The design of buildings is traditionally a very compartmentalized process. The architect establishes a footprint of the building and a tentative floor plan. Then the structural engineer builds a steel or concrete substructure for the exoskeleton of walls, doors, and windows. It’s often not until late in the design process, when the building envelope is nearly complete, that the member of the design team most versed in energy-conservation strategies, the HVAC engineer is consulted. Many building elements that affect energy use are by this time locked into the design. For the life of the building, these flaws will consume energy, affect occupant comfort, and maybe even reduce the useful lifetime of the structure. As a result, the intrinsic value and the rate of return on the investment are diminished.
One common request by the HVAC engineer, of the architect and structural designer, is for a substantial amount of insulation in the walls and roof. This is a very effective way to reduce the energy flow through the building shell. Double-paned windows are another good conservation practice, although sometimes a little harder to justify. The advantage gained by each of these, however, can be neutralized by a subtle structural feature that is often ignored by the HVAC designer: thermal bridging.
Insulation reduces the flow of heat through a wall by virtue of lowering the conduction of thermal energy. This process has a direct analogy in the flow of electricity. Wires surrounded by plastic and mineral fibers prevent the conduction that occurs if you touch a bare wire. A building is surrounded by an insulating envelope that likewise prevents the immediate flow of heat from inside out in the winter, and in the opposite direction in the cooling season. Thermal bridging is like a bare wire grounded, in that it permits the ready flow of heat through a conductive path: the building shell.
One example of a thermal bridge is framing with wood (or, worse yet, metal) studs, common in much building construction. The studs permit heat to flow quickly and with little resistance through the wall, many times faster than it can pass through the insulated portions. The R-value of the wall at the studs can be a third of the value between studs (much worse for metal studs). As a result, the ability of the wall as a whole to retard the flow of heat is greatly diminished, no matter how high the R-rating of the insulation used. Walls employing metal studs or structural-steel bracing have an even more significant impact upon the thermal quality of the wall since they conduct heat more readily than even wood.
Other examples of thermal bridges are the metal frames around windows, metal doors without any integral insulation, receptacle and junction boxes situated in outside walls, and conduit and piping coursing through the exterior walls. Virtually any item that causes the loss or lessening of the insulation in a wall can be considered a thermal bridge. Since the roof is the single largest component of the load in single-story buildings, a 15 percent increase in load can have quite an impact on energy-use characteristics. Other thermal bridges are less obvious. A balcony or ledge that protrudes beyond the envelope proper creates a more direct and conductive path than passing through the walls or portions of the slab encased on the outer exposed edge by the insulation and fenestration. Exhaust ducts from vent hoods, dryers, or restrooms also penetrate the building shell, with a metal duct that is a good conductor of heat.
Understanding thermal conduction is a bonus to building designers. It can also lead to some ingenious conservation measures. For example, landscaping can be situated to insulate exposed portions of the building slab. During a renovation project, the contractor can be instructed to carefully insulate around pipes and junction boxes in exterior walls and to be attentive in other ways to minimizing heat- transfer paths across structural and architectural thermal bridges.
These practices also reduce the possibility of air passing along this same route. Other common sources of air leakage are window and door frames, wall penetrations by pipes, and sloppy corner bracing of building walls. Proper repair of these thermal bridges and penetrations makes the envelope more airtight and the utility bills lower.
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Excerpt from Energy Conservation Projects.