FAQ #21: What Is Infrared (IR) Rejection and How Does It Cause Heat Buildup?

Q: Window film companies always talk about "IR Rejection." What is infrared radiation, and how does it actually make my house hot?

This is one of the most important things to understand about window film — and most people have never had it explained clearly. Let's fix that.

Infrared (IR) radiation is a type of electromagnetic energy that exists just beyond the red end of the visible light spectrum. You can't see it, but you feel it constantly — it's the warmth you feel when you stand in a patch of sunlight, or when you hold your hand near a hot stovetop without touching it.

About 53% of the solar energy that reaches your windows is infrared radiation. It's the single largest contributor to solar heat gain in your home.

Here's the process, step by step:

1. Sunlight hits your window. The glass is largely transparent to infrared radiation — it passes right through, just like visible light does.

2. IR energy enters your home and is absorbed by interior surfaces — your floors, furniture, walls, countertops, and even the air itself.

3. Those surfaces re-emit the energy as heat. This is called thermal re-radiation. The infrared energy that entered as radiation is now heat energy radiating from your interior surfaces.

4. The heat accumulates. Unlike a breeze that passes through, this heat has nowhere to go. It builds up in the room over the course of a sunny afternoon.

5. Your air conditioner works overtime trying to remove the heat that the sun keeps pumping in through your glass.

This is why a room with large south- or west-facing windows can feel like an oven by 3 PM even when the thermostat is set to 72°F. The AC is fighting a constant battle against the infrared energy pouring through your glass.

IR Rejection is the percentage of infrared radiation that a window film blocks before it enters your home.

A film with 94% IR Rejection intercepts 94% of the infrared energy at the glass surface — before it ever reaches your floors and furniture. The result is dramatically less heat buildup, a more comfortable room, and a significantly reduced load on your air conditioning system.

Hüper Optik ceramic films achieve IR rejection rates of up to 94% using nano-ceramic particle technology that specifically targets the infrared wavelengths. This is why ceramic films outperform older dyed or metallic films, which may block some IR but are far less effective at targeting it precisely.

IR Rejection is a powerful metric — but it only measures one portion of the solar spectrum. For the complete picture of how much total heat a film blocks, you also want to look at the Solar Heat Gain Coefficient (SHGC). See FAQ #20 for a full explanation.

Together, high IR rejection and low SHGC tell you that a film is genuinely high-performance — not just good at one part of the job.

Infrared radiation is the invisible engine of heat buildup in your home. It passes through glass silently, gets absorbed by your interior surfaces, and re-radiates as heat that your AC has to fight all day. High IR rejection film stops this process at the source — at the glass — before the heat ever enters your home.

Have a window tinting question? Browse our full FAQ or contact us for a free consultation. Sungate has served Atlanta-area homeowners for over 33 years.