Effi,
www.owenscorning.com
Yep, I visited there a lot when we were preparing to insulate our house
too...
layer 1.
Owens Corning Raft-R-MateŽ attic vents
Depending on how much depth you finally end up with, you may only need them
at each end of the rafter span where the insulation can get bunched up and
block off the air flow. But, you can certainly use them the full length of
the rafter bay if you wish. Just more cost (and slightly more restriction
to the airflow).
layer 2.
radiant barrier; yes in warmer clime
Sounds fine. I have no experience with radiant barriers.
layer 3.
one 5" piece attached every 3' or so along the roof rafter,
by means of glue and press on metal braces
I personally would not trust the press on metal braces. I know they use
them for roof trusses, but they have a big press in the factory to press
them in firmly and evenly. I don't see how you could ever achieve the same
results in the field.
If you want to use metal braces, I would get the types that have all the
little holes in them, then nail them up with joist hanger nails (short
lengths that won't go all the way through the 2x lumber).
I still think plywood supports would be cheaper and easier to install.
layer 4.
a secondary 2x4 running the length of the roof rafters
and attached to the 5" 2x4 pieces mentioned in layer 1.
I was just looking at an old article from the Journal of Light Construction
Online (couldn't find it online now) about "Energy efficient cathedral
ceilings". One of their techniques was to use 4" wide plywood gussets
nailed to the rafter with 8D nails and to the new bottom chord (2x3 in
their case). By adjusting the length of the gussets, they could adjust the
depth available for insulation. From what I could tell, they only nailed
the gussets on one side of the rafter.
If it were me, I would probably use wider gussets or even full length
strips of plywood, just for the added strength.
may need or want to add some stability 2x4s
between each new "extended rafter" if they sway
If each end is fastened securely, I wouldn't think there would be too much
sway (don't know how long your span is), but I see no real problem with
your solution.
9.75" thick air space created and used for:
1.50" or so foam attic vents
8.25" insulation
Around here, I would try for R30 minimum in the ceiling if at all possible.
But, you may not need that much in your climate. You would have to do some
checking on that.
In any case, your current plan sounds much better than your original
proposal.
Have fun!
Anthony