Question: What is the R-Value (or R-Rating) of Radiant Barrier Foil Insulation?
This will probably surprise you but radiant barrier on its own has no R-Value!
This is important to understand because radiant barrier does not take the place of traditional insulation (fiberglass, cellulose, etc.). Traditional attic insulation has R-value; this works to slow conductive heat. Radiant barriers reflect radiant heat; both conductive and radiant heat are trying to enter your home on a hot, summer day (or leave on a winter day when you are running the heat inside).
The sun heats up the roof and then the heat is transferred by radiant heat across the attic space until it hits the attic insulation. Then, the heat transfer method switches from radiant heat to conductive heat to move through the attic insulation and into your home. This is why you need BOTH Types of insulation. Traditional attic insulation and radiant barrier work together and each do their part. Radiant barrier is your first line of defense (against radiant heat) and traditional attic insulation (fiberglass or cellulose) is the second line of defense against conductive heat gain.
What is R-Value?
R-Value is a term to describe how quickly heat will pass through (or be absorbed) by a product. This testing to designate an R-value only applies to conductive heat flow, not radiant heat flow. Conductive heat will only pass through a solid object (or several objects that are in contact with one another - like the layers of your roof on your home). Radiant heat is heat that passes through a gas or a vacuum, so this is the kind of heat the sun produces that radiates to earth until it is absorbed or reflected.
Every product has a different R-Value. For example, wood has an R-Value of 1 for each inch of wood; so three inches of wood has an R-Value of 3. Styrofoam or EPS (expanded Polystyrene) has an R-Value of 3.85 per inch; therefore, three inches of Styrofoam will have an R-value of over 11. This is a great real-life example of why we don't make coffee cups out of wood, Styrofoam keeps the heat in longer!
How Do Radiant Barriers Affect R-Value?
Reflective radiant barriers reflect (or reject) 97% of the radiant heat from hitting an object. By doing this, a radiant barrier makes the r-value of a material more effective.
Here is an example: if you fill two plastic ice chests with ice cold drinks and place one in the sun and the other in the shade which one will stay cold longer? They both have the same R-Value because of the plastic. Well, by placing one ice chest in the shade you effectively put a radiant barrier over it (since radiant barrier acts like shade) and you remove the opportunity for the ice chest to absorb any radiant heat. So the answer is the chest in the shade stays cooler longer because radiant barrier works to make R-value more effective at blocking heat gain.
What about R-Value and Bubble/Fiberglass/Foam Insulation?
Bubble foil does offer some R-Value because of the bubbles (because remember, radiant barrier does not have an R-value on its own). However, we see claims of up to R-14 for some bubble products.
If you read the fine print, this rating is always achieved as part of an overall assembly, which usually includes a lot of dead air spaces. In a lab you might be able to create a perfectly tight seal on bubble foil, but in the real world it is virtually impossible to get these results.
Besides, if there is air flow around an object, the R-Value does not really matter. For example, if you have a heavy jacket and you unzip it and let air get between you and the jacket then it does not matter how thick the jacket is because you have no dead air space. So unless you can completely air-seal the bubble foil you will not get the full effectiveness of the R-value and you will be investing a lot more for no supplementary benefit.
Using Bubble Insulation in Attics
If you are installing Bubble Foil in a vented attic (meaning the attic is not sealed air tight), then you don't need bubble foil; all you need is a foil radiant barrier. You will pay extra for the bubble foil, but you will not get any additional benefit.
Would a 2" inch thick umbrella keep you any cooler on a hot sunny day than a regular umbrella? The answer is no. The thickness of the umbrella is irrelevant because the air temperature is the same on either side of the umbrella. This is the perfect example of why when we are building a home or structure, we do not put insulation/radiant barrier on the interior walls between rooms. We usually only insulate EXTERIOR walls since there is a big temperature difference between the inside and the outside of the structure.
R-Value only matters if there is a difference in temperatures on either side of the insulation.
Bubble foil is recommended for metal buildings to stop condensation and on the backside of knee walls to stop "hot walls." Other than those very specific applications, using it in an attic (for example, stapling it to the bottom of your roof deck) is overkill and furthermore, you create the potential to trap moisture since bubble foil does not allow water vapor to pass through.
Even though radiant barrier can be attached to some products (like bubble insulation, insulated sheathing, TechShield, and Polar-Ply), the product is achieving the r-value from the non-foil materials. Oftentimes the R-values are exaggerated and impossible to achieve in a vented, residential attic space.
Bottom line: Radiant Barrier does not have any R-value; therefore, you need a radiant barrier in addition to your traditional insulation to have the most effective insulating assembly.