insulating question

Greetings all from the great white north.I have an insulating question i would like to get some feedback on.I have a backsplit house 4/12 pitch just reshingled it last year.All the soffit venting and roof vents are working as they should.What i would like to do,to increase my R value is remove all the blown in fibreglass insulation then insulate with 3 inch polyiso between the trusses then blow the fibreglass back on top of it.A 6 mil poly vapour barrier is already in place(obviously under the drywall).My question is..am i setting myself up for a huge headache because im mixing the two different types of insulation?And am i going to create moisture by doing so?The reason for the polyiso is that it is no cost to me whatsoever.If it means anything i live about an hour east of Toronto and we do get some bone chilling weather here.Thanks.

You shouldn't be! The ISO on top of the ceiling below is just act as an other vapor barrier, and the 3 inches of ISO will give you R21 with no voids. The fiberglass over that will just increase the r factor and if you do in fact have adequate ventilation and air movement, it should work like a charm. by the way, what is a splitback?