Radius started building shelters in 1978. The company began with concrete shelters in compliance with government specifications. After numerous problems, Radius abandoned concrete in favor of fiberglass. Fiberglass is stronger, cleaner, brighter and safer than concrete.

McCarthy warns that concrete walls soak up water and are prone to cracks that leak. Underground shelter walls are under much more pressure than the average household basement, and are deeper under the water table. Water inside a shelter will disable life support systems. It corrodes the steel supports in a concrete shelter, limiting the shelter’s lifespan.

Fiberglass shelters are watertight structures just like boats. Radius fiberglass structures have a 300 year lifespan.

Concrete surfaces are porous, rough and pocketed, a combination that is great for mold growth. Radius fiberglass shelters have solid, smooth surfaces just like a countertop. Their non-porous material can be cleaned with common cleaning solutions and chlorine.

Concrete walls will always make the shelter feel extremely damp because the walls hold massive amounts of moisture. Radius fiberglass shelters are made of an inert fiberglass material that do not hold moisture and form a complete vapor barrier.

Shelterists buying concrete for protection from weapons of mass destruction -– especially nuclear blast protection -- should know that the ceiling of any shelter should have a gamma protection factor of at least 512. With a reference dose of 13,000 rems gamma, the inside shelter dose is 25 rems (13,000/512). To protect human occupants the concrete shelter ceiling must be 12 inches thick with 39 inches of earth cover, or the concrete itself must be 36 solid inches.

Radius fiberglass shelters achieve this level of protection with much less material. The Radius shelters have an earth protection factor of 65,536 (inside shelter dose of 0.2 rems) to 262,144 with an inside shelter gamma radiation dose of 0.05 rems.

A nuclear blast would likely create EMP, or what is known as an electromagnetic pulse. The high intensity energy collects on concrete shelters in the embedded steel reinforcing bars. EMP itself is used as a weapon as it produces damaging current and voltage surges. Radius shelters are made of structural fiberglass and so do not collect EMP.

Concrete surfaces are usually rough. While concrete walls can be painted white, the concrete itself absorbs light, making it hard to get the shelter bright enough to easily read or do enjoyable, distracting activities.

Radius fiberglass structures have a solid, white, non-porous smoothness that reflects light and makes the shelter rooms brighter. Concrete shelters are dark and moist, an environment that takes a toll on the occupants. Radius shelters protect both physical and psychological health.

The hatch at ground level to enter the concrete structure is critical and hard to make air or bug tight. When the hatch is connected to the concrete structure it should have a seismic joint to allow difference in ground movement between the hatch at ground level and the shelter, which is perhaps 15 feet deep.

The Radius shelters use a gasketed seismic joint between the shelter and the hatch at ground level. Radius shelter hatches are impervious to air, water, bugs, dirt, fire and explosions (up to two 1-megaton blasts).

Concrete shelters are permanent and cannot be moved. They therefore require a building permit. Radius shelters have no foundation and therefore re-deployable. Radius shelters do not require a building permit.

Concrete shelters have no re-sale or market value. Radius shelters are a commercially re-deployable product. Radius shelters have a commercial value and can be re-installed in another location.

When a concrete shelter is being constructed, there is an open hole for a number of weeks making it dangerous for children. The Radius structural fiberglass shelters are installed in one or two days. Radius shelters are installed and assembled on site, not constructed on site.

McCarthy acknowledges that many people are forced to build their own shelters due to the cost of a commercial shelter. "But building your own shelter," he says," makes as much sense a building your own car. It can be done and it may be cheaper, but you may not have a working 'Shelter System' with an air filtration system, method of venting heat, water tank, plumbing system, septic system, food storage."

Most of all, concrete shelters might not provide the intended protection against nuclear, biological or chemical weapons, or from the wind and water of hurricanes and tornadoes.

"In considering your family’s safety," McCarthy said, "concrete is not a viable shelter system material. Only fiberglass provides the shetlerist with a warm, dry place that is impervious to storms and to nuclear, biological and chemical weapons."