Shelter in the basement — conversion & protective ventilation
Honestly, up front: a converted home basement is not a bunker. It protects well against fragments, debris, blast waves at some distance, radioactive fallout and cold — not against a direct hit. The steps are sorted by effect per effort.
1. Choose the room (free)
- Deep and inside: as much earth and as many walls as possible between you and the outside, no or few outer walls, no windows.
- Away from hazards: heating-oil tank, gas pipes, gas boiler, chemicals, heavy shelves.
- Check for flooding (nearby streams and rivers, groundwater) — a basement that fills with water is a trap.
- Space: Swiss shelters plan with about 1 m² per person, pets in addition; with places to lie down, better double that.
2. Secure the openings (biggest effect)
- Basement windows/light wells are the weak point: on the inside a wooden board + sandbags, or fill the light well from the outside with sandbags/gravel and cover it with a board. Around 50 cm of sand/earth stops fragments and greatly reduces radiation.
- Door: solid (steel or solid wood), opening outwards.
- Second exit: emergency exit via a light well, openable from the inside. Tools in the room: crowbar, hammer, saw.
3. Ceiling & structure — only with professionals
Propping up the ceiling with building props and cross beams against falling debris is possible — ask a structural engineer first, do not alter load-bearing walls. In Switzerland, shelter construction is standard: Swiss shelter companies also advise on retrofitting.
4. Protective ventilation — air is the often forgotten problem
In a sealed room, people and animals use up the oxygen within hours, and CO₂ rises. Protective ventilation should do three things:
- supply filtered fresh air,
- create a slight overpressure in the room so that no unfiltered air gets in through gaps,
- keep running without mains power (hand crank or battery).
The most important rule
Never build the NBC/CBRN filter yourself. Home-mixed activated carbon, vacuum-cleaner, car or household filters do not protect reliably. What you can build yourself is the airtight ventilation line with overpressure around a purchased, certified NBC/CBRN shelter filter.
Layout (order of the airflow)
Outside air → weather protection/insect screen → blast valve → pre-filter (coarse filter) → certified NBC filter → radial fan (power + hand crank) → check valve → shelter (overpressure) → overpressure/blast valve on the opposite side → outside. In normal times fresh air runs through a bypass around the gas filter; only in an emergency do you switch over to filter mode.
Our own, improved drawing. Corrected compared with the original: exhaust on the opposite side (not back to the intake), blast valve added at the intake, fresh-air bypass for normal times, sizing for your household. Original infographic: blog.hennweb.de (link below).
What happens in the NBC filter
| Layer | Works against |
|---|---|
| Pre-filter | coarse dust, protects the expensive layers |
| High-efficiency particulate filter (HEPA) | radioactive dust, bacteria/spores, aerosols |
| Activated carbon (often impregnated) | chemical gases and vapours |
| After-filter | carbon dust |
How much air do you need?
Rule of thumb: 3 to 10 m³ of filtered air per person per hour — measured after the filter, not the fan's output without a filter.
| Example: 4 shelter places | |
|---|---|
| Minimum | 12 m³/h |
| good | 24 m³/h |
| comfortable (longer stay) | 40 m³/h |
→ In the example: a system for 25–40 m³/h of filtered air. Your exact values (shelter places, air volume, quantities) are calculated by the shelter shopping list from the household entered in the supplies calculator — large dog ≈ ½ place, small dog ≈ ¼ place.
Battery for the fan
Fan power (watts) × hours = watt-hours. Example: 40 W × 24 h = 960 Wh — with losses, plan 1.2–1.5 kWh of battery per day. Plus a hand crank as a reserve.
Installation notes
- Air intake not at ground level (many toxic gases are heavier than air), protected from rain, away from exhaust gases, the garage and the basement drain.
- Wall duct airtight with pipe sleeve, collar, EPDM seal and bolted flange — not just expanding foam.
- Filter housing completely airtight — no air may flow past the filter.
- Radial fan or a ready-made shelter unit — not a bathroom or PC fan (the NBC filter strongly restricts the airflow).
- Check the overpressure with a differential pressure gauge; the room must be slightly positive relative to the outside. Never underpressure — it sucks unfiltered air in.
- Store the NBC filter in its original packaging, dry and dark, and only fit it in an emergency (follow the manufacturer's instructions). Keep spare pre-filters ready.
Materials list
- certified NBC/CBRN shelter filter + matching filter housing with sealing flange
- pre-filter box with spare coarse filters
- radial fan with pressure reserve (12/24 V or mains voltage) + battery + hand-crank option
- check valve, overpressure/exhaust valve
- metal or pressure-resistant ventilation pipe, wall ducts, EPDM/silicone seals
- differential pressure gauge, CO₂ meter, CO alarm
- data sheet of the filter
Simpler alternative: a ready-made shelter ventilation unit with manual and electric operation (Swiss standard, tested). More expensive, but safer than a DIY build.
🛒 Shopping list with all parts, guide prices and links: EINKAUF-SCHUTZRAUM.html (also in the sidebar).
Final inspection: have the design and installation checked by a ventilation installer, shelter equipment supplier or building-services engineer.
5. Equipment for 2–14 days
With radioactive fallout the 7/10 rule applies: after 7 hours about 10 % of the radiation remains, after 49 hours about 1 %. The first 1–2 days are the most important.
- Water 2 l/person/day + water for animals (amount: shopping list/supplies calculator), food that needs no cooking, can opener
- Bucket with lid, bin bags, cat litter (also for other pets)
- Head torches, LED lanterns, batteries; crank radio — test reception in the basement beforehand, if necessary a throw-out wire antenna to the outside
- Camp beds/mattresses, sleeping bags; for older people a proper chair and a bed
- First-aid kit, medication for 2 weeks, FFP2/FFP3 masks
- Crowbar, hammer, saw, gloves, fire extinguisher/fire blanket
- No combustion in the room (stoves, candles over long periods, generators) — CO alarm!
- Pets: leads, muzzle, blankets, food
6. Practise
- A trial night with everyone (including the animals) — afterwards you will know what is missing.
- Time how long it takes from the alarm to the basement — with the slowest members of the household. Target: under 2 minutes.
7. Public shelters & old bunkers
Germany (as of 09/2026): Germany abandoned public shelter construction in 2007. Nationwide, 579 facilities with around 478,000 places are still designated (less than 1 % of the population) — according to the Federal Office of Civil Protection (BBK), none of them is operational. The federal government is not building new bunkers (Dobrindt, 14 Sept 2026); instead it has municipalities register underground car parks, tunnels and basements of public buildings as "protective locations" by the end of 2026. These are to be shown later in the NINA app. There is no date yet.
- Former Cold War civil-defence facilities (e.g. multi-purpose facilities in underground car parks, former auxiliary hospitals) are mostly used for other purposes today — not operational, no right of access. A directory (as of 2007) is available at geschichtsspuren.de.
- Legal route to the complete list: the Institute for Federal Real Estate (BImA) is responsible for the facilities still designated; Baden-Württemberg has the most, with 220 of 579. Freedom-of-information requests (federal IFG; state laws, e.g. LIFG Baden-Württemberg) to the BImA and to your town/municipality can be made free of charge via fragdenstaat.de.
Switzerland: shelter construction is standard there. Shelters are allocated to the residents of the respective municipality — not to residents of neighbouring countries. You can find out your allocated shelter place from your municipality.
Other countries: information is provided by your country's civil-protection authority (see region box).
Beware of maps and ruins: objects marked as bunkers in OpenStreetMap are often old military bunkers or fortifications — usually locked ("access: no/private"), not shelters for civilians. Castle and fortress ruins are not shelters either. The many "assembly points" of companies and schools are fire assembly points, no protection during an air-raid alert.
What this means for you:
- Your basement remains protective location no. 1: reachable in under 2 minutes, with everyone in the household, including older people and animals. No public bunker beats that when warning times are short.
- When out and about during an air-raid alert: go to the nearest underground car park (lowest level, away from entrances), a basement or a solid building — not into the car.
- Ask once: at your town/municipality (public order office/civil protection) whether and where protective locations or public shelters have been registered. Then note them here: ……
- Keep your country's warning app up to date (Germany: NINA — the protective locations will appear there once the federal government releases them).
Sources (Germany): BBK protective structures; Münster Aktuell 14 Sept 2026 (Dobrindt, BMI municipal conference); Correctiv 13 May 2026; civil-defence facility directory geschichtsspuren.de (as of 2007).
Order by cost
| Step | Effort |
|---|---|
| Choose the room, tidy up, remove hazards | low |
| Water, light, radio, toilet, beds, CO₂/CO alarms | low–medium |
| Light wells with sandbags/boards, emergency exit | medium |
| Solid door, ceiling props (structural engineer) | medium–high |
| Protective ventilation with certified NBC filter | high |
Source for protective ventilation (section 4): summary and conversion based on "Schutzraum Filter selber bauen DIY", HennWeb blog (blog.hennweb.de/schutzraum-filter-selber-bauen-diy), as of July 2026 (German) — original with diagrams there. Rules of conduct for gases: BBK (Germany).