Basically it works this way.  Radiant heat is heat that is transferred across either an air space or a vacuum.  This is how the heat from the sun reaches the earth.  Radiant heat acts similar to a sound wave.  By definition, you MUST have either an air space or a void for radiant heat to even exist.

If you don’t have an air space then you basically have a solid.  Heat can essentially only move through a solid by conduction. This is by direct contact.  This is how an egg cooks on a hot skillet.

So, without an air space, you cannot have radiant heat. Without radiant heat there is no way to have a “Radiant Barrier”.  In fact, because of the conductive nature of pure aluminum, if you install radiant barrier foil WITHOUT the required air space it will actually work AGAINST you and INCREASE Heat flow.

The bottom line is you MUST have an air space on one side of a radiant barrier for it to work.

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Looking for more videos on this topic? Check out my posts below.

I put together a video to help illustrate how traditional attic insulation and radiant barrier WORK TOGETHER to make a home more comfortable and energy efficient.

Remember, traditional attic insulation – fiberglass or cellulose help to reduce Conductive Heat Loss.  A radiant barrier will help to reduce Radiant Heat Loss.  In cold weather, heat is lost in BOTH ways.

Combining good attic insulation and radiant barrier will give your home the best defense to stay warm in the Winter and cool in the Summer.

Click here to view the embedded video.

I've written several other posts on this that you might be interested in. Check these posts below:

Paint applied as a Radiant Barrier

You may have recently heard some Radio/TV ads talking about an “Amazing Radiant Barrier Paint that is applied to the underside of your roof”. And how it “reflects” over 75% of the heat to keep your attic cooler and save you money on your utility bills.

Let’s get some facts straight. There is NO such thing as RADIANT BARRIER PAINT.

The term “Radiant Barrier” is supposed to describe products that “Reflect” over 90% of radiant energy (think light colored and/or shiny surfaces like aluminum, silver and gold) or have an emissivity of less than .10. This means they can only “Release” less than 10% of energy as radiant heat (think a potato wrapped with foil to keep heat from being released).

The term “Radiant Barrier” has been hijacked by the paint installers to try to confuse consumers into thinking they are the same. This is like hamburger calling itself a prime steak.

Paints like Sherwin Williams E-Barrier, STS Coatings HeatBloc or Solec LO/MIT are NOT the same as radiant barrier foil.  I’m surprised the FTC has not stopped this fraudulent advertising or a lawyer has not created a class-action suit on the behalf of customers who thought they were being sold a “radiant barrier” by installers.

Paint products technically have their own name.  They are called Interior Radiation Control Coatings or IRCC’s for short. This is not as cool as being called a “Radiant Barrier”. Ironically, most paints don’t even qualify to be an IRCC since the definition of an IRCC is a product that reflects at least 75% or emits less than 25% of radiant heat.  Here are some test results of radiant barrier paints.

Paints will NEVER perform as well as foil radiant barrier products. Foil always reflects 97% of radiant heat energy.  This is an indisputable fact.  Radiant Barrier Foil is always superior to IRCC’s. Paints run into other challenges.  Because radiant barrier paints are “applied” rather than “installed” you get inherent variables in the application process.

First, how can you tell without testing if the product was put on too thick or too thin?  What about painting unprimed wood with paint?  Common sense says it will be absorbed by the wood and reduce the “smoothness” required for a good low emissivity surface. You will also need to paint the deck AND the rafters to get the maximum benefit.

Finally, what about cheating?  These radiant barrier paints are really EXPENSIVE, like $50 per gallon.  I’ve seen guys use cheap silver paint, or mix water with the good paint to extend coverage and reduce costs, which will also reduce the effectiveness of radiant barrier paints.  I’m sure there are many honest installers, but watch out for the bad apples.

The reality is that most radiant barrier paint products end up reducing radiant heat into the attic by about 20-40%.  This is far below the claimed rates of 75%.

There are claims that foil is not effective unless you get the whole roof. This is NOT true.  Any product, whether it be foil or paint has a cumulative effect, the more coverage the better.  A tree over part of your home still helps, right?

If you compare the math, you could actually put foil (reflecting 97%) over half the home and get more heat reduction than applying paint (reflects 20%-40% in real installations) over the whole home.

Don’t believe anyone that says that radiant barrier paint is as good or as effective as radiant barrier foil.  And remember, there is technically NO such thing as radiant barrier paint.