# How to Understand Insulation R Values

By Allan Robinson

A material's insulation R value is a measure of its effectiveness as an insulator. It's commonly used in construction as one of the criteria for selecting a building material. A specially equipped research laboratory performs a test under controlled conditions to determine a material's heat transference rate. This measurement may then be used to calculate the material's R value.

Study the R value testing procedure. The technician uses a piece of test material with a standard length, width and thickness. One end of the material is heated so that there is a specific temperature differential between the two ends of the test material.

Examine the R value measurement procedure. The technician measures the rate at which heat is flowing from one end of the test material to the other at the given temperature differential. It's essential that the initial temperature differential be known because the heat flow will decrease until the temperature differential between the two ends of the test material reaches zero.

Calculate the test material's R value. This is given by the equation R = dT x A / Q where R is the R value, dT is the temperature differential between the two ends of the sample, A is the sample's surface area and Q is the heat flow at the given temperature differential.

Convert the calculated R value to the desired units of measure. R values in the United States typically measure the temperature differential in degrees Fahrenheit, the area of the test material in square feet and the heat flow in British Thermal Units (BTUs.)

Look at some minimum R values for standard materials that are one inch thick. Brick has an R value of 0.2, hard wood has an R value of 0.71 and loose-fill fiberglass has an R value of 2.5.