Insulation, Radiant and Vapor Barriers - Insualtion on 1 1/2 story

I have a 1 1/2 story house and had a couple questions on insulation.

While painting an upstairs bedroom ( southwest side of house) I had
the paper of the sheetrock rolled up on my paint roll.

The sheet rock was pretty brittle and It appears it may have been from
the heat . In Houston and it can get pretty hot here.

The East and West outside walls of the bedroom goes to the attic. On
the west side thats where the AC is. So there is plenty room to work
up in there.

I looked at the older insulation and a lot of it is torn apart
This is in between the studs Vertical.
I want to replace it but also want to put another blanket of
insulation on it. Either Horizontal or an insulation type sheet with
foil on one side.

Suggestions?

Also. The rest of the house has blown-in insulation.
The rafters for the second floor run from east to west. As it is
now............ outside air entering from the vent in Soffit can
travel through the rafters from east to west. The blown-in insulation
never covered up the spaces in between the 1st floor ceiling and 2nd
floor flooring. Does this need to be stopped up with insualtion or
does it need to breathe.

Any help would be greatly appreciated and if you have a part# or name
brand for rolled insulation or sheets please advise;

regards

MBnTX