Be wary of the relative dew point and the use of open cell insulation. Spray applied insulation at the bottom of the roof sheathing could cause moisture to condense within the insulation, or worse at the interface between the insulation and roof sheathing. This could lead to mold and issues with rotting wood. As woodgeek points out, the attic has traditionally been used as a point of control for humidity and heat loss from tempered environment of your home. Ventilation of the attic is just as important for maintaining dry insulation and a reasonable attic temperature. Not knowing exact details of your installation, it would seem to me that a better option would be in adding additional insulation to the existing insulation at the attic/ceiling interface. Depending upon your climate, elevation, latitude, etc you will want to research what is an optimal R-value goal. Where I live the utility recommends an R-38 attic insulation, however I believe R-45 to be more in line with reaching towards a net-zero value. Realize that the utility is in the business of selling energy, not saving you money or energy. Factor the buy in costs of insulation against rising utility costs (have you ever seen them go down?). Blown cellulose will also behave as an informal air barrier and provides a slightly higher R-value than shredded fiberglass. Also, cellulose is a reclaimed/recycled product whereas fiberglass is typically virgin material. I was taught that there is more embodied energy in manufacturing fiberglass than will ever be recovered as an insulation product. Lastly, there are incentives in the form of tax rebates that exist from the Federal Government in addition to the rebate offered by your utility. Cheers and good luck!