Your roof acts the same way.  It can easily reach 150º or hotter on a sunny day.  This heat will then be re-radiated through the attic and will be absorbed by your attic insulation causing high energy bills and comfort issues.

This video illustrates how a radiant barrier works – plain and simple.  Rather than absorbing the radiant heat from the roof, let’s bounce it back and keep the attic insulation cooler. If you keep the insulation cooler, then less heat enters the home and you save money and stay more comfortable.

Think of it this way:  Radiant barrier is your 1st line of defense against radiant heat and your attic insulation is your 2nd line of defense against conductive heat.  They will actually work TOGETHER for maximum efficiency and comfort.

Click here to view the embedded video.

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In an attic application, you should ALWAYS use a perforated product. Period.

Perforated Radiant Barrier Foil

Why? Solid products like bubble foil insulation are called “Vapor Barriers”.  A vapor barrier basically stops moisture from moving from point A to point B.  Or, another way to view this is that a vapor barrier will “TRAP” moisture.  I’m not going to get into the technical definition of what defines a vapor barrier (perm rating, etc), but here is an illustration of what IS and IS NOT a vapor barrier.

If you take a wet block of wood and put it inside a bag or an envelope made of perforated radiant barrier attic foil the wood block would eventually dry out.  Therefore, perforated radiant barrier is NOT a vapor barrier.  Moisture ALWAYS goes from wet to dry.  If you did the same test with the wood block inside a plastic bag the wood would still be wet months from now.  If moisture in its vapor form cannot pass through or object won’t “dry” then the product IS a vapor barrier.

Why is this important?

Virtually all (over 70%) of home issues are due to mold, mildew, rot, decay, etc. And moisture is the common theme here.  DRY products don’t grow mold, rot or decay. The bottom line is that moisture in wall and ceiling assemblies is not a good thing.  You want DRY walls and ceilings.

In cooler weather, the moisture INSIDE the home is greater than OUTSIDE.  Think cold and dry.  This is why our lips get chapped in the Winter and not in the Summer.  Since moisture will naturally move from wet (inside) to dry (outside) it will pass through sheetrock, insulation and then into a typical attic.  The LAST thing we want to do is TRAP moisture here.  Moisture, attic insulation and wood do not go well together.  A perforated radiant barrier will allow moisture to pass on through into the attic.  We want our attics to be cool and DRY.

Using a perforated radiant barrier will not change the effectiveness of the reflectivity.  Attic Foil has tiny pinholes about every ½ inch that allow for water in its vapor form to pass through (see picture).  These holes make up a TINY percentage of the surface area and will not change the effectiveness of the radiant barrier.

Solid products like bubble foil insulation are usually not perforated and are a recipe for disaster when installed inside an attic.  Solid (non-perforated) bubble foil is a great product when used correctly in applications like metal buildings.  The problem is that solid bubble foil products are often MISUSED in residential attics. This is especially true if the bubble type reflective foil products are laid directly over the attic insulation.  Moisture will pass through the sheetrock and will get trapped in the insulation below the bubble foil insulation.  This moisture will accumulate until it either condensates (turns to water) or freezes (turns to ice).

This is why it is critical to use a perforated tarp-like radiant barrier product. It will give you all the benefits of reflective insulation without the potential for moisture to get trapped.

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

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.

Click here to view the embedded video.

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

Click here to view the embedded video.

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

The answer: ZERO

Heat moves in 3 different methods:  1) Conductive 2) Radiant 3) Convective.  R-Value is a term used to measure how resistant a product is to CONDUCTIVE HEAT ONLY.  The best way to illustrate this is to picture an ice chest with a big block of ice inside.  Common sense tells us that WHAT the ice chest is made of and how THICK the walls of the ice chest are will determine how long it will take for the ice to completely melt.  An ice chest made of foam will obviously keep the ice from melting longer than an ice chest made of metal.  This is R-Value.

R-Value examples:  Wood has an R-Value of about 1 per inch of wood. So, three inches of wood has an approximate R-Value of 3.  Styrofoam has an R-Value of about 3-4 per inch resulting in an R-Value of about 11 for 3 inches.  This is why we don’t make coffee cups out of wood.  The coffee would not stay hot very long.

Radiant Barrier has NO R-value.  Why? Because R-Value is a measurement to determine the effectiveness of a material to slow CONDUCTIVE heat flow. It is an apples to oranges comparison. This is like asking how effective is an umbrella to protect you against the wind?  Obviously, an umbrella is designed to stop rain and not wind.  Just like a radiant barrier is designed to stop radiant heat as compared to conductive heat.

So, if radiant barrier has no R-Value, how is it effective?

Think of radiant barrier as SHADE.  Does shade have R-Value? No, but it sure is nice to have on a sunny day to help keep cool and comfortable.

Here is an example:  Let’s take two identical ice chests and fill them with ice.  Then, put one ice chest in the sun and the other in the shade. Which one is most effective in keeping the ice from melting?  The one in the shade, right?  Although the AIR TEMPERATURE will be exactly the same in the shade and the sun, the OUTSIDE SURFACE TEMPERATURES of the ice chest will be higher in the sun.  This will cause the ice chest in the sun to be less effective than the one in the shade.

If you were to wrap an ice chest with radiant barrier foil and put it in the sun, the foil would act like shade by reflecting 97% of the radiant heat from hitting the ice chest.  Decreasing the outside surface temperature is as effective as adding more insulation or more R-Value in most circumstances. Therefore, although radiant barrier by itself has NO R-value, it WILL increase the effectiveness of the existing insulation or R-Value.

Your home is very similar.  Regular insulation is like the ice chest and a radiant barrier will act like shade to keep the box cooler.  Install a radiant barrier to make the insulation in your home even more effective. Combining the two will maximize comfort and efficiency by slowing BOTH conductive and radiant heat flow.

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