What “97% Reflective” Actually Means

The Physics Behind Radiant Barrier Foil — and Where the Technology Came From

We say on our own website that our radiant barrier foil reflects 97 percent of radiant heat. That is a real specification rather than a slogan, but almost nobody explains what it actually describes — or where the technology behind it came from.

So here is the honest version, because a Houston homeowner deciding whether to spend money on an attic deserves to understand what they are buying. What the number means, the patent it traces back to, what the space program proved about it, and the one thing about your house that will determine your result more than the foil itself.

What the 97 Percent Actually Describes

It describes a property called emissivity, and it is worth two minutes of your time because it is the whole reason this product works.

Every surface both absorbs and radiates infrared energy. How much it radiates from its own surface is its emissivity, expressed from 0 to 1. Aluminum foil has an emissivity of roughly 0.03 — meaning it radiates about three percent of the heat energy it could, and reflects the remaining 97 percent. Those two numbers are the same fact stated two ways.

That is a genuinely unusual material property. Most building materials sit near 0.90. Your roof decking, your framing, your existing insulation — they all radiate heat readily. A sheet of aluminum does not, and that difference is what building codes actually regulate: California’s code, for example, requires a radiant barrier to have an emittance of 0.05 or lower to qualify as one.

One important clarification, since it comes up constantly: this is thermal reflectivity, not visible shine. It is not about how bright the foil looks. It is about infrared energy you cannot see.

What it is not

A radiant barrier has no R-value, and any company that quotes you one is quoting a number that cannot be measured. The EPA has stated that radiant barrier products do not carry an R-value because convective air movement in an open attic cannot be meaningfully quantified, and the Federal Trade Commission has stated that no generally accepted test procedure exists to determine one.

That is not a weakness. R-value measures resistance to conducted heat — a completely different mechanism. Foil addresses radiant heat. Fiberglass and spray foam address conduction. They are different tools for different jobs, which is why we sell all of them and why the right answer for your attic depends on what your attic is actually doing.

A 1925 Patent, Not a 1960s Invention

Radiant barrier is often sold as space-age technology. The truth is more interesting: it is considerably older than the space program.

In 1925, Ernst Schmidt and Eduard Dyckerhoff filed a patent describing metal foil held at a distance from the surface being protected, using the bright metal face to reflect heat radiation while the thinness of the foil limited conduction through it. That is the complete concept — a low-emissivity surface facing an air gap — and it was on paper thirty-three years before NASA existed.

By 1939 it was a commercial product in the United States. A patent granted that November to Roy J. Scott, assigned to the ALFOL Insulation Company, described crumpled aluminum foil sheets arranged in spaced layers to create insulating air spaces. Foil insulation was being sold to American builders before the Second World War.

So this is not an experimental material. It is a hundred-year-old principle that has been manufactured, installed and measured for the better part of a century.

What the Space Program Proved About It

NASA did not invent radiant barrier. What NASA did was prove it under conditions no Houston attic will ever approach.

A spacecraft has no atmosphere around it to carry heat away. In orbit there is no convection and essentially no conduction to the outside — virtually all heat transfer happens by radiation. That makes low-emissivity reflective film the primary thermal control tool rather than a supporting one, and NASA has used aluminized reflective materials for spacecraft thermal control since the earliest days of the space program.

NASA also states that the metallized emergency “space blanket” now found in every first-aid kit was first developed by the agency in 1964, and it lists radiant barriers among the documented spinoff technologies in its own Spinoff publication.

Two things worth being straight about, because this industry abuses both. NASA does not endorse commercial products and does not verify any manufacturer’s performance claims. There is no such thing as NASA-approved insulation, ours included. And a spacecraft application does not transfer directly to a house — your attic has air in it, which means conduction and convection are still very much in play. That is exactly why foil alone is not a complete attic solution.

Why Houston Is Where This Technology Earns Its Keep

Radiant barrier is one of the more heavily measured products in American residential construction, and the research is consistent about one thing: climate decides almost everything.

The Department of Energy’s Building America program, through Pacific Northwest National Laboratory, reports that radiant barriers can cut cooling costs between 8 and 12 percent in hot climates, while noting they are seldom cost-effective in cold or marine climates. A DOE and Oak Ridge National Laboratory field study of Florida homes — a climate close to ours — measured 9 percent cooling energy savings and a 16 percent reduction in peak cooling load after a retrofit. Oak Ridge’s 2010 analysis put annual savings in the hottest US climate zones, which include Houston, at roughly $50 to $150 per year.

Peak load is the figure Houston homeowners should notice. That is the worst stretch of an August afternoon, when your system is running hardest and least efficiently. Reducing what the attic dumps onto your ceiling during those hours is worth more than the annual average suggests.

Gulf Coast humidity adds a wrinkle that drier climates do not have, and it affects installation rather than performance. Foil laid flat across attic-floor insulation can trap water vapor and cause condensation underneath it. That is one of several reasons we install to the roof framing instead — the reflective face stays open to its air gap, and moisture is not being sealed against your insulation.

The One Thing That Decides Your Result

Across every dataset, one variable moves the numbers more than the product itself: whether your HVAC ducts run through the attic.

Oak Ridge found that duct location and condition changed radiant barrier savings by roughly a factor of two. The reason is easy to picture. Ducts in an attic are carrying cooled air through a space that passes 130 degrees on an August afternoon in Houston. Every foot of that run is fighting the attic temperature around it. Bring the attic down and you are not only protecting your ceiling — you are protecting the air inside those ducts on its way to your rooms, and taking load off equipment that is already working hard.

Attic ductwork and attic-mounted air handlers are extremely common in Houston-area homes. If that describes yours, you are in the group the research consistently shows the strongest results for. Measured attic temperature reductions from a properly installed radiant barrier run from 3 to 23 degrees depending on the home.

It also explains why two neighbours can get noticeably different results from the same installation. It is not the foil. It is the house.

Where Foil Stops and Something Else Starts

Radiant barrier handles one of the three ways heat moves. It does nothing about the other two, and pretending otherwise is how homeowners end up disappointed.

Conduction — heat moving through the ceiling assembly — is the job of fiberglass attic insulation or spray foam insulation. If your attic floor is thin, uneven or disturbed, that is the first thing to fix, and foil will not compensate for it.

Convection and air movement — hot attic air being pulled into your living space through ceiling penetrations, and superheated air sitting in the attic itself. Air sealing addresses the first. Solar attic fans can help move the second.

And when existing material is wet, contaminated or badly deteriorated, none of the above matters until it comes out. Insulation removal is sometimes the honest first step, even though it is the one nobody wants to hear.

We install all of it, which means we have no particular reason to push you toward one. What we do have is a reason to get the diagnosis right, because the fastest way to lose a customer in Houston is to sell them foil when their real problem was six inches of settled fiberglass and a leaking return duct.

Radiant Barrier Questions Houston Homeowners Ask

What does 97 percent reflective actually mean?

It describes emissivity. Aluminum foil has an emissivity around 0.03, meaning it radiates about three percent of the heat energy it could and reflects the other 97 percent. Most building materials sit near 0.90. It refers to infrared energy, not visible shine, and it is the property building codes regulate — codes typically require an emittance of 0.05 or lower for a product to qualify as a radiant barrier.

Did NASA invent radiant barrier?

No. The principle was patented in 1925 by Ernst Schmidt and Eduard Dyckerhoff, and foil insulation was a commercial building product in the United States by 1939. NASA adopted the same physics for spacecraft thermal control, where radiation is essentially the only way heat moves, and lists radiant barriers among its documented spinoff technologies. NASA does not endorse commercial products and does not verify manufacturers’ performance claims.

Does radiant barrier have an R-value?

No, and neither does anyone else’s. The EPA has stated radiant barrier products do not carry an R-value because convective air movement in an open attic cannot be meaningfully quantified, and the FTC has stated no generally accepted test procedure exists to determine one. R-value measures resistance to conducted heat; radiant barrier addresses radiant heat. Any company advertising an R-value for foil is quoting a number that cannot be measured.

Will it lower my Houston electric bill?

In this climate it can meaningfully reduce cooling load. DOE’s Building America program reports 8 to 12 percent cooling cost reductions in hot climates, and an Oak Ridge field study in Florida measured 9 percent cooling energy savings with a 16 percent cut in peak load. What no honest contractor can do is promise you a specific number before seeing your attic — your roof, existing insulation, ductwork and how you run your system all move the result.

Does Houston humidity cause a problem with foil?

It can, if the foil is installed in the wrong place. Laying it flat over attic-floor insulation can trap water vapor and cause condensation on its underside, which matters more on the Gulf Coast than in a dry climate. Installing to the roof framing keeps the reflective face open to its air gap and avoids sealing moisture against your insulation.

Should the foil go on the rafters or on top of my insulation?

On the rafters, in nearly every case. Beyond the moisture issue, foil laid across attic-floor insulation collects dust — and dust raises emissivity, which is precisely the property that makes it work. DOE’s fact sheet notes a horizontal application can lose roughly half its effectiveness over time as dust settles. Gravity works against the horizontal install and barely touches the vertical one.

Find Out What Your Houston Attic Is Actually Doing

The attic assessment and written estimate are free. We get into the attic, measure what is actually there, and tell you whether radiant barrier foil, fiberglass, spray foam, insulation removal or a combination is the right answer for your home — including when the answer is that your attic is already in decent shape. Serving Houston, Katy, Sugar Land, The Woodlands, Cypress, Spring, Pearland, Richmond, Missouri City, Kingwood and the surrounding communities.