Will aluminum foil on windows keep heat out?

Most of us scramble for relief when summer heat waves hit, desperately seeking ways to cool our homes without skyrocketing energy bills. Is aluminum foil really the affordable hack it claims to be—or could it be doing more harm than good?

Placing aluminum foil on your windows can reflect up to 95% of sunlight and temporarily reduce indoor heat, but it comes with risks to window glass and overall aesthetics, making it best for short-term use rather than permanent solutions.

Many homeowners, contractors, and even local window dealers have asked me: does this classic DIY trick truly have benefits, or is it mainly a myth propped up by viral Internet advice? Let’s break down what the science (and industry experience) says about aluminum foil, and explore better options for year-round comfort.

Can you put foil on your windows to keep the heat out?

If you’ve ever considered taping foil to your windows in a heatwave, you’re not alone. But is it actually safe or practical?

Adding aluminum foil to your windows’ exterior can effectively reflect sunlight and block heat, but putting it inside—especially on double-glazed or modern windows—risks overheating glass and possible damage. Experts recommend using cardboard as a barrier for extra protection.

Let’s get technical for a moment. Aluminum foil’s main asset is its high albedo¹—the ability to reflect radiation. If you tape it on the outside of your windows, it bounces much of the sun’s heat away before it even gets to your glass. But from my experience and industry reports, taping it inside creates a gap where heat can build up, turning the space into a mini greenhouse. On double-pane windows², this leads to overheating, which can break the window’s seal—or even crack the glass. Here’s a breakdown of the pros and cons:

Placement	Benefit	Risk/Drawback
Outside	High reflection; safer	Harder installation, visible from street
Inside	Easier to set up	Glass overheating, window damage possible

My tip? If you must use aluminum foil, attach it to a piece of cardboard that fits your window and place it on the sun-facing exterior. Always use heavy-duty foil (like Reynolds Wrap Heavy Duty)—it’s more durable and effective for short-term emergencies. Avoid taping directly onto the glass for anything other than a quick fix.

What is the best way to block heat through windows?

Frustrated with relentless indoor heat even with the foil up? There are better, longer-lasting solutions to keep your space cool.

The most effective way to block heat through windows combines exterior shading, specialized coatings like Low-E glass, and professional-grade window films. These options balance aesthetics, performance, and comfort while reducing cooling costs year-round.

Let’s compare your choices:

Heat-Blocking Solutions Table

Solution	Effectiveness	Longevity	Aesthetics	DIY Friendly?
Aluminum Foil + Cardboard (outside)	High (short-term)	Temporary	Fair to poor	Yes
Reflective (One-Way) Window Film3	High	Long-term	Good	Moderate
Low-E Double/Triple Glazing4	Very high	20+ years	Excellent	No (professional)
Exterior Shutters/Awnings	High	Long-term	Excellent	Yes/Pro
Shade Trees/Natural Cover	Moderate-high	Permanent	Excellent	No (nature)

As a manufacturer at HOGLAD, I strongly recommend Low-E glazing combined with argon-filled double or triple-pane windows, plus exterior sunscreens or adjustable shutters for lasting comfort. This isn’t just sales talk—energy-efficient windows lower both your bills and your environmental impact. If you’re after a quick, budget fix, foil or blackout curtains will do in a pinch. For anything long-term, invest in technology designed specifically for your climate and lifestyle needs.

Why do people put foil on windows?

It’s more common than you think—especially during heat waves or in homes without air conditioning. But what’s driving this trend?

People cover windows with foil to reflect sunlight, reduce indoor temperatures, save on air conditioning costs, increase privacy, and block UV rays that fade furnishings. It’s a quick, cost-effective response to extreme heat or sunlight exposure.

During heat emergencies[^5] (and, frankly, on budget-strapped renovation projects), I’ve seen clients turn to aluminum foil for its immediate effects: cooler rooms, more privacy, and less glare on screens. There’s even a bonus for people living in apartments facing intense afternoon sun—foiling windows can help keep valuables and interiors from fading over time. However, foil isn’t a perfect solution. It rarely looks good from the street, and in some communities, it can lower curb appeal or even violate building rules. For temporary relief—like a sudden heat wave—it’s a creative workaround. For real performance, other solutions are superior.

What side of aluminum foil reflects heat?

You might have noticed that aluminum foil has a shiny side and a dull side. Does orientation really matter?

The shiny side of aluminum foil reflects up to 88% of radiant heat and should face outward to maximize sunlight and heat deflection, making your DIY heat barrier as effective as possible.

Here’s the science: the shiny side⁵ is created by the final rolling process in foil production. Its smooth, reflective surface is what sends sunbeams bouncing back out. If you put the dull side⁶ out, you lose a big chunk of that benefit. Always face the shiny side outward and keep the foil flat and tight against your cardboard or frame (but not glued tight to the glass), to get the most from your makeshift sunshield.

Foil Orientation	Heat Reflection
Shiny Side Out	Highest (up to 88%)
Dull Side Out	Lesser (still good)

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Conclusion

Aluminum foil can offer quick, temporary relief from summer heat, but for lasting comfort and property value, modern window technology and well-designed shading solutions reign supreme.

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