Calculating the R-value: How to calculate the R-value? (formula + units) (2023)

value R oThermal-Rexistenceis the basic metric for insulation materials in construction. This R-value is calculated based on multiple measurements covering the area of ​​a two-dimensional barrier(wall, insulating blanket, polyiso, etc.), the temperature difference between inside and outside and time.

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

  1. How R-Value is Calculatedabout measurement.R-value calculation includes ASHRAE measurementsHeat flow over different insulating materials.and determine the R-value of the insulation based on how quickly (or slowly) heat flows through the material. This is how we arrive at R-values ​​for different materials;Here is a detailed example of R-values ​​for 51 materialsused for insulation in construction.
  2. How to find the R-value for walls, attics, etc.More often we want to know how to determine the R-value of the insulation in our house.Example:How to calculate the R value of our attic? What is the R value of the walls? To calculate these R values ​​we need to specify the total insulationmaterials used, their thickness and specific R-value per inch, and the total for each individual layer of insulationto get a general R-value of a wall, attic, roof, etc.
    Calculating the R-value: How to calculate the R-value? (formula + units) (1)

Calculating R values ​​can be quite complex. Therefore, we will simplify it. We'll follow a structured step-by-step approach to illustrate how the R-value is calculated. Here's a quick overview:

  1. R-value formula.We'll start by explaining what the R-value actually is by using a formula to calculate the R-value. This includes usingThermal conductivity (k value of insulation); The R-value is basically an inverse of the k-value (don't worry, we'll explain all that for you). In addition, we will see how to use theHeat Flux R-Value Formula(Used less often in practice.)
  2. R value units.Let's see what R-10, R-20, R-49, etc. really mean. in terms of the units used to express the R-value.
  3. How to calculate the R-value of insulation.We explain how to find the R value of walls, attics, basements, etc. All the R values ​​of the individual layers that make up these components are summed.

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

R-value formula (with k-value and heat flux)

Here is the basic R-value equation:

valor R = 1 / valor k

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

The k value is a measure of thermal conductivity.; that is to say. the ability to transfer heat from one end of the insulating material to the other. Rather, the R-value is a measure of thermal resistance; that is to say. How can an insulating material prevent heat transfer from one end of the insulating material to the other?

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

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  • Through1 square metermade of homogeneous material (surface area).
  • this thing is1 inch thick(thickness component).
  • In1 hour(time component).
  • For1 degree temperature differencebetween indoor and outdoor temperature (temperature component).
Calculating the R-value: How to calculate the R-value? (formula + units) (2)

Let's say we have a board that is 1 square foot and 1 inch thick. The temperature inside is 73°F and the temperature outside is 72°F (we have a difference of 1 degree). We measure how much heat is transferred from the inside to the outside in 1 hour. If we measure a heat transfer of 1 BTU/hour, that means that the thermal conductivity or k-value is equal to 1. This also means that the R-value is equal to R-1 since R-value = 1 / k-value; If the value k is 1, we have 1/1 = 1.

Alternatively, this RSI (SI unit with metric units) value equation 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 inside and outside. θ is a measure of heat flux expressed in W/m2; That is, heat is measured in watts instead of BTUs, and surface area is measured in m instead of square feet.2.

Before we examine how to find the R-value for walls, attics, windows, roofs, etc., let's see how the R-value equation can be used in practice:

Examples of calculating R values

Let's say we have a wall. The outside temperature is 72°F and the inside temperature is 73°F. We take a 1-square-foot, 1-inch-thick section of this wall and measure how much heat is transferred from the inside to the outside in 1 hour. Realistically, we'll measure around 10 BTUs of heat transferred.

This means that the k value or thermal conductivity is 10 (since 10 BTUs of heat were transferred when we used a 1 square foot, 1 inch thick wall, 1 degree difference and 1 hour). So 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

This 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 R-value of the insulation would be R-1.

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Use:ASHRAE performs extensive measurements of heat loss through various building materials and calculates the R-value for us. They typically present these metrics in large r-value charts.

R-0.1 is a very low insulation R-value. To increase the overall R-value of the walls, we use wall insulation. Examples of wall insulation are insulation mats and solid wall insulation such as EPS, XPS or ISO. CanCheck here some R-values ​​of rigid insulating materials.

Now let's say that weAdd an insulating material such as polyiso. Calculating the R-Value for Polyiso follows exactly the same process as calculating the R-Value for a brick wall.

Here's how we can calculate the R-value of polyiso: Let's say we have 1 square foot and 1 inch of polyiso. The temperature difference is 1 degree and we measure the heat loss for 1 hour. Well, ASHRAE does these measurements and they tell us that one of those polyiso plates lost 0.25 BTUs in 1 hour.How to determine the R-value of polyiso?

Simply. We now know that the k value of this polyiso sheet is 0.25. We use the R-value formula to calculate the R-value as follows:

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 an insulating capacity 40 times greater than simple brick walls; This is exactly the reason why we use it as an insulation material.

R-value units (feet2×°F×h/BTU)

In many cases, it is very helpful to understand what R-10 or R-19, etc., actually means in units.

The units for measuring the values ​​of R arepie2×°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 BTUs (British Thermal Units).

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R-10 only tells us that we have 10 feet2×°F×h/BTU Insulation. Those R-value units look complex, don't they? This is exactly what the R-value was invented for.It is much easier to write that it has R-10 insulation than it is to write that it has 10 feet2×°F×h/BTU Insulation.

Now that we understand how R-value is measured, we can examine how the R-value of attics, roofs, walls, etc. is determined:

How do I find the R value? (sum of all R values)

Suppose 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 2.5-inch thick polyiso (this polyiso has an R-value of R-4 per inch). To find the R-value of the insulation in such a wall, we must add up all the R-values ​​of the materials that make up the wall. In this case we have 2 materials that make up the wall:

  1. 12-inch brick.Bricks have an R-value of R-0.1 per inch. Therefore, for a 12-inch brick wall, the contributing R value is R-1.2.
  2. 2.5 inch polyiso.We know that polyiso has an R-value of R-4 per inch. This means that the contributing R-value for 2.5" ISO is R-10 (since 2.5" × R-4 per inch = R-10).

To calculate the overall R-value for such a wall:

Total R-Value (Brick + ISO Wall)= R-1,2 + R-10 =R-11.2

As you can see, we add up all the R values ​​for each layer. A general equation to calculate the R-value of these multilevel structures is the following:

Valor R total = valor R (material 1) + valor R (material 2) + valor R (material 3) + … + valor R (material n)

Using this equation, we can calculate the R-value of any building object. As an example, let's see how to find the R-value of the attic:

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How to Find the R-Value of the Attic

Penthouses typically have the highest R-values, ranging from R-30 to R-49. If you have an attic and you want to calculate the R-value, you need to add up all the individual layers that make up the attic. Here is an example of attic insulation and building materials:

  1. tejas, asphalt, 1 inch thick.Valor R = 0,45.
  2. Spray with closed cell foam, 2 inches thick.Valor R = 10,6.
  3. fiberglass mats, 2.5 inches thick.Valor R = 11,2.
  4. plasterboard, 1 inch thick.valor R = 1,1.
  5. plywood, 3 inches thick.Valor R = 2,9.
  6. wooden beams, 8 inches thick.Valor R = 9,8.
Calculating the R-value: How to calculate the R-value? (formula + units) (3)

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

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 main problem that we usually face when calculating the R values ​​in our houses is getting them to be calculated correctly.Collect all R values ​​of each layerthat make up the loft, the wall, the ceilings, you name it. There are no shortcuts or hints here. You just have to check what layers you have and what are their R values.

Hopefully, you now understand how R-values ​​are calculated, what units are used to express R-values, and can calculate R-values ​​yourself.

table of Contents

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Related Posts:
  1. What is the R-value of the insulation? (+ Insulation R-value table)
  2. Insulation R-Value Table: R-Values ​​Per Inch (For 51 Materials)
  3. Explanation of the 9 types of insulation (materials, R-values, applications)
  4. Rigid insulation R value per inch (+ EPS, XPS, ISO thickness tables)
  5. ISO R-Value Table: Polyiso Insulation Thickness (1-20 inches)


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