Windows Fogging from Inside? Understanding Condensation Causes and Effective Solutions

Why Do New, Expensive Windows Also “Sweat”? Understanding the Paradox of Airtight Windows

One of the most surprising and frustrating experiences for homeowners is when, after replacing old windows with modern triple-glazed units or high-performance aluminium windows, the condensation problem doesn’t disappear but actually intensifies. This phenomenon has a logical explanation in building physics and doesn’t indicate a defect in the new product.

Old, drafty windows functioned as an uncontrolled ventilation system. Through numerous gaps, poor seals, and weak weatherstripping, not only did enormous amounts of valuable heat escape (often 30-40% of the building’s energy losses), but also excess moisture produced by occupants. This “natural ventilation” masked the real problem of excess water vapor in the home.

Modern energy-efficient windows feature nearly perfect airtightness, which represents their fundamental advantage from an energy perspective. Air infiltration rates in new windows are often 10 times lower than in old constructions. However, this same airtightness means that all moisture produced inside the home—from breathing, cooking, washing, drying, houseplants, or pets—remains “trapped” in the rooms.

An average family of four produces about 2.6-3.2 gallons (10-12 liters) of water vapor daily. In an airtight home, this vapor has no escape route, leading to a systematic increase in relative humidity. Combined with the fact that the glass surface remains the coldest element in the room (despite better insulation than old windows), condensation becomes inevitable.

Physics in Practice: 3 Factors Creating Water on Your Windows

Relative Humidity – The Invisible Effect of Daily Living

Relative humidity is the fundamental parameter determining air’s tendency to condense water vapor. Expressed as a percentage, it shows how much water vapor air contains relative to the maximum amount it can hold at a given temperature.

Optimal relative humidity in homes should range between 45-55%. Values above 60% significantly increase condensation risk, while exceeding 70% virtually guarantees foggy windows. The problem is that everyday activities easily raise humidity levels:

• Cooking can increase humidity by 10-15% within an hour
• Showering or bathing raises bathroom humidity by 20-30%
• Drying laundry indoors can increase humidity by 5-10% throughout the apartment
• Each person produces about 1-1.5 liters of water vapor daily through breathing
• Houseplants, depending on type and size, can add 0.5-2 liters of vapor daily

Dew Point – The Magic Condensation Boundary

The dew point is the critical physical parameter determining the temperature at which air with given humidity reaches 100% saturation with water vapor and must release excess as condensate. The higher the relative humidity in the room, the higher the dew point temperature.

Practical examples showing the relationship between humidity and dew point at standard room temperature of 20°C:

• At 45% humidity, dew point is 8.2°C
• At 55% humidity, dew point rises to 10.6°C
• At 65% humidity, dew point reaches 13.2°C
• At 75% humidity, dew point reaches 15.4°C

This means: the higher the humidity in your home, the warmer the surfaces where condensation will occur. At high 75% humidity, even a relatively warm glass surface at 16°C will be covered with water droplets.

Glass Surface Temperature – Key to Dry Windows

The window glass surface is typically the coldest element in a room during heating season. According to physics laws, this is where air first reaches dew point and condensation begins.

The inner glass surface temperature depends on several key factors:

• Overall window U-value (thermal transmittance)
• Glass U-value
• Outside temperature
• Quality of warm-edge installation
• Presence of thermal bridges

The basic relationship is simple: if glass surface temperature drops below the room’s dew point, condensation is inevitable. Therefore, a permanent solution must simultaneously raise glass surface temperature and/or lower room humidity.

Quick Diagnosis: What Exactly Is Your Window Telling You?

The type of observed condensation provides valuable diagnostic information helping identify the problem cause and select appropriate solutions:

Condensation Diagnostic Table

Condensation Type	Location	Meaning	Action
Internal Condensation	On glass surface room-side	Systemic problem – excessive humidity and/or cold glass surface	Systemic solution required
External Condensation	On glass surface exterior-side	Good sign – indicates excellent window insulation	No action – natural phenomenon
Between-panes Condensation	Between glass panes in sealed unit	Critical failure – damaged seal integrity	Immediate warranty claim/replacement

Internal condensation is the most common problem type requiring a comprehensive approach. External condensation, observed mainly during transitional periods (spring, autumn) in morning hours, paradoxically indicates high window insulation quality—the outer glass pane is so effectively isolated from interior heat that its temperature drops below the external dew point.

Between-panes condensation is an alarm signal indicating sealed unit failure. The seal has been compromised, the desiccant is saturated, and insulating gas (argon/krypton) has escaped. Such a window has irreversibly lost its thermal properties and is subject to warranty claim.

Quick Fixes: Honest Analysis of “Fast Solutions” and Their True Costs

Ventilation – Simple, But Cost-Free?

Regular ventilation is the most commonly recommended method against excess moisture and indeed provides temporary relief. Exchanging humid indoor air for dry outdoor air (in winter) effectively lowers relative humidity in the room.

Effectiveness: The method works short-term and can stop condensation during ventilation and for several hours after.

Hidden costs and drawbacks:

• Energy losses: Expelling heated air means direct heat losses that can reach 25-30% of heating costs with intensive ventilation
• Thermal discomfort: Winter ventilation drastically lowers room temperature
• Need for constant control: A manual solution requiring systematic action from occupants
• Limited effectiveness: In conditions of high external humidity (autumn, rainy weather), the method loses effectiveness

Dehumidifiers – Electric “Band-Aids”

Electric dehumidifiers are mechanical devices that effectively remove excess moisture from air. They offer immediate relief and quickly stop the condensation process.

Effectiveness: Very good at mechanically removing water vapor. A dehumidifier with 20 L/day capacity can maintain 50-60% humidity even in problematic rooms.

True operating costs:

• Electricity: A typical 400-600W dehumidifier running 6-8 hours daily generates monthly costs of $60-120 (at $0.15/kWh electricity rate)
• Annual costs: $720-1,440 accumulating over years of use
• Noise: 40-50 dB sound level can be disturbing, especially at night
• Maintenance: Regular tank emptying (daily or every few days)
• Limited scope: Solves only part of the problem—removes excess moisture but doesn’t affect glass surface temperature

Window Vents – Is a Hole in the Window the Solution?

Window Trickle Vents – Intelligent Humidity Management

Window trickle vents are mechanical airflow regulators mounted directly in the window frame, enabling controlled ventilation without opening sashes. Modern solutions like the Maco Vent systems used by BWS represent the latest generation of humidity-controlled systems that automatically respond to room relative humidity levels.

The Maco Vent system features a polyamide tape as humidity sensor, eliminating the need for electrical power or complicated operation. The mechanism automatically regulates airflow across a wide 30-70% relative humidity range, adapting to actual room needs. When humidity rises above optimal levels, the vent gradually opens, allowing fresh air intake. When humidity drops to acceptable levels, the system automatically limits flow, minimizing energy losses.

The key advantage of Maco Vent vents is their intelligent approach to drafts and energy losses that plague standard ventilation solutions. Through precise flow regulation, the system minimizes perceptible drafts even in windy locations, eliminating the common problem of users “taping over” vents during winter. Energy losses are limited to the necessary minimum—just 2-8% of heating costs, while standard vents can generate losses of 15% or more. The discrete design allows full integration with the window profile, maintaining aesthetic appeal while ensuring full ventilation functionality.

System Solutions: Modern Window Technology Against Condensation

U-Value – The Key Thermal Insulation Parameter

The thermal transmittance coefficient (U-value) is the fundamental parameter indicating how much thermal energy escapes through the entire window (glass, frame, and connections) over time. Expressed in W/(m²K) or BTU/(hr·ft²·°F), it indicates heat losses: the lower the value, the better the insulation properties.

Direct connection to condensation: Better thermal insulation means significantly higher inner glass surface temperature. Maintaining this surface above dew point temperature is the basic mechanism for preventing condensation.

Comparison of thermal transmittance coefficients:

Old windows from the 1990s and earlier had catastrophic U-values of 1.8-3.0 W/(m²K), meaning enormous energy losses and very cold glass surfaces. Standard windows from 2000-2020 achieved U-values of 1.1-1.6 W/(m²K), which still didn’t satisfactorily solve the condensation problem.

Modern window technology enables much better parameters. Contemporary PVC windows using advanced profile systems like GEALAN S9000 with 6-chamber construction at 83mm depth achieve U-values of 0.73 W/(m²K). The GEALAN S8000 system with 5-chamber profile at 74mm depth ensures parameters of 0.8-0.9 W/(m²K), already guaranteeing significant glass surface temperature increase.

In the aluminium window segment, systems with advanced thermal breaks offer the best results. Yawal TM 102HI is currently one of the most efficient systems on the market, achieving an exceptional U-value of 0.58 W/(m²K), placing it among passive house solutions. The Aliplast Genesis system also offers excellent thermal parameters thanks to glass-fiber-reinforced polyamide thermal breaks.

Glazing Units – Evolution of Insulation Technology

The thermal heart of every window is the insulated glass unit. The evolution from single glazing through double glazing to today’s triple glazing is the story of significant energy efficiency improvement.

Comparison of Ug values (glazing only):

• Basic double glazing: Ug ≈ 1.1 W/(m²K)
• Coated double glazing: Ug ≈ 1.0 W/(m²K)
• Standard triple glazing: Ug ≈ 0.7 W/(m²K)
• Premium triple glazing: Ug ≈ 0.5 W/(m²K)

Glass Surface Temperature Comparison Table – BWS Standard vs. Old Windows

Outside Temperature	Double-glazed Window (Ug=1.1)	BWS Triple-glazed (Ug=0.5)	Condensation Risk (55% humidity, dew point 10.6°C)
-10°C	approx. 8°C	approx. 14°C	Yes / No
-5°C	approx. 10°C	approx. 15°C	Yes / No
0°C	approx. 13°C	approx. 17°C	No / No
+5°C	approx. 15°C	approx. 18°C	No / No

Calculations for 20°C indoor temperature

The table demonstrates the advantage of triple glazing used as standard in BWS windows—under typical winter conditions, the glass surface remains safely above dew point, eliminating condensation risk.

Warm Edge Spacers – Eliminating Thermal Bridges

Warm Edge Spacers – Eliminating Thermal Bridges at Glass Perimeter

Thermal bridges at the insulated glass unit perimeter are an often underestimated but critical cause of window edge condensation. Traditional aluminum spacer bars, still used in many budget windows, create a massive thermal bridge around the entire glass perimeter. Aluminum conducts heat over 1,000 times better than air, causing drastic cooling of glass edges by 5.4-9°F (3-5°C) compared to the center. This is why condensation almost always starts with characteristic droplets in window corners and along edges, creating an unsightly “mourning frame” of moisture.

Modern “warm edge” spacers made from low thermal conductivity composite materials completely eliminate this problem. Various technologies can be used: from polycarbonate through stainless steel with thermal break to advanced glass-fiber-reinforced plastics. In BWS production practice, Swisspacer warm edge spacers are used as standard in all insulated glass units, regardless of the chosen window system. This means every window—whether PVC GEALAN S8000, GEALAN S9000, or aluminium Yawal TM 102HI—leaves the factory already equipped with a solution eliminating thermal bridges at no additional cost to the customer.

The benefits of warm edge spacers are multifaceted and directly translate to user comfort. Raising edge temperature by 3-5°C effectively prevents condensation at the glass unit’s most problematic area, eliminating mold formation in window corners. Additionally, warm edge spacers improve the overall window U-value by up to 10-15%, translating to measurable energy savings throughout the service life.

The Critical Role of Installation: Why the Best Window Can Fail

According to industry data, 80% of condensation problems with new windows result from installation errors. Even the world’s best window will fail if installed incorrectly. With newly installed window systems, the biggest thermal bridge often isn’t in the window but around it—at the junction between frame and wall.

Limitations of “Foam-Only Installation” – Why It’s Insufficient

The traditional foam-only installation method, relying solely on filling the expansion gap with polyurethane foam, has fundamental flaws:

Moisture absorption problem: Unprotected installation foam acts like a sponge, absorbing moisture from both inside (water vapor) and outside (rainwater). Wet foam completely loses its insulating properties.

Material degradation: Under freeze-thaw cycles and UV radiation, foam degrades, becomes brittle and crumbles, creating gaps leading to drafts and heat loss.

Most common installation errors:

• Leaving foam without protective layers
• Incorrect window positioning relative to insulation layer
• Inadequate reveal preparation
• Incorrect expansion gap sizing

Consequences: Creation of cold zones around the window where surface temperature drops below dew point, leading to condensation and mold growth despite excellent window parameters.

“Warm Installation” – Professional Installation Technology

Warm installation (layered installation) is an advanced installation technology ensuring the window-to-wall connection is as airtight and warm as the window itself. The system is based on the principle: “tighter inside than outside.”

Three-layer protection system:

1. Inner layer – vapor barrier: Applied from the room side, creates an absolute barrier for warm, humid air. Prevents vapor penetration into the insulation layer.
2. Middle layer – thermal insulation: Polyurethane foam carefully filling the space between frame and wall, fully protected from both sides.
3. Outer layer – breathable membrane: Protects against wind and rainwater while allowing any residual moisture to evaporate outward.

Results: Complete elimination of thermal bridge at window-wall junction, prevention of insulation material degradation, and blocking of uncontrolled drafts and moisture infiltration.

Choosing Contractor and Manufacturer: Key Success Criterial

Professional window installation and appropriate product selection form the foundation of a permanent condensation solution. Therefore, special attention should be paid to selection criteria for both contractor and window manufacturer. Key element is knowledge of modern installation technologies, particularly warm installation, which as demonstrated accounts for 80% of success in eliminating condensation. Best results are achieved by companies offering complete solutions—from window production with appropriate thermal parameters to certified installation performed by authorized teams with years of experience in advanced sealing technologies.

An important indicator of manufacturer professionalism is product standards that include warm edge spacers as standard equipment, not an optional extra. Companies aware of condensation issues, like BWS, incorporate Swisspacer warm edge spacers into standard equipment on all their products, guaranteeing thermal bridge elimination at the production stage. Equally important is the availability of triple glazing as standard rather than an exclusive option, and the possibility of equipping windows with intelligent ventilation systems like Maco Vent.

A particularly telling indicator of manufacturer confidence in their technology is the warranty scope offered for installation seal integrity. While standard market warranty is 2 years, companies confident in their technology, like BWS offering a 5-year warranty on warm installation seal integrity, demonstrate full confidence in the effectiveness of their solutions and readiness to take responsibility for long-term work results.

Available Market Solutions Against Condensation

Comprehensive Approach to Condensation Problems

Effective condensation problem solving requires a holistic approach considering both window thermal parameters and intelligent humidity management in rooms. For residential buildings with high humidity problems, the optimal solution combines PVC windows with low thermal transmittance and the Maco Vent system. The GEALAN S9000 system with U-value of 0.73 W/(m²K), combined with automatic humidity regulation ensures full control over internal conditions without compromising energy savings.

For investors planning large glazing areas or panoramic windows, where condensation risk is particularly high due to larger contact surface with cold external air, the best available thermal parameters are crucial. Yawal TM 102HI aluminium windows with U-value of 0.58 W/(m²K) currently represent the pinnacle of technology available on the market, enabling spectacular glazing while maintaining full safety from condensation.

In renovation projects where budget may be limiting but condensation problems require definitive solution, PVC GEALAN S8000 windows offering parameters of 0.8-0.9 W/(m²K) are an excellent compromise. This level of energy efficiency, especially combined with standard triple glazing and Swisspacer warm edge spacers, guarantees significant glass surface temperature increase and condensation elimination while maintaining attractive value for money.

Summary: How to Effectively Solve Condensation Problems

Key Conclusions

Condensation is a physical phenomenon resulting from glass surface temperature dropping below the dew point characteristic of room air humidity. Understanding this relationship enables informed action.

Quick fixes (ventilation, dehumidifiers, vents) are temporary solutions with high operating costs and limited long-term effectiveness. They can provide support but don’t eliminate the problem cause.

A permanent solution requires a systemic approach combining proper window parameters (low U-value, triple glazing, warm edge) with professional warm installation. Only this comprehensive approach guarantees definitive problem elimination.

The statistic that 80% of condensation problems with new windows result from installation errors shows the crucial role of proper installation. The best window can fail if poorly installed.

Practical Guidelines for Solution Selection

Choosing the appropriate anti-condensation solution should begin with thorough analysis of offered products’ thermal parameters. The thermal transmittance coefficient (U-value) is the basic indicator of window effectiveness against condensation—values below 0.9 W/(m²K) can be considered the minimum ensuring effective protection. Best results are achieved by PVC windows using advanced GEALAN profile systems with parameters of 0.73-0.9 W/(m²K), while for the most demanding applications, Yawal aluminium windows with parameters reaching 0.58 W/(m²K) are available.

An essential element of every modern anti-condensation window is the use of triple glazing combined with warm edge spacers. This combination, which should be standard rather than an optional extra, ensures optimal glass surface temperature even in extreme external conditions down to 14°F (-10°C). Equally important is checking the installation technology offered by the contractor—only professional warm installation with a three-layer sealing system guarantees full utilization of the best windows’ energy-saving potential.

It’s also worth considering supplementary solutions that can significantly assist in combating excess room humidity. Humidity-controlled vents like the Maco Vent system offer automatic humidity management without significant energy losses, providing the ideal complement to airtight, energy-efficient windows. The comprehensiveness of offered solutions should include the ability to choose the optimal system for specific applications—from economical GEALAN S8000 solutions to premium Yawal TM 102HI—plus guaranteed professional execution backed by multi-year seal warranty.

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This article is educational in nature and presents general principles regarding condensation on windows. In case of persistent moisture problems, we recommend consulting a window industry professional who will conduct a detailed diagnosis of the situation in your home and propose the optimal solution.