Calculating R-Value: How To Calculate R-Value? (Formula + Units) (2024)

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:

  1. How the R-value is calculated via measurement. Calculating R-value involves ASHRAE making measurements of 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.
  2. 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-Value: How To Calculate R-Value? (Formula + Units) (1)

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:

  1. 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).
  2. 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.
  3. 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).
Calculating R-Value: How To Calculate R-Value? (Formula + Units) (2)

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/m2; that is heat is measured not in BTUs but in watts and the surface area is not measured in sq ft but in m2.

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 (ft2×°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 ft2×°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 ft2×°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 ft2×°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:

  1. 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. 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:

  1. Roof shingles, asphalt, 1 inch thick. R-value = 0.45.
  2. Closed-cell foam spray, 2 inches thick. R-value = 10.6.
  3. Fiberglass batts, 2.5 inches thick. R-value = 11.2.
  4. Gypsum board, 1 inch thick. R-value = 1.1.
  5. Plywood, 3 inches thick. R-value = 2.9.
  6. Wood beams, 8 inches thick. R-value = 9.8.
Calculating R-Value: How To Calculate R-Value? (Formula + Units) (3)

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:
  1. What Is Insulation R-Value? (+ Insulation R-Value Chart)
  2. Insulation R-Value Chart: R-Values Per Inch (For 51 Materials)
  3. All 9 Types Of Insulation Explained (Materials, R-Values, Applications)
  4. Rigid Insulation R-Value Per Inch (+ EPS, XPS, ISO Thickness Charts)
  5. ISO R-Value Chart: Thickness Of Polyiso Insulation (1-20 Inches)
Calculating R-Value: How To Calculate R-Value? (Formula + Units) (2024)

FAQs

Calculating R-Value: How To Calculate R-Value? (Formula + Units)? ›

How Do You Determine R-Value? In order to calculate the R-value of a particular piece of insulation, you divide the thickness of the material by its thermal conductivity. Thermal conductivity measures the rate at which temperatures transmit through an insulating material.

What units are in the R-value formula? ›

The R-Value is an imperial system unit of measurement (ft2·°F·h/BTU) and the RSI Value is a metric system unit of measurement (m2·K/W). The higher the resistance value, the slower the rate of heat transfer through the insulating material.

What is the R-value units? ›

A product's R-value measures its thermal resistance in units of m²K/W. By dividing a material's thickness (in metres) by its lambda value, you can discover how well it resists heat transfer at a specific thickness.

How to calculate total R-value? ›

Total R-Value calculation

In general, the R-Value of a material is determined by dividing the thickness of the material (in metres) by the thermal conductivity (in W/m.K). The Total R-Value of the construction is the sum of the thermal resistance of each layer, surface air film resistance, and any bridged layers.

How do I find my R-value? ›

Use the formula (zy)i = (yi – ȳ) / s y and calculate a standardized value for each yi. Add the products from the last step together. Divide the sum from the previous step by n – 1, where n is the total number of points in our set of paired data. The result of all of this is the correlation coefficient r.

How do you find the unit of R? ›

We can calculate the value of R using the ideal gas equation, PV=nRT. The units and the values of the gas constant 'R' depends on the units of pressure, volume and temperature. To calculate the value of R in SI units, the values of P, V, T also have to be taken in SI units.

How do I decide whether to use 0.0821 or 8.314 for R? ›

The R's are actually the same just calculated with differnt units, typically you use R=0.0821 for PV=nRT but if you want J then you would use R=8.314. It's all about units.

What is the unit conversion for R-value? ›

The Americans use BTUs per hour instead of Watts (a BTU is a British Thermal Unit!), square feet instead of square metres, and degrees Farenheit instead of Centigrade or Kelvin. So their unit for R-value is hr-ft2-°F/Btu. So, to convert from the SI R-value to the American R-value, multiply by 5.678.

What does R unit mean? ›

The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol R or R. It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per amount of substance, rather than energy per temperature increment per ...

What is the set of units of R? ›

The set of units of R forms a group R× under multiplication, called the group of units or unit group of R. Other notations for the unit group are R, U(R), and E(R) (from the German term Einheit).

How do you calculate R formula? ›

r = ∑ ( x i − x ¯ ) ( y i − y ¯ ) ∑ ( x i − x ¯ ) 2 ∑ ( y i − y ¯ ) 2 . Label your variables x and y as it is easier to work with letters compared to names of variables. In this example denote 'test score (out of 10)' by x and 'hours playing video games per week' by y .

How do you calculate R total? ›

"2. The total resistance of a series circuit is equal to the sum of individual resistances."
  1. RT = R1 + R2 + R3.
  2. 2 + 2 + 3 = 7 Ohms.
  3. R total is 7 Ohms.

How do you calculate the value of R? ›

R = 8.31 x 10⁷ergs mol⁻¹K⁻¹. Therefore, the value of R in the CGS system. 2: Calculate the Value of R in L(atm) mol⁻¹K⁻¹. ⇒ R = 0.0821 L(atm) mol⁻¹K⁻¹ is the required answer.

What is the calculated value of R? ›

The value of R at atm that is at standard atmospheric pressure is R = 8.3144598 J. mol-1. K-1.

What is an example of the R-value calculation? ›

To calculate R-value, divide 1 by the U-value figure. For example, a U-value of 0.10 equals an R-value of 10 (1 divided by 0.10). To calculate U-value, divide 1 by the R-value—a 3.45 R-value equals a U-value of 0.29.

How to find R-value on calculator? ›

You need to turn your diagnostic on Press: 2nd, 0 to open catalog Press: x-1 to jump to the “D” section and scroll to “DiagnosticOn” Press: Enter twice and “Done” will appear Start at Step 3 again, and “r” will appear this time.

What unit is R measured in? ›

The gas constant R is 8.314 J / mol·K. Convert the numerical value of R so that its units are cal / (mol·K). A unit conversion table will tell you that 1 cal = 4.184 J. Make sure you know where to find it.

What are the units for R in statistics? ›

The correlation coefficient r is a unit-free value between -1 and 1.

What is the R-value in PV NRT units? ›

P = Pressure (atm) V = Volume (L) n = moles R = gas constant = 0.0821 atm•L/mol•K T = Temperature (Kelvin) The correct units are essential. Be sure to convert whatever units you start with into the appropriate units when using the ideal gas law.

Top Articles
R. Kelly is sentenced to 30 years in prison
R. Kelly trial: A look at the key moments after star found guilty
State Laws Held Unconstitutional Declaring A Law Illegal By The State
Body Rub Fort Wayne
First Look at the P-8A Poseidon’s New Multi-Mission Pod
8x20 one trip shipping container (NEW CONTAINERS) - general for sale - by dealer - craigslist
Generation Church Clayton
2023 GRASSHOPPER 225 Na Prodej in Gretna, Nebraska
Version 1.20 is here! | Lunar Client
Zoe Mintz Adam Duritz
Https //Compassmobile.dollartree.com Login
Target Fireplace
Ashley Store in Manchester, CT
Directions To Rite Aid Near Me
Archive | Concur | Vanderbilt University
Pixieproject
The Times - 27 September 2024 - Educação e Diversidade
Prodigy Starter Pets Evolutions
Culver's Grandview Flavor Of The Day
Lane Kiffin to Jalen Milroe, my 8 candidates for EA Sports video game cover | Toppmeyer
Until when are you going to stay here?
2018 Suzuki DR-Z400SM - motorcycles/scooters - by dealer - vehicle automotive sale - craigslist
All You Need to Know about the Naruto Theme Park – NARUTO & BORUTO Shinobi-Zato
Thespicemustfly
Refined Corn Oil by Hardy Consultant Limited. Supplier from United Kingdom
Hudson-Rimer Funeral Chapel | Edina, Missouri
Gamevault Agent
Sam's Club Gas Prices Today
Craigslist North Bay Motorcycles
Weve Got You Surrounded Meme
Groupon Meow Wolf Denver
Craigslist Duluth Mn Free
Florence Al Craigslist
Jetblue Live Flight Tracker
Citymd Southern Blvd Photos
La définition du licenciement pour motif économique | Travail-emploi.gouv.fr | Ministère du Travail et de l’Emploi
Cables & Cable Assemblies
Dr Oz Protandim Reviews
Publix Org Passport Login Mobile
Annual San Antonio Football Game
ApTask hiring Solaris Administrator T2 in Baltimore, MD | LinkedIn
Imouto Wa Gal Kawaii - Episode 2
Jamie Michael Andrews · 12611 N Community House Rd Ste 100, Charlotte, NC 28277-3817 · Physician Assistant
Craigslist Buses For Sale
Basis Independent Brooklyn
Christie Brimberry’s Strange Life Twists - Net Worth Post
Aid Office On 59Th Ashland
How To Start An Lmtv
Meet Content Creator Sam Vicchiollo: The Tiktok Superstar You Need To Know About - The LA Note Magazine
Bernadette Peters Nipple
Gang Signs Letters Decoding The Secret Language Blood Gang Letter C
Asian Massage Nuru Jax
Latest Posts
Article information

Author: Otha Schamberger

Last Updated:

Views: 5946

Rating: 4.4 / 5 (55 voted)

Reviews: 94% of readers found this page helpful

Author information

Name: Otha Schamberger

Birthday: 1999-08-15

Address: Suite 490 606 Hammes Ferry, Carterhaven, IL 62290

Phone: +8557035444877

Job: Forward IT Agent

Hobby: Fishing, Flying, Jewelry making, Digital arts, Sand art, Parkour, tabletop games

Introduction: My name is Otha Schamberger, I am a vast, good, healthy, cheerful, energetic, gorgeous, magnificent person who loves writing and wants to share my knowledge and understanding with you.