I am going to bring my attic insulation up to the current recommendation for my area which is R-48. I now only have about an R-9 or so of old fiberglass insulation and will blow in the appropriate amount of cellulose insulation on top. The house has the original gable vents and a newer ridge vent but very few soffit vents as you can see in the picture below. I plan to install plastic rafter vents but notice that there is a large gap from where the wall's top plate is and the rafter vent and the cellulose insulation will fill this void. It seems that this will clog the few soffit vents that exist. My questions are:
Is it okay to fill this gap with the insulation?
If not, do I just cover the gap with something like 1/4" luan?
Should I cover the gable vents to allow better airflow from the soffits to the ridge vent?
Do I have to add more soffit vents?
Any sound advise is greatly appreciated.