An open cell spray polyurea foam for use in an insulation layer in a wall structure may include a polyurea. The polyurea may be a reaction product of an isocyanate compound and water. The open spray polyurea foam may also include a filler. The majority of the filler may exist in the spray foam formulation as an unreacted first fire retardant. The spray foam formulation may further comprise a second fire retardant, and the insulation layer may exhibit a fire retardancy sufficient to pass Appendix X and/or ASTM E-84.