Most buyers compare bunker builders on square footage and finishes. The questions that matter are about soil, air, water and fuel, and most companies hope you never ask them. Print this list and bring it to every meeting, including ours.
How to use this list
These twelve questions come from the standards that govern shelters and emergency systems: Swiss civil protection, FEMA, the EPA, NFPA 110 and the ventilation research done at Oak Ridge National Laboratory. A builder who works to those standards will answer each one with a number, a standard or a document. A builder who answers with reassurance is telling you something too.
Print this checklist and take it to every builder you interview.
1. What will you learn about the soil and groundwater before you design?
A buried structure is a boat in the ground. Groundwater pushes up and in, wet clay swells, and rock changes how and where you can dig. None of it shows on a site visit.
A good answer sounds like
A geotechnical investigation with test borings, a report on the water table and soil type, and a design that responds to it, before a price is final.
2. Who engineers the structure for buried loads, and will the plans carry a licensed engineer's stamp?
Earth is heavy, and it pushes on walls as well as roofs. Shipping containers, the most common shortcut, carry their strength in the corner posts; their corrugated walls and roof are not designed for soil load.
A good answer sounds like
A licensed structural engineer designs for your soil, depth and surcharge, and the stamped drawings are yours to keep.
3. How will the structure stay dry, and how will you control humidity?
A few feet down, the ground stays roughly 40 to 70°F all year, according to the Department of Energy. That steady temperature is an advantage for comfort and a challenge for moisture: warm, humid air meeting cool walls means condensation.
A good answer sounds like
A named waterproofing system, perimeter drainage that relieves water pressure, and dehumidification sized for the space and occupancy.
4. What is the fresh-air rate per person, in cfm, at full occupancy in summer?
Oak Ridge's shelter research found that people in a full shelter generate enough heat and moisture to become dangerous within hours in warm weather. FEMA's historic minimum was 3 cubic feet per minute per person; Navy tests at 12 cfm per person still became difficult; the Oak Ridge guidance recommends about 40 cfm per person in hot weather.
A good answer sounds like
A specific number per person, sized for your household at full occupancy in July, with the blower capacity and filter pressure drop to match.
5. What is the complete air-filtration chain, from intake to exhaust?
Switzerland, which has a shelter place for essentially every resident, specifies the chain: protected air intake, blast valve and pre-filter, ventilation unit, NBC gas filter, and an overpressure and explosion-protection valve, with airlocks on larger shelters.
A good answer sounds like
Every element named, the filter class stated, positive pressure explained, and a manual or hand-crank backup for the blower.
6. How is carbon dioxide measured and managed?
In a sealed room, carbon dioxide from breathing becomes a problem long before oxygen runs short. Even in cold weather, roughly 1 cfm of outside air per person is needed just to keep exhaled carbon dioxide from building to dangerous levels.
A good answer sounds like
Continuous CO2 monitoring with an alarm, and a ventilation plan that keeps levels in the normal indoor range at full occupancy.
7. What protection factor does the design give against fallout, and how was it calculated?
Fallout radiation drops quickly: for every sevenfold increase in time after a detonation, the exposure rate falls tenfold, so 400 R/hr at two hours becomes about 40 R/hr at fourteen hours. Shielding is what carries a family through those first hours and days. Each halving thickness of mass, about 2.2 inches of concrete or 3.3 inches of packed earth, roughly halves gamma radiation.
A good answer sounds like
A protection factor for the sleeping and living spaces, with the overhead and wall mass that produce it. Try our Shelter Configurator to see the math.
8. Will you test for radon and build in mitigation?
Radon is the leading cause of lung cancer among people who have never smoked, and the EPA estimates about 21,000 radon-related lung cancer deaths a year in the U.S. Below-grade spaces are where radon collects. The EPA's action level is 4 picocuries per liter, and it suggests considering a fix between 2 and 4.
A good answer sounds like
Radon testing of every below-grade space, a sub-slab mitigation path built in during construction, and a post-occupancy test.
9. Where does the water come from after the tanks run out?
Government guidance is one gallon per person per day for drinking and sanitation, for at least three days. At that bare minimum, a family of six needs 1,080 gallons for 180 days, before cooking, cleaning, livestock or a garden.
A good answer sounds like
A permanent source, a producing well or spring with secure water rights, plus stored reserve, treatment and a way to pump without grid power. See water independence.
10. What fuel runs your backup power, and how is it tested and rotated?
Fuel ages. NFPA 110, the standard for emergency power systems, requires a fuel-quality test at least annually. Diesel is commonly cited as lasting about 6 to 12 months stored cool and dry; gasoline about three months; propane does not degrade.
A good answer sounds like
A fuel choice matched to how long it will sit, a testing and treatment schedule, and tank capacity tied to a number of days of runtime.
11. Where do the batteries live in January?
Lithium batteries should not be charged below freezing. Charging a lithium battery below 32°F causes lithium plating, which permanently reduces capacity, and the damage accumulates.
A good answer sounds like
Batteries in a conditioned or below-grade room that stays above freezing, with low-temperature charge protection in the battery management system.
12. What standard is the safe room built to?
Storm shelters and safe rooms have a real definition. Under FEMA P-361 and ICC 500, they are designed for wind speeds up to 250 mph and debris impact from a 15-pound 2x4 traveling at 100 mph.
A good answer sounds like
A named standard, FEMA P-361 or ICC 500, with the door, frame and anchorage tested to it. Read safe rooms explained.
Three more about the company
- Who pulls the permits? Most below-grade spaces are permitted as part of the residence. A builder should know how your county will classify the work before you buy the land.
- How do you protect my privacy? Ask who sees your plans, whether crews and suppliers sign confidentiality agreements, and whether your project will ever appear in marketing.
- What happens after you leave? Filters, fuel, batteries, water treatment and radon all need attention on a schedule. Ask who provides it.
Read the Styrkholm Spec for how we answer every one of these questions.
Key takeaways
- Ask for numbers, not adjectives: cfm per person, protection factor, pCi/L, gallons, days.
- Every buried structure should be designed for its soil and groundwater and carry an engineer's stamp.
- Air, not walls, decides how long a family can stay below grade.
- Radon and humidity are the everyday hazards of underground space.
- Stored water, fuel and battery capacity all run out; ask what replaces them.
- Nuclear War Survival Skills (Oak Ridge National Laboratory), Ch. 7: Ventilation and cooling
- Swiss Federal Office for Civil Protection: Shelters for the population
- U.S. EPA: Health risks of radon (archived)
- DHS Center for Domestic Preparedness: the 7:10 rule of thumb
- Ready.gov: Water (archived)
- Clean fuel for generators: a drill-down of NFPA 110
- REDARC: Why you should not charge a lithium battery below 0°C / 32°F
- FLASH: FEMA P-361 and ICC 500 safe rooms
- U.S. Department of Energy: Geothermal heat pumps
- Conexwest: How deep can you bury a shipping container?
This guide is general information to help you plan. It is not engineering, legal or water-rights advice, and requirements vary by state and county. Every Styrkholm project is designed by licensed professionals for the specific site.