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Ventilation Requirements for Storm Shelters

Ventilation Requirements for Storm Shelters

Ventilation Requirements for Storm Shelters

Hi there!

I'm new to the forum. Would appreciate some help with clarifying ventilation requirements for storm shelters.

We're working on a school storm shelter that is being provided with mechanical ventilation via two supply fans (supplying outside air only). The storm shelter area includes several classrooms. During normal operation, the classrooms are provided with ventilation via rooftop units. Per ICC 500, section 702.1.2, "Tornado shelters that rely on mechanical ventilation shall be provided with the minimum mechanical ventilation rate of required outdoor air in accordance with the applicable building code provisions for the normal use of the space." Per the 2015 IMC, the ventilation rate for classrooms is 10 CFM/person. This results in a pretty large OA CFM requirement for the spaces, which seems excessively high for a storm shelter.

My question is whether the requirement to size the storm shelter ventilation systems for normal use of the spaces still applies when the spaces are provided with two separate ventilation systems between normal operation and emergency mode. It makes sense the system should be sized for normal operation if a single system is being used in both normal and emergency mode, but it doesn't seem it would be necessary to size the ventilation system to normal operation when a separate emergency ventilation system is provided.

Any thoughts on this would be appreciated. Thanks!

RE: Ventilation Requirements for Storm Shelters

In the 2020 version the minimum for mechanically ventilated shelters is 5cfm per design occupant.
I have not designed a shelter, but offer my approach for discussion.
If the shelter is also a classroom:
  • When normal power is available, ventilate as a classroom, for the classroom expected occupancy count
  • Check if the normal ventilation provides 5 cfm per person when the space is at it's shelter capacity.
  • Consider how shelter capacity ventilation will be provided following loss of utility power.
I suggest you review with your AHJ before finalizing your approach

RE: Ventilation Requirements for Storm Shelters


Thanks for the feedback! I had not been aware that the ICC 2020 gave a minimum of 5cfm per person since we have been going off of the 2015 ICC. For the other design considerations:

- The space is ventilated as a classroom when normal power is in use.
- Shelter ventilation is powered by a UPS during storm mode.

Thanks again!

RE: Ventilation Requirements for Storm Shelters

Be careful using a code more recent than invoked in your jurisdiction. You might need to request a variance to use the latest code, it is best to discuss with the AHJ before submitting plans for review.
https://codes.iccsafe.org/codes/collections has current - location specific versions posted.

I looked at the 2014 version of ICC 500 https://codes.iccsafe.org/content/ICC5002014, my interpretation of the ventilation requirement in that code is that the ventilation must be suitable for the normal use of the space per the applicable building code. I would infer that I do not need to consider the additional occupants during shelter use in calculating the ventilating requirements, but that the ventilation provided must be maintained during a power outage. I think your approach is compliant and should work, I would have the calculations for 5 cfm per occupant in reserve and provide if requested.

RE: Ventilation Requirements for Storm Shelters

Thanks for the link!

That was my interpretation as well in reading through the 2014 version. Again, appreciate your help on this!

RE: Ventilation Requirements for Storm Shelters

Have you check any of the NFPA codes?

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