Definition, Abbreviations, and Nomenclature

Ohio Timed: Advanced Fabricated Building Definition, Abbreviations, and Nomenclature

°F: degrees Fahrenheit
°C: degree Centigrade
Note that °C = (°F – 32) x 0.556. Thus 32°F = 0°C and 68 °F = 20°C

Building thermal envelope: the barrier between the interior and exterior of the home, it includes the walls, ceiling, roof , attic, windows, doors, and insulation. This barrier must be designed to minimize heat flow between the home interior and the outside.

Climate Zone: an area of a country with similar climate conditions. The US has 8 Climate Zones, seven of which are shown in Figure 1.8, with Zone 8 being in Alaska. Note that Florida has two Climate Zones, Zone 1 and Zone 2. Zone 1 is quite small and covers Miami-Dade and Broward counties. The remainder of Florida falls into Zone 2, along with much of the southeast Gulf coast.

**Figure 1.8 Climate Zone map of the U.S. excluding Alaska which is in Climate Zone 8.**

fenestration: the glass components of a building’s façade including windows, glass doors, and storefronts.

glazing: another name for fenestration, that is, the glass components of the building envelope.

IC-rated: recessed lighting with this rating can be safely installed in a ceiling and surrounded by insulation. Non-IC rated lighting means the recessed light cannot be installed surrounded by insulation and must have at least a 3 inch air gap around it. See Figure 1.9.

**Figure 1.9** A properly installed IC-rated recessed or ‘can’ light which can be safely covered with insulation.

radiant barrier: a foil material used to reflect solar radiation, usually installed on the interior surface of the roof. Radiant barriers can greatly reduce heat being transferred into the house, thus reducing the cost of air-conditioning.

R-value: the thermal resistance of a material in units of hours × ft² × °F/Btu. The higher the R-value, the lower the amount of heat that can flow through the material per hour. In most ofFlorida, outside of Miami-Dade and Broward counties, walls must have an R-value of 13 or more and ceilings have an R-value of 30 or higher. This is more compactly stated as walls must have a minimum R-13 and ceilings must have a minimum R-30 performance (see Figure 1.10).In terms of heat transfer, windows are the weak link in the building façade because they generally have fairly low R-values. For example, a window that meets the requirements of the Florida Energy Code will have a R-value of just 1. As a result, the trade off between having more or less glass in the home has to be assessed because more glass provides additional day lighting,reducing lighting costs. On the other hand, more glass also means more heat being transfer red into the home by both conduction and radiation, increasing the cost of air-conditioning.

**Figure 1.10** The picture above depicts how many inches of various materials are needed to produce a R-22 value. It is clear that insulating materials like cellulose fiber and fiberglass have much higher thermal resistance than wood or brick.

**Figure 1.11** This window has a SHGC of 0.39 which means that it allows just 39% of the heat portion of the solar spectrum to pass through the glass into the interior of the home. VT stands for visible transmittance as is described below.

Solar Heat Gain Coefficient (SHGC): the ratio of solar radiation passing through a window to the total solar radiation striking the window. It is a measure of how much heat energy is passing through glass. SHGC has a minimum value of 0 and a maximum value of 1. A low SHGC, closer to 0, means that most of the heat energy of solar radiation is reflected back to the exterior rather than being transmitted inside (see Figure 1.11).

U-factor: a measure of the resistance to heat transfer through an assembly. It is roughly theinverse of the R-value, meaning that the lower the U-value, the higher the resistance to heatflow. This is just the opposite of the R-value because the higher the R-value the greater theresistance to heat flow.

In general: U = 1/R where R is the sum of the thermal resistance of the components of, for example a wall, which has an outer surface and an inner surface, that together sandwich the insulation, a moisture barrier, and the wall structure together in a single unit. It also includes the effects of air movement inside and outside the surface which can contribute to heat transfer.

Heat Flow = U-Factor x Surface Area x temperature difference between the two surfaces
or
Q (heat flow) = U x A x ΔT

**Figure 1.12** Windows carry a National Fenestration Research Council (NFRC) label that indicates the U-Factor for the window assembly as a whole and also provides additional information about the window performance. In this case the U-Factor is 0.35 and the Solar Heat Coefficient is 0.32.

Visible Transmittance (VT): the fraction of the visible light spectrum that passes through the glass. Generally, windows with a lower SHGC will have a lower VT because the films in the low SHGC glass that block solar heat from passing through the window also prevent some visible light from being transmitted.

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