How Are Conductivity, R-Value and U-Value Read?

• One material: thermal conductivity
• A layer of material: R-value
• A window: U-value

What you will learn

• Say what each figure describes and what it applies to.
• Read the good direction of each number correctly.
• Link the R-value to thickness and the U-value to the R-value.

Before you start

Know that heat moves from warmer places to cooler ones and that insulation slows this down.

What does thermal conductivity tell you?

Thermal conductivity measures how easily a material conducts heat. Boston University's notes on heat transfer call the thermal conductivity k a constant that depends only on the material. That is what lets us compare materials.

The notes give two contrasting examples. Copper, a good thermal conductor, has a thermal conductivity of 390 J/(s·m·°C), which is why some pots and pans have copper bases. Styrofoam, a good insulator, has only 0.01 J/(s·m·°C). For an insulating material, low conductivity is the good direction.

• Good conductor (like copper): high conductivity
• Good insulator (like foam): low conductivity

What does the R-value tell you?

The R-value tells how well a whole layer of material resists heat flow. University of Maryland Extension writes that insulation effectiveness is measured by its R-value, and the higher the R-value, the better the insulation. Illinois Extension adds that the R-value measures resistance to conductive heat flow.

Unlike conductivity, the R-value depends on thickness too. According to Illinois Extension it depends on the type of insulation, its thickness and its density. Two types can have the same R-value but differ in thickness because of how they are made and their density.

• Thin layer: lower R-value
• Thick layer: higher R-value

Materials are often rated by R-value per inch of thickness. Maryland Extension gives fiberglass batts about 3.2 per inch and loose-fill cellulose about 3.7 per inch. To reach R-49 you would need about 15 inches of fiberglass batts.

What does the U-value tell you?

The U-value tells how much heat a window or assembly conducts. A University of Alaska Fairbanks guide to choosing windows says the U-factor is the reciprocal of the R-value and measures how well a window conducts heat. The lower the U-factor on the sticker, the higher the R-value and the more efficient the window at keeping a home warm.

The NFRC consumer guide glossary adds that the U-factor may be given for the glass alone or for the entire window, including the effect of the frame and spacer materials.

Apply the reciprocal relationship to the same object: the U-value of a whole window goes with the R-value of that whole window, rather than a single pane or insulation layer. Also check the unit system before taking a reciprocal; the R-values per inch above use the US convention in the cited guides.

• Window letting a lot of heat through: high U-value
• Window letting little heat through: low U-value

How do the three figures read together?

Each describes a different aspect, and the good direction differs.

Real-world example

Compare two insulating layers for a roof. One is fiberglass batts at about 3.2 R-value per inch, the other loose-fill cellulose at about 3.7 per inch. At the same thickness cellulose has the higher R-value. To reach R-49, the Maryland guide says you would need about 15 inches of fiberglass batts. The R-49 figure is the total for the whole layer, not per inch.

Common misunderstandings

A bigger number is always better. True only for the R-value. For conductivity and U-value, lower means better insulation.

The U-value and R-value are unrelated. The U-factor is the reciprocal of the R-value.

Two materials with the same R-value are equally thick. They can differ in thickness because of how they are made and their density.

Frequently asked questions

What does low thermal conductivity mean?

The material conducts heat poorly, so it is a good insulator. Styrofoam has a far lower conductivity than copper.

What does a high R-value mean?

The layer resists heat flow better, so it insulates better.

How does the U-value differ from the R-value?

The U-value is the reciprocal of the R-value: the lower the U-factor, the higher the R-value.

Does the R-value depend on thickness?

Yes. It depends on the type of material, its thickness and its density.

Summary

• Conductivity describes a material, the R-value a layer, the U-value a window or assembly.
• Lower conductivity and U-value are better; a higher R-value is better.
• The U-value is the reciprocal of the R-value, and the R-value depends on thickness.