I bought a new house the past summer.  It's a 1920's single car, two story garage that was converted into a house. It's a strange house, but I live alone and the price was right. I've been getting it ready for the winter and was crawling around the attic this past weekend.  I was patching up all the air leaks that I could see and repairing the mesh covering the gable end vents.  I noticed that there was a flat roof from the original building still up there.  They built the current roof on top of the parapet, so I have the flat roof with flattened R7 insulation, then 16 inches of parapet (on three sides), then begins the current stick framed roof.  The summer was brutally hot upstairs, which after going into the attic, I understand why.  I have a window unit AC for the bedroom, and don't really intend on getting central air.

I was planning to buy some blow in cellulose insulation from Home Depot and using the machine to essentially fill my attic up to the depth of the top of the parapet.  Would it also behoove me to also install a radiant barrier on the rafters?  I really don't know anyone who has used it.  The rolls are cheap enough on amazon and it's just my time.  I kind of feel as though going from the essentially R nothing to a R45 or so from the blow in cellulose would be enough.  Due to my house's orientation I get sun all day and I have no trees that block any of the sun.  I do have a light colored roof. Any thoughts?  Thanks.

I would be wary of that affecting the ventilation.

also make sure you do not clog the edge vents with the blown in.

This.  The radiant barrier will help (can't say how much, but it will help).

I'm not too worried about blocking the soffit vents since there are none currently.  I have a tiny attic, about 400 sq feet.  In the future I would like to add vented soffit when I have the siding redone.  I have a ridge vent cut into the roof and also two gable end vents that are at the peaks.  I threw together a quick picture on CAD to show what I have now, roughly.  The roof rafters are notched and sit on the parapet.

Attached File

From my understanding to have the ridge vent function as intended you need to have soffit vents with baffles traveling from the soffit to the ridge vent. Otherwise it won't ventilate properly. To just completely take away all ventilation might not be the best idea unless using spray foam insulation. In that case the ridge vent is not needed. When in doubt have someone come over and give you any estimate and pick their mind. See what they would do and why.

open cell foam on the bottom of the roof deck

They make radiant barrier either pierced and solid.

I applied radiant foil barrier material to the underside of the rafters and I'm convinced it helps. No ridge vent yet, just an attic vent fan with Hardie ventilated soffit all around.

If you are using blown cellulose, get some prices.  I found that it was cheaper to have a contractor to do it than do it myself.

I followed several steps and this stuff works, i first installed pink soffit baffles from home depot between the rafters at floor level. I then applied atticfoil.com barrier to my rafters in 2014 and it helped tremendously. Then last year i had a new roof installed, ridge vent II ridge vent installed, blocked my gable end vents, had roofers install nice new vented soffits , and i blew in over a foot of pink insulation, anywhere from R60 to R72 depending. . Now my house is nice and warm in the winter, and cool in the summer. The attic breaths well, stays cold in the winter preventing snow from melting in the winter, and vents hot air out the ridge vent in the summer, and the barrier keeps radiant heat from penetrating into the house.

Well worth it.

Edited to add, my 1971 raised ranch was an OVEN in the summers on the main floor upstairs before thses changes. And with a mere r11 in the attic, it was bleeding heat up there and causing minor ice dams in places i did not want them.