Thermal Capacity of Walls

Thermal Capacity

• The thermal capacity of walls depends on the materials used to build the wall so that it can conduct or resist heat. For example, when the weather is cold, it is desirable to have a home with a high thermal capacity to block cold air from penetrating through the exterior walls and seeping through to the interior walls. This capacity is referred to as thermal conductivity. And when the weather is warm, it is a desirable that the home has a thermal capacity to keep warm air from entering the home, and is referred to as thermal resistance. The materials used to develop the exterior and interior walls of the home, and the insulation materials used between the cavities of the exterior and interior walls form what is called the “envelope” of the home. The combination of the materials used determines the thermal capacity of the home when the weather is cold and when the weather is hot.

The R-Value

• The R-value is the standard term used to measure a home’s thermal capacity. The main components of a home, from the exterior sheathing to the studs and the insulation materials have an R-value. The R-value measures the capacity of the material to insulate the home. Materials are rated on a scale from 0.01 and higher. The higher the R-value of the material, the higher the insulation value of the material.

Exterior Walls

• The R-value of the sheathing material, the cladding material, wall studs, insulation and the house wrap film determine the thermal capacity of the exterior walls. According to R-values developed by the Colorado Energy Commission, a typical exterior wall assembly might have ½-inch plywood sheathing with an R-value of 0.63, 3 ½-inch wood studs with an R-value of 4.38, 3 ½-inch fiberglass insulation with an R-value of 13.0, and house wrap film with an R-value of 0.17. The combined R-value of these equals 18.01. This total can be used to compare the difference in the thermal capacity of a brick home versus one clad with vinyl siding. The R-value of brick is 0.44, and the R-value of vinyl siding is 0.61. Therefore, the thermal capacity and R-value of the brick home would be 18.45 and the vinyl-clad home would have a thermal capacity and R-value of 18.62.

Interior Walls

• The R-value of the studs, the drywall sheathing, windows, and the insulation used in the wall cavity area is used to calculate the thermal capacity of interior walls. A typical room might have gypsum board drywall with an R-value of 0.56, 3 ½-inch wood studs with an R-value of 4.38, and double-pane windows with a ½-inch gap have an R-value of 2.04, and 3 ½-inch fiberglass insulation with an R-value of 13.0, giving the room a total R-value of 19.98.

  Insulation plays a major role in improving the thermal capacity of a home by providing what is called “thermal bridging.” The insulation forms a barrier which reduces the ability of cold air and warm air from penetrating from the outdoors to the indoors. As a result, the insulation plays a tremendous role in maintaining thermal efficiency and keeping the R-value of the home consistent during all types of weather.