Here is a more detailed scenario for retrofitting a house with insulation.
It's a 2000 sq ft slab on grade in texas, 8' ceilings, 5' attic at peaks, currently has 4" of fiberglass betwwen the ceiling rafters in the attic.
Was going to add cellulose for benefits: better r value than fiberglass; 30 to 40% radiant barrier; "seal" of attic (which fiberglass cannot do).
Now it looks like cellulose has these problems: invites mold; decomposes over 20 years; loses its inflamability over 20 years; makes a constant layer of dust in the house.
xxxxxxxxxxxxxxxxxxxxxxxxxx So back to the original situation, wanting to increase insulation significantly, can this be done by fiberglass and how many layers can be added in an attic? Also there is no radiant barrier, should one be added? xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
What is rock wool and would it be better than fiberglass?
Also needing to insulate a 4" airspace with 2" of insulation but cannot find anything to do it with at retail outlets, except maybe use an electric carving knife to split some 4" thick insulation but then the r value is minimal...the insulation manufacturers make a fiberglass board insulation with aluminum facing, but it cannot be purchased by the general public, only licensed contractors can buy it supposedly.