guys,

I have been researching ATTIC heat invasion in the past 3 months. ( i
am in TEXAS)

some findings.. when the temp outside is 100F.. the roof shingles are
reaching 135 -140 F...
now the inside roof temp is 130-135 F... and the attic floor or
surroundings is also. 120-130F...

I have started installing a Radiant Barrier
which I purchased a TRIAL roll 500 sf ft for 80$ from here

http://www.afs-foil.com/radiantbarrier.html

and I am impressed with it.. becuase.. the area underneath the barrier
is 100F while others remain the same i.e 130 F

a rule of thumb is that around 30% our Electricity bill is from AC
costs .. and 30% of AC costs is cooling the heat invasion from the
Roof.. so.. it should get me a 9% savings ROI in my monthly peak
bills..
but I found lot of interesting research saying that it cud be anywhere
from 2% to 15%.. lot of factors.. first of all I dont have access to
all portions of my attic or there is no attic space over the cathedral
Living and one BDR.. also. I dont have any TURBINE vents.. or any
vents.. need to get that installed too

http://www.ornl.gov/sci/roofs+walls/radiant/rb_01.html

also..i have 2 HVAC units and both are in the attic so I think I will
save a bit there.. as they are all going to be 30F cooler...

BTW, I bought the 25 inch roll and am stapling it underneath the
rafters.. and it is taking me around 100 Sq feet per 2 hours or so..
trick is to get as much covered up as possible.