity, between the two lites, and it does not reach the frame, the wall or the interior finishes. If you have staining at a window head or at the sill corners, that is a flashing and water intrusion problem in the wall assembly, which is a separate and more urgent finding. The two get conflated constantly, and the fixes are completely different.
Should I replace all the windows or just the fogged ones?
Start with the failed units, unless the frames themselves have failed. Where the sash accepts a replacement sealed unit, glass-only replacement is the least expensive path and preserves the existing frames. Whole-window replacement is warranted when sashes are distorted or will not latch, when balances and hardware have failed, when the glazing is not removable, or when you actually want the performance upgrade. Budget with the understanding that units of the same age and exposure tend to follow each other.
What does the Texas inspector have to report about my windows?
Insulated windows that are obviously fogged or show other evidence of broken seals; deficiencies in glazing, weather stripping and glazing compound; the absence of safety glass in hazardous locations; the absence of fall protection where the sill is less than 24 inches from the finished floor and more than 72 inches above finished grade; the absence of performing emergency escape openings in sleeping rooms; and missing or damaged screens. The inspector is not required to inspect exhaustively for broken seals, to check glazing labels exhaustively, or to identify specific locations of damage.