Abstract: Failure to kill all grain-infesting insects when fumigating a storage structure occurs for several reasons: leaks in the facility, poor gas distribution, emergence of surviving insects following a poor treatment, and insect resistance. Fumigation can be improved by using a closed-loop fumigation (CLF) system where the base ducts are pressurized to force the gas mixture upward through the grain mass which provides better gas distribution through the grain. Before using CLF, the structure must be well sealed. Materials used for sealing include closed-cell foam, silicone, gray tape, 6 mil plastic attached with adhesive spray and elastomeric roof coating. Areas to seal on metal bins include aeration fan motor connections, fan inlets, fan transitions, roof eaves, sidewall seams, bin base joints, side and roof entry doors, roof vents, and CLF piping. On concrete silos, also seal downspouts, inner and outer connecting vents and manhole covers. A correctly installed CLF system will use less fumigant, save on labor costs after initial installation, and cause less worker exposure which will lead to less health costs. In a test at a concrete facility that was divided into two sections where one half received conventional treatment and the other half had a CLF system, phosphine levels reached over 2,000 ppm on the CLF side and only a maximum of 100 ppm on the conventionally treated side. Sealing and using a CLF system will save money and result in better control of insect pests.