Why your double‑glazed units are fogging – and what to do about it

For many asset owners and facility managers, “fogged” or cloudy double‑glazed units (DGUs) are one of the most visible and frustrating facade defects. Tenants complain about spoiled views, the building starts to look tired, and there’s often uncertainty about whether this is just a cosmetic issue or a sign of something more serious.

Understanding what a fogged DGU actually means – and how to respond – can help you make better decisions about maintenance budgets, risk and long‑term performance.

What a fogged double glazed units (DGU) actually is

A modern DGU (also called an IGU – insulated glass unit) is made up of two or more panes of glass separated by a spacer and sealed around the edge to create an insulated cavity. That cavity is usually filled with dry air or an inert gas and contains desiccant to control moisture.

When you see condensation, streaking or a milky haze inside the cavity (between the panes, where you cannot wipe it off), it usually means the edge seal has failed. Moisture has entered the unit, the desiccant has become saturated, and the system can no longer keep the cavity dry. From that point on, the fogging will tend to worsen as temperature and humidity cycles continue.

Why fogging matters beyond appearance

The most obvious impact is visual. Fogged glass degrades the appearance of the facade, affects views and daylight, and can undermine the perceived quality of the asset. For premium office, hotel and residential buildings, this can flow through to tenant satisfaction and brand.

Less visible, but just as important, is performance. Once the seal has failed and the cavity is compromised:

• Thermal performance can deteriorate, changing how that zone behaves in terms of heat gain and loss

• Condensation and moisture can begin to affect internal glass surfaces and spacer components

• Long‑term durability of coatings or internal surfaces can be reduced

In isolation, a single failed DGU may not move the needle on whole‑building performance. But patterns of failure – for example on a particular elevation, level or batch – can point to wider issues that will affect comfort, energy use and maintenance costs over the life of the building.

Common causes of DGU fogging

Fogging is not always the result of one single mistake. It is usually a combination of design, manufacturing, handling and installation factors, such as:

• Edge seal or spacer design that is not well suited to the building’s environment or movement

• Manufacturing defects in the seal or spacer system

• Damage to corners or edges during transport, storage or installation

• Movement or distortion in the frame that puts stress on the edge seal over time

• Poor drainage or weeping in the framing system leading to standing water around the unit

For asset owners and facility managers, the key is not to guess the cause, but to document the pattern of failures and get a specialist assessment. That way you can distinguish between isolated, age‑related failures and more systemic problems that may need broader intervention.

When is a fogged DGU a “must fix”?

Not every fogged unit needs to be replaced tomorrow. The decision is usually based on a mix of:

• Location and visibility – Is it in a high‑profile area, premium tenancy or critical view line?

• Extent and trend – Are you seeing just one or two units, or a growing pattern in a particular zone?

• Performance impact – Is there associated condensation, draughts, or comfort complaints?

• Risk and liability – Are there related issues with framing, fixings or glass that could affect safety?

In many cases, a pragmatic approach is to monitor isolated failures while planning staged replacement in more affected zones. However, without a structured inspection and condition assessment, it is very easy for fogging issues to be treated as purely cosmetic and left until tenants or owners “can’t live with it anymore”.

Access, replacement and methodology

Once you decide to address fogged DGUs, access becomes a key part of the conversation. On high‑rise facades, there are several common approaches:

• Rope access – Often the most efficient option for targeted replacements, especially where units are sized and weighted appropriately and access from roof or terrace levels is available.

• BMUs and building maintenance systems – Useful where coverage is good and the system is properly maintained, but may not reach all areas or be suitable for more complex handling.

• Scaffolding or mast climbers – More appropriate for large‑scale replacement programs or where there are multiple related facade issues to resolve at once, although not always practical on operating and tenanted buildings.

• Internal access – Possible in some configurations, particularly where glass can be handled safely from inside without overloading the facade or compromising safety.

The right method depends on unit size and weight, elevation, building use, surrounding public areas and your tolerance for disruption. A specialist facade and glazing contractor can help you choose the best combination for your asset.

Getting ahead of the problem

The most effective way to manage fogged DGUs is to treat them as part of a broader facade condition and risk picture, rather than as one‑off nuisances. That usually means:

• Conducting a structured facade inspection that records the location, extent and severity of DGU failures

• Looking for patterns – by elevation, orientation, age, frame type or manufacturer

• Prioritising replacements based on risk, visibility, tenant impact and program opportunities

• Reviewing glass and framing specifications, QA documentation and warranties where available

• Integrating DGU replacement into planned maintenance and refurbishment programs

This approach helps you avoid piecemeal, reactive replacement and instead make informed decisions about when and where to invest.

When to bring in a specialist

If you are starting to see fogged DGUs on your building, or you suspect a pattern is emerging across part of your facade, it is worth getting a specialist inspection rather than waiting until complaints escalate.

A facade and glazing specialist can:

• Assess the extent and likely causes of fogging

• Advise on the performance and risk implications

• Recommend access methods and replacement strategies suited to your building

• Help you build a staged, budgeted plan for remediation

For many owners and facility managers, that expert input turns what feels like a vague, visual annoyance into a clear, prioritised set of actions.