Understanding R Values

Understanding R-values and Automotive Heat Insulation

Understanding R-Values and Automotive Heat Insulation

Proper car insulation is the best defense against burning your forearm on the center console (looking at you, Corvette restorations) and getting hot feet when accelerating. There’s a lot of buzz about what materials to use, what their R-values are, and what that actually means for you and your passengers — we’re here to set the record straight.

Read on to learn everything you need to know about car heat insulation, including good insulators (closed cell foam), bad insulators (foil-faced bubble wrap), and what next steps to take for a more comfortable ride.

Proper insulation in your car is the best defense against burning your forearm on the center console (looking at you, Corvette restorations) and getting hot feet when accelerating. There’s a lot of buzz about what materials to use, what their R-values are, and what that actually means for you and your passengers — we’re here to set the record straight.

Read on to learn everything you need to know about automotive heat insulation, including good insulators (closed cell foam), bad insulators (foil-faced bubble wrap), and what next steps to take for a more comfortable ride.

What is Insulation?

Simply put, insulation is material designed to slow the flow of heat. It is important to know that insulation is only truly effective when it attacks the type of heat it was meant for. Not all insulators are equal, and every product isn't effective against every type of heat. Yes, I'm going to explain what that means.

There are three ways heat is transferred: conduction, convection, and radiation.

  • Conduction is when heat moves between solid materials, like when you put a pot on top of a hot stove.
  • Convection is the naturally rising flow of heat through liquids or gases. Remember learning about heat rising? Hot air is less dense than cold air, so gravity acts on it less. Hot air rises and cold air sinks. That movement is convection.
  • Radiated heat is electromagnetic infrared waves that travel through the air. When an object is in the direct pathway, it heats up and then radiates heat to other objects, like when your car seats become hot from sitting in the sun.
radiation, convection, and conduction

The Difference Between Heat Insulation and Heat Shields

If you’ve experienced unpleasant heat in your restoration hot rod, luxury sports car, or heavy truck, you’re probably looking for some new insulation. How can you be sure what you need?

Heat Insulation: High-quality insulators like closed cell foam slow down the heat transfer process many drivers experience in their cars. Closed cell foam and open cell foam both fight heat transfer by conduction and convection, which is usually a big by-product of aftermarket engines or performance packages. If your car runs hot, you need foam materials to resist that heat transfer.

Heat Shields: Although effective against radiated heat, most aluminum foil heat shields are not good insulators. Heat shields should be installed facing a heat source, so they can reflect the radiant heat away from the car’s metal (hood, firewall, underbody). You should not install a car heat shield on interior metal and expect it to help at all. However, when paired with a closed cell foam insulator in certain areas of your vehicle (floor, doors, roof), the two products together produce in a night-and-day difference when it comes to controlling temperature.

The Bottom Line

The takeaway here is that insulating your car is your best tool for defense from unwanted heat or cold. As is the case with car sound deadening, closed cell foam or open cell foam heat insulation works best when it’s part of an overall solution to the problem, so keep in mind that you might need a radiant heat barrier, too. We have a full suite of options that are detailed at the bottom of this article. Scroll if you want or stick around to learn about R-Values.

What is an R-Value? How Much Do I Need?

R-value is a way of measuring a material’s resistance to conductive heat flow. In simple terms, the higher the R-value, the better the material is at resisting the natural flow of heat. The three factors that play the biggest part in determining R-value are:

  1. Type of insulation
  2. Thickness of material
  3. Density of material

Beyond material, thickness, and density, the R-value of an insulator can be affected by age, temperature, and amount of moisture. Like with other materials found in your car, if the insulation material is old and damp, it will be less effective. Your car’s insulation also can be affected by how you treat it and where you install it. For example when open cell foam is compressed, it’s R-value is substantially reduced because the air pockets inside the foam are a substantial part of its ability to resist heat. You also shouldn’t expect insulation added to the floor to keep our the sun’s heat. Sounds like common sense, but you never know…

The key is to remember that the purpose of an insulator is to slow the heat transfer process — not stop it completely. In the summer, this usually means helping to keep cool air inside the car and hot air outside. In the winter, it’s the opposite – keep warm air in and cool air out. By slowing the flow, your car’s heating and cooling systems don’t have to work as hard and will be more effective.

heat transfers from hot to cold

What's a Good R-Value?

A “good” R-value is subjective, but it can be helpful to know how it’s calculated. You’ll most commonly see it described as R-value per inch. This means that for every inch of material thickness, you can expect XX amount of R-Value.

R-Value = R-Value per Inch X Material Thickness

That’s why you see terms like R-13 for fiberglass insulation in your home. The R-value per inch is usually between 2.2 and 3.8, but the material is ~3.5 inches thick because it’s designed to fit into a 2-by-4 wall cavity.

R-13 = 3.71 R-Value per Inch X 3.5 Inches

Inversely, a common 4” clay brick has an R-value of only 0.44 per inch. Heavy weight is rarely an indicator of good insulation. It can even be counterproductive if you’re compressing your other insulating materials.

Materials to Use for Automotive Heat Insulation

There are three materials that most often get talked about for heat insulation in a car.

Foam

Both open cell and closed cell foams are good insulators. The advantage of using open cell foam is it’s lighter and more flexible than closed cell foam. It’s less dense and can be manipulated into tight spaces easier, though compression reduces its R-value. The advantage of using closed cell foam is that it’s more durable and usually has a higher R-value per inch. Closed cell foam is also more water resistant (although our open cell foam is hydrophobic and the exception to that rule).

JUTE

Jute is the most common OEM insulator. It’s lightweight, absorbs sound, and has a high R-value. Our jute is made of recycled cotton fibers that are treated for mold and mildew resistance. It all-natural (so no itching), class A fire rated, and is sandwiched between two aluminum foil barriers to make it a good radiant heat shield too.

FOIL

WATCH OUT! It's honestly misleading that foil products are even called “Foil insulation”. Foil should only be used as a radiant heat barrier. Even then, the key feature you're looking for is how reflective the foil is. Higher quality products tend to be backed by another material like organic fibers, jute, or foam. Foil on its own will not insulate. We shed a single Smokey the Bear tear whenever we see someone lining the entire inside of their van conversion with foil-faced bubble wrap. It seems like a cheap solution but it’s basically useless as an insulator (although it will block out the sun’s radiant heat through the windows!).

 

Now that you know which insulation materials to look for and how they’re best used, take a look at some of our favorite automotive heat insulators.

Product Material R-Value/In. Thickness Primary Use Secondary Use

Closed cell foam

4.33

3/8"

Decoupling

Heat Insulation

Open cell, hydrophobic melamine foam

4.16

1/2"

Sound Absorbing

Heat Insulation

Mass loaded vinyl + closed cell foam

4.45

3/8"

Heat Insulation

Recycled cotton fibers + reinforced foil

3.91

3/8"

Heat Insulation

Heat Shield

Insulate Your Vehicle Today!

If you’re tired of excessive heat in your vehicle, there is a solution. Look for a high-quality closed cell foam or open cell foam and only use foil as a radiant heat barrier. If you can remember that, you can make your car more comfortable in no time at all. Get started with Second Skin Audio heat insulation products to get started today!