Systems guide

How much air a bunker needs

Ventilation is the system that decides how many people a bunker can actually hold. People make heat and moisture as well as carbon dioxide, and in summer the heat is the harder problem.

The short answer
  • About 1 cfm of outside air per person keeps carbon dioxide in check in cold weather.
  • In hot weather, Oak Ridge guidance is about 40 cfm per person.
  • Size the air system for a full house in the hottest month.

The numbers

  • About 1 cfm per person: the floor in cold weather, to keep exhaled carbon dioxide from building to dangerous levels.
  • 3 cfm per person: FEMA's historic minimum for fallout shelters.
  • 12 cfm per person: in a Navy summer test, still not enough; conditions became difficult and about a third of the men developed heat rash.
  • About 40 cfm per person: the Oak Ridge guidance for hot weather, to hold the temperature rise to a couple of degrees.

Why a vent pipe is not enough

A buried room has no wind and little temperature difference to drive air through a pipe. With people inside, the air warms and grows humid quickly. A powered blower, with a hand crank for when power fails, moves a known amount of air no matter the weather.

Filtration

Filtration and fresh air are not the same job. Filters clean the air coming in; the blower sets how much comes in. Many filtered systems move less air through the filters than they do in plain ventilation mode, so check both numbers for the full household.

What to ask any builder

  • How many cfm per person at full occupancy, in July?
  • How many cfm through the filters?
  • What happens to the blower when the power goes out?
  • Where do the intake and exhaust surface?

Questions

How many cfm does a family of six need?

In cold weather, at least about 6 cfm for carbon dioxide. In summer heat, plan toward about 240 cfm, which is 40 cfm per person by the Oak Ridge guidance.

Does a CUBE Bunker include air filtration?

Every complete CUBE Bunker includes the Air system sized for its household. Higher tiers add NBC filtration. You choose the tier in the Builder.

Sources
  1. Nuclear War Survival Skills (Oak Ridge National Laboratory), Ch. 7: Ventilation and cooling

CUBE Bunker prices are preliminary until engineering for the cube system is complete and are confirmed in writing after a site visit. This guide is general information, not engineering, legal or medical advice. Updated 2026-11-02.