How to Calculate R-Values

Choosing the right insulation is one of the most important decisions for your project. But with so many products on the market, how do you compare them fairly?

R-Values are the key to doing this, since they provide you a way to measure thermal performance far more directly than what would otherwise be possible.

With that in mind, let’s take a look at how you can calculate the R-Value of any given insulation product.

Using Thermal Conductivity and Thickness to Calculate R-Values

To calculate the Thermal Resistance (R-Values) you can use the following formula…

𝑅=𝑑𝜆

Where:

• 𝑅 is the Thermal Resistance of the material in m²K/W; 
• 𝑑 is the thickness of the material in metres (m); and, 
• 𝜆 is the Thermal Conductivity of the material in W/mK.

Applying the Formula: Collecting Data

To make use of this formula, we’ll need to collect the right information about the insulation materials we’re calculating the R-Values for.

For example, if we wanted to compare the R-Values of Glass Wool and SuperFOIL SF40, we simply need to make sure we’ve got the right information to use our formula.

Let’s break the process down…

Thickness:

SuperFOIL SF40 has a set thickness: 65mm. And that means, for any comparison against it, we’ll need to use that same thickness for our calculation.

However, given that our formula uses metres (m) for thickness, which means we need to convert our 65mm into that unit too.

Fortunately, that’s a simple thing to do, just divide 65 by 1000, giving us a thickness of 0.065m.

Thermal Conductivity:

With our thickness in hand, we can turn our attention to the other half of our formula. 𝑑, our Thermal Conductivity. Fortunately, this shouldn’t be too difficult to find either.

After all, Thermal Conductivity is how most traditional insulation options will measure their performance, meaning it won’t be hard to find a number to slot in here.

For example, this paper provides us with a typical thermal conductivity value of 0.0343 W/mK for Glass Wool, which we can plug straight into our calculation, giving us:

𝑅 = 0.065 / 0.0343

We could leave it like that, of course, but R-Values tend to be given in terms of numbers and decimals, rather than fractions.

So, why don’t we go ahead and simplify that fraction down to 3 decimal places, which gives us an R-Value of 1.895 m²K/W for Glass Wool at the same thickness as SuperFOIL SF40.

Making Comparisons

Since we know the R-Value of SuperFOIL, this now enables us to make direct comparisons.

For example, when installed in a wall, SuperFOIL SF40 has an R-Value of 3.90 m²K/W; which is over 2 m²K/W higher than the R-Value provided by the same thickness of Glass Wool.