If the existing insulation is fiberglass batts, I would definitely add cellulose over it to stop the wind-washing and convective loops heat loss you have now. 7" (R-20) of blown cellulose at 1.5# per cubic ft. would add 1# over each square foot compressing the batts, losing some R-value. Take two full12oz. pop cans and lay them 12” apart on the insulation, measure the area between them, the decrease in height of the batt compared to unloaded. This would be the same as adding 5” of cellulose. Next, take the compression distance and compare it to the chart for compression, underlined at the bottom: http://saveenergy.owenscorning.com/2...nd_your_2.html
Basically, 2” compression = R-5 reduction, 4” compression = R-9 reduction.
The added insulation acts as a blanket covering the more air permeable batts similar to blown-in loose fill low-density (0.5) fiberglass. Test results were 30-50% loss of R-value at 60-70* inside when temperature dropped outside: http://www.homeenergy.org/archive/he...92/920510.html