What is R-Value?

This is the first in a series of blogs I’ve written for Rockwool and the R-Class Builder Program.  If you are not already a member, you can join at this link, Rockwool R-Class.  The R-Class program is free.

What is R-value?  I write often about different insulations, how they perform, where they should and shouldn’t be used.  I think this blog post should go back to the basics and talk about what is R-value, how it is calculated, and how much is needed.

This concrete foundation is not insulated, concrete has a much lower resistance to heat flow (R-Value) than the insulated wall. This becomes evident when using thermal imaging. Heat is leaving this structure through the concrete. Insulation will slow this heat loss.

R-value is how we quantify thermal resistance.  Insulation is designed to slow the movement of heat through a building.  The physics of heat movement tells us heat moves from something or someplace warm to something or someplace cold.  A good example of this is standing in front of a large picture window on a cold day.  Being from a very cold climate, this is something I’ve experienced many times.  Standing in front of that window can give you a chill, what is happening is the heat of your body is leaving and moving to the cold surface of the window.  This loss of heat is what makes you feel cold.  If you wear a heavy jacket, hat and mittens while standing in front of that window, you no longer feel cold.  You’ve slowed the heat moving from your body to the cold glass.  Notice how I said, slowed the heat movement.  You didn’t stop it completely, but you’ve slowed it enough so that you no longer feel cold.  The additional clothing has some quantity of R-value, it resists or slows the movement of heat.

Insulation in our homes has the same purpose.  In a hot climate, the heat outside wants to move toward the cooler, indoor portion of the home (if the house is air conditioned and cooler than the outside temperatures.)  In a cold climate, during the heating season, heat wants to leave the inside and move towards the colder outside.  We slow this process with insulation.  More is better, sometimes what is even more important is the location of the insulation.  (Joseph Lstiburek from Building Science Corporation says it best, we should be wearing that jacket, hat, and mittens instead of eating it.  A topic for another blog post.) 

R-values are denoted by a numeric value, the higher the number, the slower heat movement through the insulation.  As an example, let’s look at ROCKWOOL Comfortbatt® cavity insulation.  This product, which is designed as cavity insulation, has an R-value of approximately 4 per inch.  How is that value measured?  There is a quantifying test procedure developed by the American Society for Testing and Materials (ASTM) called the ASTM-c518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus.  (If you find yourself having difficulty falling asleep, reading any of the ASTM test standards should do the trick.)  The test is conducted using a device called a heat flow meter.  A sample of insulation is placed between two plates, one plate is cold and the other is hot.  The equipment measures the heat flow from the hot plate, through the insulation, to the cold plate.  The measured resistance is the insulation’s R-value.  An insulation’s R-value might be listed per inch, but insulation comes in various thicknesses and the full R-value of the insulation is typically listed on the product.  For example, Rockwool’s Comfortbatt® insulation designed for a 2 x 6 wall cavity is listed at R-23.

So, how much is enough?  Well, that depends on several factors.  Where you live is the biggest variable.  (Building codes require minimum insulation values depending on your area.)  What are your expectations for comfort and operational costs (the cost to heat and/or cool the home)?  Insulation can improve durability, helping to make the home last for centuries instead of decades.  There are also insulation requirements for achieving certification for some programs such as energy star, zero-energy ready and passive house.  Sometimes using more insulation than required by code will be necessary to meet a homeowner’s expectations.  All these are topics outside the scope of this blog post but will be discussed in future postings.  The answer of how much is one that you will hear often from me, it depends.

Cavity insulation being installed in a conditioned storage building. This is 7.25″ of Comfortbatt® insulation with an R-Value of 30.

All insulations need some sort of quantitative value, so we know how much to use for specific circumstances.  We will be covering much more information on those topics in future blog posts, until then, I’ll leave you with some words from my good friend Dan, Rock on!  Get it, ROCKWOOL…Rock on.  I know, you’re smiling.

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