Steady-state heat flow assumes that temperatures on both sides of a building envelope element (while different) are held constant for a sufficient period of time so that heat flow on both sides of the assembly is steady. The steady-state heat flow method is a simplification, because in the real world, temperatures change constantly. It can, however, predict average heat flow rates over time, and is used by Standard 90.1 to limit conductive heat losses and gains. For example, heating equipment is commonly sized using the steady state heat flow out of a building during peak heating conditions. Many energy codes specify minimum building insulation levels in terms of steady state heat flow. Because they are easy to understand and use, the terms for steady-state heat flow are part of the basic vocabulary of building energy performance.
Framing Effects Most construction assemblies include more than one material in the same layer. For example, a wood stud wall includes cavity areas where the insulation is located and some areas where there are solid wood framing members. The wood areas have a lower R-Value, and conduct heat more readily than the insulated areas. It is incorrect to neglect framing members when calculating the U-Factor for the wall, roof, or floor assembly. The correct U-Factor includes the insulation portion of the wall and the U-Factors through the solid portion of the wall. Energy efficiency standards require that the U-Factor of each envelope assembly be calculated taking into account framing and other thermal bridges within the construction assembly. Masonry walls have some similarity to frame walls. Many masonry walls are partially hollow, with webs connecting the inside and outside faceshells. Furthermore, some of the cores may be filled with grout. Over the face of the wall, there are significant areas that are solid and hollow, and each area has a different thermal transmittance. |
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U-FactorThe U-Factor is the rate of steady-state heat flow. It is the amount of heat in Btu (British thermal units) that flows each hour through one square foot, when there is a one degree temperature difference between the inside air and outside air. The heat flow can be in either direction, as heat will flow from the warmer side to the cooler side. Each layer of a building assembly, such as the sheathing and the insulation, has its own conductance, or rate of heat transfer. The conductance for an individual layer is like the U-Factor, and it has the same units. The difference is that it is only for a single element or layer. The U-Factor includes the conductance of every element of the building assembly, including the air films on the interior and exterior surfaces of the construction assembly. The surface conductance's quantify the rate at which heat is transferred between the surface of the construction assembly and the surrounding environment. The air films depend on wind speed and the roughness of the surface and can contribute significantly to the overall insulating qualities of a wall or roof. For light frame walls, the steady-state U-Factors provide an adequate description of heat transfer. For heavy concrete and masonry walls, however, this is only true under constant or average temperature conditions. The dynamic heat storage properties of the concrete and masonry alter the thermal behavior of the wall, and the U-Factor becomes less accurate as a predictor of heat flow rates. |
R-Value R-Values are also used to describe steady-state heat flow, but in a slightly different way. The R-Value is the thermal resistance to heat flow. A larger R-Value has greater thermal resistance, or more insulating ability, than a smaller R- value. The big advantage of R-Values is that they can be added together. The total R-Value of a construction assembly is the sum of the R-Values of each of the layers. The layers should include the sheathing and finishes, the insulation and weatherproofing elements, and the surface air films. The U-Factor is the inverse of the total R-Value.
The R-Value is widely recognized in the building industry and is used to describe insulation effectiveness. The insulation R-Value is not the total R-Value of the wall, however. It only describes the thermal resistance of the insulation material. The R-Value of the entire wall assembly can be significantly lower when metal framing penetrates the insulation. See the discussion of calculation methods below for more details. |
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