Insulation: R-Value: R-value describes a material's ability to PREVENT heat loss, while U defines the amount of heat that will be LOST through a material or building component. R-Value is printed on many products used in building construction, when those products are intended for use in heat-losing areas. R-Value is a relative measure, with no units. It is not like "pounds of cement" or "gallons of oil". However, it can still be used in deciding which material to use. For example, if a one-inch thick sheet of material A has a value of R5, and a one-inch sheet of B is rated R3, then A is a far better insulator than B. There are other considerations, of course. For example, extruded Polystyrene (pink or blue), resists moisture well, while expanded Polystyrene (white, beads are obvious) breaks down in moist situations. On the question of cost, there may be a temptation to buy the less expensive material, but the extra cost of the better insulator will be repaid many times over during the life of the structure. The table below presents R values for many common building materials. Material R-Value Air, perfectly still 5.95 U-value: U-value is the more important measure for the designer, and it has units. U-value allows us to calculate how much heat will be lost by a wall or window, how big the boiler should be for this house, and what we might save by upgrading the ceiling insulation. To see how U-value is used in building design, see Figuring Heat Loss. U-values are used for assemblies of materials, or "systems", as with a double-glazed window, which has two panes of glass, an air space inside, and two air films clinging to the outside of the glass. The U-value of a “system” defines: the number of BTUs that will pass through one square foot of that system, in one hour, for every degree (F) difference between the temperatures inside and outside of the system. Heat energy will "drive itself" from the warmer area toward the colder, and U-value tells how fast that will happen. It is calculated by adding the inverse of the sum of all the R-values in the assembly. A double-glazed window would work out like this, starting with R-values of the components: Glass 2
x 1/8 x 2.44 =.61 4.33 divided into 1 (the inverse) equals .23, and the U-value of that window assembly is .23 BTUs per sq. ft. per hour per Deg. F Diff. Table of U-values: Component U-Value 4-inch wall, un-insulated .131 ** The air-space should be at least 1/2 inch wide. This U-value
differs slightly from the previous example, because the source
used a different value for air films. Net wall: (8x60) - (6x2.5x4) x U x DT = Windows: Total Heat loss of wall = 1908 BTUs/Hr Note that the double-glazed window, in the North wall, with no compensating solar gain, loses 5 times as much heat per square foot as a typical 4” insulated wall.
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