|QUESTION 1: Is natural ventilation allowed
to accomplish the indoor air quality requirements of OSSC Section 1203.3
for Group R Occupancies?
DETERMINATION 1: Yes. OSSC Section 1203.3 requires either natural
ventilation or mechanical ventilation in Group R Occupancies. Due to some
ambiguity in OSSC Section 1203.3.2, the following errata is appropriate
to clarify the intent, testimony and actions that occurred in the adoption
of that code section:
1203.3.2 Ventilation for Group R Occupancies, three stories
or less in height. If a mechanical ventilating system is
EXCEPTION: Buildings of ordinary tightness may meet one
of the following two requirements in lieu of documenting achievement
of required occupancy ventilation load. [The rest as written.]
be provided, such system shall be capable of providing outside
ventilation air as specified in Table 12-A.
QUESTION 2: Can the ventilation required by OSSC Section 1202.2.1
and 1203.3 be provided by a combination of natural and mechanical ventilation?
DETERMINATION 2: Yes, provided each type of ventilation applies
to a separate space. For example, natural ventilation may be proposed
for an individual bedroom or individual office, while mechanical ventilation
is proposed for a dining room or employee break room.
QUESTION 3: Can the mechanical ventilation system flow rates of
OSSC Table 12-A, as required from OSSC Section 1202.2.1, be reduced by
up to 50%?
DETERMINATION 3: No, unless design calculations based on approved
ASHRAE standards are submitted. The allowance of a 50% reduction in Section
1202.2.1 is only applicable to the occupancy load used in Table 12-A.
As a clarification, the occupancy load may be reduced to one-half [50%]
the MINIMUM specified in OSSC Section 1002.
ANALYSIS: This advice has been reviewed by the Building Codes
Structures Board's Energy Committee and BCD staff. The committee and staff
reached consensus on the determinations above. Further analysis follows:
OSSC Section 1203.3.2 provides an exception to the mechanical ventilation
system design requirements, if natural ventilation is not the design
method chosen. The exception provides two prescriptive path methods. The
prescriptive paths originated as a proposal in June, 1993 from the Indoor
Air Pollution Task Force. The task force developed these prescriptive
paths to be used in lieu of a designed system for low rise residential
structures. Such structures are commonly not designed by an engineer or
architect. However, the exception was developed when a mechanical ventilation
system was proposed for all new residential buildings.
The December, 1994, Hearings Officer's report then proposed that either
mechanical or natural ventilation is acceptable for Group R Occupancies.
The Building Codes Structures Board recommended to the Administrator that
the Hearing's Officer's recommendations, with minor revisions, be approved.
The first printing of this code language was in the 1996 OSSC.
Additionally, staff and the committee advise that OSSC Section 1202.2.1
should read: "the average ventilation occupancy load used shall not be
less than one-half the minimum
in Section 1002."
Staff advise that editorial mistakes occurred in revising OSSC Section
1203.3.2 to reflect the intent, testimony and actions in the adoption
of OSSC Chapter 12. Several editorial code changes are anticipated for