R-value or *thermal Resistance* is the basic metric for insulation materials in construction. This R-value is calculated based on several measurements that include the area of a 2-dimensional barrier

*(wall, insulation batt, polyiso, etc.)*, the temperature difference between indoor and outdoor, and time.

When homeowners want to know how to calculate R-value, they usually have one of these two things in mind:

**How the R-value is calculated**Calculating R-value involves ASHRAE making measurements of*via measurement*.**heat flux over different insulation materials**and determining the R-value of insulation based on how quickly (or slowly) this heat flows through the material. That’s how we come up with R-values for different materials; here is an extensive example of R-values for 51 materials used for insulation in construction.**How to find R-value for walls, attics, and so on.**More often, we want to know how to determine the R-value of insulation in our house.*Example:*How to calculate the R-value of our attic? What is the R-value of walls? To calculate these R-values, we have to specify all the insulation**materials used, their thickness, and specific R-value per inch, and sum all individual layers of insulation**to get an overall R-value of a wall, attic, ceiling, and so on.

Calculating R-values can be quite complex. That’s why we going to simplify it. We are going to take a structured step-by-step approach to illustrate how to calculate R-value. Here is a quick outline:

**R-value formula.**We will start by explaining what R-value actually is by using a formula for calculating the R-value. This involves using**thermal conductivity (insulation k-value)**; R-value is basically an inverse of the k-value (don’t worry, we’re going to explain all of this). On top of that, we will look at how to use the*heat flux R-value formula*(less often used in practice).**R-value units.**We are going to look at what does R-10, R-20, R-49, and so on actually mean in terms of units used to express R-value.**How to calculate the R-value of insulation.**We will explain how to find the R-value of walls, attics, basem*nts, and so on. This involves summing all the R-values of individual layers that make up these construction elements.

Let’s start with the R-value formula and how is R-value calculated:

Table of Contents

## R-Value Formula (With k-value And Heat Flux)

Here is the basic R-value equation:

**R-Value = 1 / k-Value**

As we can see, the R-value is the *inverse of the k-value* (1 divided by the k-value). To fully understand what R-value is, we have to look at the k-value.

**k-value is a measure of thermal conductivity**; ie. the ability of heat to be transferred from one end of insulation material to the other end. Inversely, R-value is a measure of thermal resistivity; ie. how can an insulation material prevent the transfer of heat from one end of insulation material to the other end.

This thermal conductivity is a measure of the amount of heat that flows (we have 4 metrics here):

- Through
**1 square foot**of hom*ogenous material (surface area component). - This material is
**1-inch thick**(thickness component). - In
**1 hour**(time component). - For
**1 degree of the temperature difference**between indoor and outdoor temperatures (temperature component).

Let’s say we have a 1 sq ft and 1-inch thick board. The indoor temperature is 73°F and the outdoor temperature is 72°F (we have a 1-degree difference). We measure how much heat is transferred from indoors to outdoors in 1 hour. If we measure a heat transfer of 1 BTU/hour, that means that thermal conductivity or k-value is equal to 1. That also means that R-value is equal to R-1 since R-value = 1 / k-value; if k-value is 1, we have 1 / 1 = 1.

Alternatively, this RSI-value equation (SI unit using metric units) can be written using heat flux and temperature difference (instead of using the k-value to determine the R-value):

**RSI-Value = ΔT / θ**

Here the ΔT represents the temperature difference between indoors and outdoors. θ is a measure of heat flux, expressed in W/m^{2}; that is heat is measured not in BTUs but in watts and the surface area is not measured in sq ft but in m^{2}.

Before we check how to find R-value for walls, attics, windows, ceilings, and so on, let’s look at how the R-value equation can be used in practice:

### Examples Of How To Calculate R-Values

Let’s say we have a brick wall. The outdoor temperature is 72°F and the indoor temperature is 73°F. We take a 1 sq ft 1-inch thick section of that wall and measure how much heat is transferred from indoors to outdoors in 1 hour. Realistically speaking, we are going to measure about 10 BTU of heat being transferred.

This means that the k-value or thermal conductivity is 10 (since 10 BTU of heat was transferred when we used a 1 sq ft 1-inch thick wall, 1 degree of difference, and 1 hour). Here is how we can calculate the R-value of this wall from the known k-value:

**R-Value (Brick Wall) = **1 / k-Value = 1 / 10** = 0.1**

That means that such a brick wall has an R-value of R-0.1 (this is for a 1-inch brick wall). If we have a 10-inch thick brick wall, the insulation R-value would be R-1.

*Note:* ASHRAE makes extensive measurements of heat loss across different construction materials and calculates the R-value for us. They usually present these metrics in big R-value charts.

R-0.1 is a very low insulation R-value. To increase the overall R-value of walls, we use wall insulation. Examples of wall insulation are insulation batts and rigid wall insulation like EPS, XPS, or ISO. You can check some R-values of rigid insulation materials here.

Now, let’s say that we **add an insulation material like polyiso**. Calculating the R-value of polyiso follows exactly the same procedure as calculating the R-value of a brick wall.

Here is how we can calculate the R-value of polyiso: Let’s say we have 1 sq ft 1-inch polyiso. The temperature difference is 1 degree and we measure the heat loss for 1 hour. Well, ASHRAE makes these measurements, and they tell us such a board of polyiso lost 0.25 BTU in 1 hour. *How to determine R-value of polyiso?*

Simple. We now know that the k-value of this polyiso board is 0.25. We use the R-value formula to calculate the R-value like this:

**R-Value (polyiso) = **1 / k-Value = 1 / 0.25** = 4**

As we can see, the calculated R-value of polyiso is R-4. This means that polyiso has 40 times higher insulation capability than simple brick walls; that’s precisely why we use it as insulation material.

### R-Value Units (ft^{2}×°F×h/BTU)

In many cases, it’s quite useful to understand what R-10 or R-19 and so on actually means in terms of units.

Units for measuring R-values are **ft ^{2}×°F×h/BTU**. Essentially, the R-value of R-1 is equal to 1 square foot times 1 degree Fahrenheit times 1 hour divided by BTU (British Thermal Units).

R-10 simply tells us that we have 10 ft^{2}×°F×h/BTU insulation. These R-value units seem complex, right? That’s exactly why R-value was invented. *It is much simple to write you have R-10 insulation than to write you have 10 ft ^{2}×°F×h/BTU insulation.*

Now that we understand how R-value is measured, we can check how to determine the R-value of attics, ceilings, walls, and so on:

## How To Find R-Value? (Summation Of All R-Values)

Alright, let’s say that you want to calculate the R-value of a wall. We will use some data from the previous two examples.

*Example:* We have a 12-inch brick wall insulated with a 2.5-inch thick polyiso (this polyiso has an R-value of R-4 per inch). To find the insulation R-value of such a wall, we have to summarize all the R-values of materials that constitute the wall. In this case, we have 2 materials the wall is made out of:

**12 inches of bricks.**Bricks have an R-value of R-0.1 per inch. In a 12-inch brick wall, the contributing R-value is thus R-1.2.**2.5 inches of polyiso.**We know that polyiso has an R-value of R-4 per inch. That means that the contributing R-value for 2.5-inch ISO is R-10 (since 2.5 inches × R-4 per inch = R-10).

Here is how to calculate the overall R-value of such a wall:

**Overall R-value (Brick + ISO Wall)** = R-1.2 + R-10 = **R-11.2**

As you can see, we are adding up all the R-values of individual layers. A general equation for calculating the R-value of these multilayer construction objects looks like this:

**Overall R-Value = R-Value (Material 1) + R-Value (Material 2) + R-Value (Material 3) + … + R-Value (Material n)**

With this equation, we can calculate the R-value of any construction object. Let’s look at how to find the R-value of the attic as an example:

### How To Find R-Value Of Attic

Attics usually have the highest R-values, ranging from R-30 up to R-49. If you have an attic and want to calculate the R-value, you have to summarize all the individual layers that the attic is made out of. Here is an example of attic insulation and construction materials:

**Roof shingles**, asphalt, 1 inch thick.*R-value = 0.45.***Closed-cell foam spray**, 2 inches thick.*R-value = 10.6.***Fiberglass batts**, 2.5 inches thick.*R-value = 11.2.***Gypsum board**, 1 inch thick.*R-value = 1.1.***Plywood**, 3 inches thick.*R-value = 2.9.***Wood beams**, 8 inches thick.*R-value = 9.8.*

When we have gathered all the materials that make our attic and roof, we can calculate the R-value of the attic like this:

**R-Value (Attic)** = R-0.45 (Material 1) + R-10.6 (Material 2) + R-11.2 (Material 3) + R-1.1 (Material 4) + R-2.9 (Material 5) + R-9.8 (Material 6) = **R-36.05**

This is just one example of how you can find an R-value.

The key problem we usually have calculating R-values in our homes is to correctly **gather up all the R-values of individual layers** that make up the attic, wall, ceilings, you name it. There are no shortcuts or hints here. You just have to check what layers you have and what R-values they have.

Hopefully, now you know how to calculate R-values, you know which units are used to express R-values, and will be able to calculate R-values yourself.

##### Related posts:

- What Is Insulation R-Value? (+ Insulation R-Value Chart)
- Insulation R-Value Chart: R-Values Per Inch (For 51 Materials)
- All 9 Types Of Insulation Explained (Materials, R-Values, Applications)
- Rigid Insulation R-Value Per Inch (+ EPS, XPS, ISO Thickness Charts)
- ISO R-Value Chart: Thickness Of Polyiso Insulation (1-20 Inches)