The Science of Solar Heat Gain: How Window Film Reduces Hot Spots

Have you ever walked into your living room on a sunny afternoon and noticed one corner is unbearably hot while the rest of the house feels fine? These “hot spots” aren’t just uncomfortable—they’re the result of solar heat gain, and they can wreak havoc on your energy bills and your home’s comfort.

Fortunately, there’s a simple, science-backed solution: solar control window film. By addressing the physics of solar heat gain, window film helps you reclaim your comfort, even in rooms that feel like saunas every afternoon.

Let’s break down exactly what solar heat gain is, why it creates hot spots, and how window film solves the problem.

What Is Solar Heat Gain?

Solar heat gain is the increase in temperature inside your home that results from sunlight passing through windows and being absorbed by your walls, floors, and furnishings.

Here’s how it works:

1. Solar radiation hits your window. Sunlight contains three main types of energy:
   • Visible light (about 43%) – what we see.
   • Infrared radiation (about 49%) – what we feel as heat.
   • Ultraviolet (UV) radiation (about 8%) – invisible rays that cause fading.
2. Glass lets much of this energy through. Standard residential windows are poor insulators. They block some UV but allow a significant portion of visible and infrared light to pass through.
3. Your interior absorbs and radiates heat. Once that solar energy enters, it’s absorbed by furniture, carpet, and walls, which then radiate the heat back into the room.

The result? Rising indoor temperatures, particularly in areas directly exposed to the sun.

Why Do Hot Spots Form in Homes?

Not every part of your house gets equally hot. That’s because several factors influence where solar heat gain is strongest:

• Window orientation. South- and west-facing windows get the most direct sunlight in the U.S., making those rooms prone to overheating.
• Window size. Larger panes mean more solar energy pouring in.
• Glass type. Standard clear glass allows more heat transfer compared to Low-E or treated glass.
• Room design. Open layouts or spaces with less airflow can trap heat in certain areas.

This is why your home office may feel perfectly cool in the morning but unbearable by mid-afternoon, or why your upstairs bedroom seems hotter than the rest of the house.

The Science Behind Window Film

So, how does window film step in to solve the solar heat gain problem?

Window films—especially ceramic solar control films like Concord’s ComforTech™ Ceramic Series—are engineered to filter out the parts of sunlight that cause heat while letting in plenty of natural light. 

Here’s the breakdown:

• Infrared rejection. Ceramic particles embedded in the film block up to 82% of solar energy hitting the glass – including infrared rays which are the primary cause of heat buildup.
• UV protection. Films block up to 99% of UV radiation, preventing both heat damage and fading of your furnishings.
• Visible light balance. Unlike reflective or tinted films of the past, ceramic films reduce heat without overly darkening your home. You still enjoy daylight without the hot spots.

Essentially, window film acts like a filter—selectively letting in what you want (natural light) and blocking what you don’t (excess heat and UV). They achieve this by reflecting some of the energy and absorbing some of the energy:

How Window Film Reduces Solar Heat Gain & Hot Spots

Now let’s connect the science to what you actually feel in your home.

1. Temperature balance. By blocking infrared heat before it enters, window film prevents certain rooms from overheating, creating a more even temperature across your home.
2. Lower HVAC demand. Without hot spots, your air conditioner doesn’t have to overcompensate for “problem rooms.” That means less cycling, less strain, and lower bills.
3. Glare reduction. Hot spots often coincide with intense glare. Window film softens light, reducing eye strain and improving comfort in rooms with TVs or computer screens.
4. Energy efficiency. By reducing heat gain in summer and helping retain warmth in winter, film makes your home more efficient year-round.

In short: less sun in, less heat out, and a more comfortable home office, bedroom, or living space.

How to Understand Window Film Performance

To be able to compare different window films and brands, you need to know the key metrics window film is measured on. 

Window film is rated by the third-party rating agency: the National Fenestration Rating Council. This agency rates both windows AND window films. The main factors measured by the NFRC are Visible Light Transmittance, Solar Heat Gain Coefficient, and the U-Factor of the filmed window unit. 

Visible Light Transmittance is just what you would think: the amount of visible light that the filmed glass allows through into the space. 

Solar Heat Gain Coefficient (SHGC) is a bit more complicated. The SHGC is the sum of incident solar energy that is transmitted inside through the glazing system. It includes solar energy directly transmitted and the part of absorbed solar energy that is released inwards. SHGC is the fraction of incident solar radiations that actually enters the building through window glass. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. 

You can see the performace specs of our Concord’s ComforTech™ Ceramic Series window film below. You can see that as the film gets darker, the solar control increases. So darker films will naturally reduce more of the solar heat gain in your home:

Real-World Example

Imagine a west-facing home office with a large picture window. Without protection, the afternoon sun pours in, raising the temperature in that room by several degrees compared to the rest of the house. Your AC kicks on more often, but the office still feels hot and stuffy.

Now add solar control window film:

• Infrared rays are blocked at the glass.
• UV rays are eliminated, protecting your desk and floors.
• Visible light still brightens the room, but without harsh glare.

Result? The office stays within a degree or two of the rest of the house, your AC doesn’t work overtime, and you can comfortably work at your computer without squinting through glare.

Additional Benefits Beyond Hot Spot Reduction

While the main goal is reducing solar heat gain, window film delivers several side perks:

• Furniture preservation. UV rays cause fading in hardwood floors, rugs, and upholstery—film helps preserve your investments.
• Safety and privacy. Depending on the film type, you can add shatter resistance or increase daytime privacy without heavy blinds.
• Eco-friendly living. By lowering energy use, window film reduces your carbon footprint.

These extra benefits make it a smart all-around home upgrade, not just a comfort fix.

Why Choose Ceramic Window Film?

There are many types of window films, but ceramic technology stands out. Unlike older metallic or dyed films, ceramic film is:

• Dual-reflective. Won’t give your home a mirrored look on the inside. Read more about dual reflectivity here.
• Color-stable. Won’t fade or discolor over time.
• Professional-grade. Ceramic film is what professional installers use, but available DIY through Concord Window Film.

Our ComforTech™ Ceramic Series offers a range of shades, so you can choose the level of light transmission that fits your space while still reaping the heat-blocking benefits. 

Read more about ceramic window film for your home and check out our video on how our ceramic film is made:

Conclusion: Science Meets Comfort

Solar heat gain is a simple scientific process, but it has a big impact on your comfort and energy bills. Those hot spots around your home aren’t just bad luck—they’re the result of sunlight streaming through untreated glass.

By applying solar control window film, you harness advanced ceramic technology to filter out heat-causing infrared rays, reduce glare, and create a more balanced, comfortable living environment.

If you’re tired of battling hot spots in your home, explore Concord Window Film’s ceramic film collection. It’s the easiest DIY upgrade to bring the science of solar control into your home—without sacrificing natural light or style.