In a previous blog, we discussed the Home Energy Rating System (HERS), a number between 0 and 150 which quantifies the energy efficiency of a home. The lower the number, the more energy efficient the home. The target for the rehabs I've been part of in Detroit is a HERS rating of 85 or lower.
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But even if a HERS rating meets the above standard, it does not insure that heating and cooling is properly distributed throughout the home. HERS ratings reflect the amount of energy leaking through the thermal envelop. It does not reflect how well heating and cooling is distributed within it's envelop.
Duct pressurization is key to providing sufficient heating and cooling to every room. Every room is designed to receive an minimum amount of heated or cooled air per minute in order to maintain it within a comfortable temperature range. The volume of air is measured in cubic feet per minute or cfm. Furnaces are sized to push enough air through a home's ductwork so that each room receives its designed amount of cfm. The level of cfm produced at the furnace reduces as it is pushed through turns and curves within a ductwork system. Pressure losses due to these twists and turns is accounted for in furnace sizing calculations, but excessive leakage due to poor construction is not. Minimizing duct pressure losses insures that the most remote corner of an upper floor bedroom is properly heated and cooled.
Energy codes address the quality of duct sealing, but many jurisdictions are just beginning to enforce them. Energy-Star qualified homes are required to have a Duct Blaster test performed to confirm a minimum level of duct pressurization loss.
But before inspections or tests are performed, builders have a choice between two methods to seal duct connections:
Mastic
Mastic is a gooey, non-hardening material that is spread over duct seams with a paint brush, putty knife or simply with glove covered fingers. If gaps are greater than 1/16 or 1/8 inches wide, then a fiberglass reinforcing mesh tape should be embedded in the mastic. To be effective, the mastic should be applied to at least a nickel thick.
In rehabs, mastic can be difficult to apply to existing ducts which tightly fit between floor joints and sheathing. But if applied properly, mastic is the most effective duct sealing method.
Tape
Now everyone jokes about the usefulness of duct tape. But when it comes to actually using tape for sealing duct joints and connections, not all duct tapes are created equal. Your typical cloth-backed rubber adhesive tape fails fairly quickly when exposed to time and temperature changes.
Most energy codes require any tape used on ducts must be labeled in accordance with UL 181A or 181B. But since the UL181 listing was created as a smoke and flame test, the presence of the label is no guarantee of its effectiveness as a sealant. A tape's sealant capacity is better reflected by its base material. Two base materials have the best sealing capabilities: butyl and oriented polypropylene (OPP). When searching for a duct tape, one should select UL181 labeled tapes with base materials of butyl or OPP.
As energy codes improve, the importance of duct pressurization is also increasing. Though only Energy-Star qualified homes are currently required to have duct leakage measured, the requirement will soon be expanding to other types of housing . Therefore, the materials and methods used by builders in making duct connections will play primary roles in meeting future code requirements. But more important than the code, properly constructed connections insure the comfort and happiness of the homeowner.
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Holladay, Martin, "Sealing Ducts: What's Better, Tape or Mastic?", August 6, 2010, GreenBuildingAdvisor.com.
