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Home Energy Magazine Online May/June 1999
trends
in energy
California Building Codes Change
Last 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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