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State of Oregon
Building Codes Division Staff 
Technical Advisory

The following advice applies to current code and conditions described in the question. Applications for similar situations may have different determinations.
PROGRAM: Structural Program
SUBJECT: Indoor Air Quality
SOURCE: 1996 Oregon Structural Specialty Code (OSSC)
REFERENCE: Chapter 12
DATE OF ISSUE: June 16, 1997 
PREPARED BY: C. Darin Wilson, P.E., Code Specialist, (503) 373-1354
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 isshall be provided, such system shall be capable of providing outside ventilation air as specified in Table 12-A.
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.]

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 maximum specified 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 this chapter.