Explaination of Thermal Conductivity & R 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 homogenous 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).

Example of a concrete insulating block with roughly these specs. ASHRAE uses these standard-sized materials to measure the heat flux.

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.

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 XPS, ROCKWOOL or FIBERGLASS.

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:

Finally, R-values are additive.  For instance if you have a material with an R-value of 12 attached to another material with an R-value of 3, then both materials combined have an R-value of 15.

Well, please feel free to contact us if you want to get more information about this topic. We also have a automatic formulation that can help you calculate what your material R value exactly is.