ICC Interpretation Clarifies Ventilation Rate Calculations for New Construction

[jar546]

An interpretation from the International Code Council has addressed how occupant loads should be determined when calculating mechanical ventilation rates, particularly in cases where IBC and IMC provisions appear to conflict.

In the example presented, a new temple was designed with an occupant load of 235 people per 2018 IBC Section 1004, while the mechanical engineer calculated ventilation rates for only 140 occupants based on the 2018 IMC Table 403.3.1.1. The question was whether the IBC number should override the IMC figure for HVAC design.

According to the ICC, IMC Table 403.3.1.1 provides specific occupant load density values for ventilation purposes, and these values apply to all occupancies except certain smaller residential uses. Section 403.3.1.1 of the IMC directs that the table’s occupant load densities be used to determine mechanical ventilation rates. This specific provision takes precedence over the more general IBC Section 1004 occupant load requirements when calculating ventilation needs.

The IBC and IMC both state that specific provisions override general provisions. In practice, this means IBC Section 1004 occupant loads govern means of egress, plumbing fixture counts, and other general code applications, while IMC Table 403.3.1.1 occupant loads govern mechanical ventilation calculations. For ventilation design, the IMC table values supersede the IBC occupant load numbers.

 

[jc_ks]

Interesting. I have a specific client who always refuses to provide the IMC ventilation rates claiming that our allowed exceptions for IBC rates should control. Do you know where I can find this interpretation?

 

[Paul Sweet]

IBC occupant loads are based on the maximum number of people that can reasonably be expected to be in a room or building. IMC occupant loads are based on average occupancy.

It's like the difference between Christmas & Easter church services and regular Sunday services.

 

[Yankee Chronicler]

An interpretation from the International Code Council has addressed how occupant loads should be determined when calculating mechanical ventilation rates, particularly in cases where IBC and IMC provisions appear to conflict.

In the example presented, a new temple was designed with an occupant load of 235 people per 2018 IBC Section 1004, while the mechanical engineer calculated ventilation rates for only 140 occupants based on the 2018 IMC Table 403.3.1.1. The question was whether the IBC number should override the IMC figure for HVAC design.

According to the ICC, IMC Table 403.3.1.1 provides specific occupant load density values for ventilation purposes, and these values apply to all occupancies except certain smaller residential uses. Section 403.3.1.1 of the IMC directs that the table’s occupant load densities be used to determine mechanical ventilation rates. This specific provision takes precedence over the more general IBC Section 1004 occupant load requirements when calculating ventilation needs.

The IBC and IMC both state that specific provisions override general provisions. In practice, this means IBC Section 1004 occupant loads govern means of egress, plumbing fixture counts, and other general code applications, while IMC Table 403.3.1.1 occupant loads govern mechanical ventilation calculations. For ventilation design, the IMC table values supersede the IBC occupant load numbers.

This is the way I have always understood and applied the codes. I can't imagine why any code official or design professional would think the occupant loads from IBC Chapter ten apply to the ventilation design under the IMC, when the IMC provides clear direction on how to calculate the design occupant load.

 

[jar546]

Interesting. I have a specific client who always refuses to provide the IMC ventilation rates claiming that our allowed exceptions for IBC rates should control. Do you know where I can find this interpretation?

It is a premium answer when you have the ICC online codes.