Attic ventilation and insulation

[Daox]

One of my projects that I am really wanting to get done before it gets too cold to work outside is adding additional insulation to the attic. It currently has about 1.5" of rockwool insulation and 4" of fiberglass over it. This gives me roughly an R-17 value for attic insulation. This is fairly low compared to the recommended R-50 rating it is suggested to have! So, I have spent the past few weekends and extra minutes here and there cleaning up the mess that the roofers made while replacing the roof this year.

Now I'm getting to the point at which I need to start doing something about the ventilation. The roof currently has a ridgeline vent put on, but nothing else. So, I was going to replace the current soffit with soffit with vents in it.

However, this now brings about another problem. My roof is a bit odd. It sits lower than a normal roof and creates a chamfered angle in each of the rooms similar to a 1.5 story, but no nearly to that effect. The chamfer is about 1.5 ft down from the top of the ceiling. See the picture below for a wall section view. My problem is that I need to insulate that small area the best I can. I'd really like to use some rigid foam in there to get higher R-values, but I have to leave room for venting. Ideas?





I also have another idea/problem. I'm planning on blowing cellulose over the fiberglass. This is going to pack the fiberglass down to some degree and decrease its insulating value. I'd like to avoid wasting what I have and preserve this insulation. I was thinking of laying down some plastic over it and stapling it down, or cardboard or something to prevent the cellulose from derating the fiberglass. Got any ideas for me?