This article was originally published in the May/June 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
Home Energy Magazine Online May/June 1999
California Building Codes ChangeLast November, the California Energy Commission approved the 1998 version of the Alternative Calculation Methods (ACM) section of Title 24, which regulates the energy efficiency performance of new residential construction. The ACM is the primary method used to design new homes, because it allows designers to trade off different features and achieve the desired level of overall energy efficiency in a flexible way. This set of standards, which is expected to take effect in July 1999, will control new residential construction in the State of California for the next five years. The most far-reaching changes in this revision center on the conditioning and delivery of air--either for providing thermal comfort or for controlling indoor air quality--and on the control of envelope leakage.
Field research conducted by Lawrence Berkeley National Laboratory (LBNL) has shown that thermal distribution systems in California houses, which consist mainly of ducts, waste 20%-40% of the energy that goes through them. (For more information, see http://ducts.lbl.gov.) This research has been evaluated and incorporated into the new Title 24 requirements. Previous versions of Title 24 implicitly assumed a much higher duct efficiency. Appendix F of the ACM, which is an adaptation of the draft ASHRAE Standard 152 for use in California, provides an accurate and reliable method for estimating duct efficiency.
Duct losses can easily be cut in half with available technology. Testing of duct-sealing methods at LBNL revealed that the most common method for sealing ducts--taping them with duct tape--usually fails over time. The new ACM standard does not allow thermal distribution credit to be taken if duct tape is used as the primary sealant.
Although it does not represent as big an energy loss as duct leakage, building envelope leakage can produce an appreciable loss. For the first time, the ACM standard now contains an accurate algorithm that reflects how much envelope leakage contributes to energy losses. Designers may get credit either by demonstrating the level of tightness or, prescriptively, by including house wrap products. To get full credit for either duct or envelope leakage improvements, airtightness testing is required.
A major barrier to increased duct or envelope tightness levels had been the concern that indoor air quality could be compromised at the expense of energy efficiency. ASHRAE Standard 62-89 (Ventilation for Acceptable Indoor Air Quality), now in the code, ensures that minimum ventilation rates can be provided. Special provisions reduce the incidence of backdrafting of combustion appliances. The ACM standard also now accurately incorporates the interaction of infiltration with mechanical ventilation systems so that those wishing to have a whole-house, mechanical ventilation system will not be unfairly penalized.
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