Why Do Windows Streak? The Science Behind Residue, Water, and Technique

If you've ever spent time cleaning windows—only to step back and see streaks—you're not alone. As someone who’s overseen countless commercial window cleanings across New England, I can tell you this: streaks aren’t just annoying; they’re often a sign of an incomplete cleaning process.

So what causes them? The answer lies in the science of water, residue, and evaporation.

I’ll walk you through the chemistry and technique behind window streaks and share what Jan-Ex does differently to keep glass surfaces spotless.

What Causes Window Streaks?

1. Hard Water Minerals

In many parts of New England, especially around Boston and Providence, water contains high levels of calcium and magnesium. This is known as "hard water."

When tap water is used to clean windows, it can leave behind mineral deposits. As the water evaporates, these minerals cling to the glass, creating cloudy streaks and spots. These deposits are especially visible in sunlight and can etch into the glass over time if not removed properly.

Tip: This is why professional glass companies use deionized (DI) or purified water systems that remove these minerals before they ever touch the glass. Deionized water, stripped of all ionic impurities, dries without leaving a trace.

2. Residual Soap or Chemical Buildup

Using too much detergent or the wrong kind of cleaner can cause streaking.

Most commercial glass cleaners contain surfactants—compounds that lift dirt and oil. But if they’re not completely rinsed or wiped away, these surfactants leave a thin film that catches light and creates visible streaks. Some cleaners even contain dyes or fragrances that further contribute to residue.

The Fix: At Jan-Ex, we use pH-neutral, streak-free formulas and microfiber cloths to minimize residue. We also avoid ammonia-based cleaners in sensitive environments where odor or chemical sensitivity is a concern.

3. Evaporation Before Drying

Especially in warmer months or direct sunlight, water can evaporate before it's been fully wiped off. When that happens, any contaminants (soap, dirt, or minerals) get left behind in streaky patterns.

Best Practice: Clean windows early in the morning or late in the day to avoid this problem. We also section our work and squeegee methodically from top to bottom to control drying time and residue.

4. Dirty or Low-Quality Tools

Rags, squeegees, or cloths that are already dirty can simply redistribute grime—and cause streaks. Tools that have accumulated cleaning solution residue or lint are ineffective at removing moisture evenly.

Also, using paper towels (yes, even the expensive ones) often leaves behind fibers and lint, which stick to the damp glass and leave visual imperfections.

The Jan-Ex Difference: We use commercial-grade microfiber cloths that trap dirt without scratching surfaces and professional-grade rubber-edged squeegees that deliver consistent pressure and coverage.

Glass surface under a microscope

5. Glass Itself Isn’t Smooth

Surprise: glass isn’t perfectly smooth. Under a microscope, it’s full of tiny pits and valleys that trap grime, oils, and detergent residue. On a microscopic level, glass is actually full of tiny irregularities: microscopic pores, and scratches. 

These tiny valleys and pits act like miniature traps for dirt, oils, moisture, and even cleaning residues. When glass is cleaned, especially with improper techniques or harsh chemicals, these contaminants aren’t always fully removed. Instead, they settle into these micro-crevices—leading to hazing, streaks, and that frustrating “film” that seems to reappear no matter how often you clean.

At Jan-Ex, we treat glass like a precision surface. Personally, I’d recommend Glance® NA Glass Cleaner—which evaporates quickly and leaves virtually no streaks or buildup behind. It’s specifically formulated for commercial glass surfaces.

How Jan-Ex Prevents Streaking on Commercial Glass

Jan-Ex has invested in the equipment, products, and training to deliver truly streak-free window cleaning. Here's how:

• We use purified water systems to avoid mineral deposits that cause clouding and streaks.

• Our crews are trained in streak-free technique—squeegee angle, pressure, and cloth placement all matter.

• We clean in the right conditions to reduce premature evaporation, especially in sun-exposed lobbies or glass walls.

• We avoid residue-heavy products, especially near sensitive environments like healthcare facilities, daycares, and restaurants.

  
  

Streak-Free Glass: Why It Matters for Your Facility

• Professional Appearance: Streaky windows can make an otherwise clean space look neglected. Clean windows demonstrate attention to detail.

• Health Perception: In healthcare and childcare settings, visible cleanliness reinforces safety. Parents, patients, and staff equate clean glass with clean hands and hygienic protocols.

• Natural Light: Clean glass allows more light into your space, boosting morale and reducing the need for artificial lighting.

• Longevity of Windows: Regular cleaning reduces mineral buildup and etching, extending the life of the glass.

  
  

Ready for Clear, Clean Windows?

Jan-Ex offers professional window cleaning services as part of our full-service commercial cleaning packages. Whether you need one-time service or ongoing maintenance, we’ll help you create a cleaner, more polished space.

🔗 Schedule a Free First Window Cleaning** or email cstanza@jan-ex.com to get started.

Free Window Cleaning Requirements!

1. Offer valid for new commercial clients only. (One free cleaning per company, per location.)

2. Office must be under 2,000 square feet and located within our Massachusetts or Rhode Island service area.

3. Includes interior glass only, and possible first floor exterior

4. Glass surface area must not exceed 400 sq. ft. (Roughly 15–20 standard windows.)

5. Windows must be accessible and free of obstructions (e.g., blinds, signage, furniture). We reserve the right to decline service if the area isn’t safe or accessible.

6. Offer includes basic window cleaning only. (Does not include restoration, paint removal, post-construction cleaning, or screen cleaning.)

7. Subject to availability. Limited slots per week.

Call us if you have any questions about this offer! ‪(617) 294-9815‬