Walls & Ceilings Architect/October 2007

blowing against a building is an example of a gas moving over a surface. There are two types of convection, forced and natural. Natural convection occurs when the movement of liquid or gas is caused by density differences. For example, we’re all aware that warm air rises. This happens because it has a lower density than the surrounding cool air, and that’s what causes a hot air balloon to rise. Cool air does the opposite and drops. This heating and cooling creates convection loops adjacent to both the interior and exterior surfaces of a wall. Convection can also take place inside of empty cavities. One example is the movement of air in a double pane window. For example, in winter, air is heated on the inside surface of the window cavity causing the air to rise. The air adjacent to the outside surface cools and drops. What results is a convection loop inside the window cavity that transfers heat from the inside to the outside. A second type of convection is known as forced convection. Here, the movement of the liquid or gas is caused by outside forces. If winds are blowing, the air movement across the outside of the wall will be higher, increasing the rate of heat transfer. The rate of heat transfer by convection depends on the temperature difference, the velocity of the liquid or gas and what kind of liquid or gas is involved. For instance, heat transfers more quickly through water than through air. This thermographic image shows convective heat transfer due to liquid mixing. Radiation Radiation is the third type of heat transfer. Radiation heat transfer is by invisible electromagnetic waves from one object to another. Heat transfers from areas of higher temperature to areas of lower temperature. One common example of radiation heat transfer is from the sun. When you walk outside on a sunny day, you immediately feel the warmth from the sun even if the air is cold. Heat from the sun is being transferred through space by radiation. Radiation also plays a role in heat transfer in a building. If you stand in front of a window on a cold day, your body radiates heat to the cold surface of the window and the result is you feeling colder. Likewise, if you stand in front of a window with the sun streaming in, you will feel warm as a result of the incoming solar radiation. This type Metals conduct 300 to 1,000 times more heat of energy—solar radiation—is primarily short-wave radiation. Glass is nearly than most building materials. The thermal impact of transparent to this short-wave radiant a metal stud in a framed cavity is greater than the energy from the sun, and as a result, once sunlight enters a room, the sun’s actual surface area of the stud. energy is absorbed by the walls and the contents of the room and is converted to heat. At the same time, the warm objects in the room also emit radiant energy. standard is ASTM C 518. Here, a heat-ﬂow apparatus measures heat transfer through homogeneous materials, such as insulation. Several material properties, includBREAKING THERMAL BRIDGING ing thermal resistance, conductance and conductivity, Thermal bridging is the name given to the path that can be determined from temperature, heat ﬂ ux, area offers smooth travel for heat transfer in poorly insuand thickness data. Another standard, ASTM C 1363 lated buildings. To throw an obstacle in this path and Hot Box measures the thermal performance of building slow down heat transfer, we need to put some insulators envelope assemblies. Measurements include the effects between the conductors. Commercial insulation consists of thermal bridging due to structural components, as of cavity insulation, which occupies space inside the wall well as insulated cavities. cavity, and insulation sheathing, which is on the outside of the external walls. There are a variety of materials that can be used for cavity insulation, including ﬁberglass, mineral wool, cellulose, open and closed-cell foam plastics, reﬂective insulation and radiant barriers. Insulating sheathing is usually made from expanded polystyrene, extruded polystyrene, polyisocyanurate (ISO board) or ﬁberglass board. Here’s a run-down of how thermal properties of various materials and systems are rated. As mentioned earlier, insulation materials and building envelope systems are characterized by their resistance to heat flow. Material performance can be rated according to thermal conductivity (k), thermal conductance (C) and thermal resistance (R-value). In the case of system performance, total thermal resistance is shown as RT and thermal transmittance is shown as U-factor or Uvalue. With material surface performance, emissivity ratings are indicated by the symbol “e” and reﬂectivity is indicated by the symbol “r”.When it comes to measuring thermal properties of building materials, the October 2007 | Walls & Ceilings Architect | 27

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Contents Trade News Silver Certiﬁed Airtight The Men of Steel Thinking Thermal Firestop 101 ICFs Create a Tight Envelope The Finish Line The Green Thumb Cracking the Code

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