Increased emphasis on energy use in buildings has lead many to believe that highly insulated walls are necessary to energy efficiency. It is true that well designed walls save energy costs and improve occupant comfort, but energy performance concerns should not dominate the design and selection of wall systems. Other concerns, such as durability, maintainability and long term value, should be given their due in the selection process.
As wall insulation (R-Value) increases, the energy savings benefits level out. Beyond a certain point, heating energy savings are outweighed by the extra costs of the insulation. This point varies, depending on building type and location, and on insulation cost.
Walls play a significant, but not commanding, role in the overall energy performance of a building. In cold climates, walls are one of the more important components in reducing heating loads in commercial buildings, usually following the mechanical, glazing and roof systems. In heating residential buildings, the walls are even more important for saving energy. In warmer climates, the roof, windows, internal heat gains, and other components of the building contribute to the cooling energy requirements, but it's the windows that usually let in the most heat.
Recent studies to quantify the thermal mass effect have concentrated on three wall insulation strategies: interior, integral, and exterior. Interior insulation isolates the mass from the interior, reducing the ability of the thermal mass to moderate the indoor temperature. Integral insulation refers to thermal mass on both sides of the insulation, as with an insulated masonry cavity wall or insulated concrete panel, or to insulation and mass materials well mixed, as in a log wall. Exterior insulation refers to mass exposed to the interior, isolated from the exterior conditions by insulation. This last is the most thermally effective way to insulate the envelope of a thermal mass building.
Measurable Properties of Thermal Mass
The thermal mass of a wall, roof, or floor is directly related to its weight and to the specific heat of the material. The weight and specific heat are important to the energy performance of buildings and are recognized by most energy efficiency standards such as California's Title 24.