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Sir ano po specifically na insulation ang bibilhin ko? no idea po kac...and, what do you mean by "thicken the leafs of the door"?
Insulation - glass fiber 16 kgm batt inside the door.Thicken leaf - put 3/8" plywood on one side; 3/4" plywood on the other side of the door.
It's of no use. Put insulation inside your hollow core door, thicken the leafs of the door and make sure you have gasketing around the edges of the jamb to prevent sound from coming in. Refrigerator seals are good seals. You can also check the Raven brand from Australia. They are weather door seals but can be used as acoustic seals as well. They are available at True Value, Acme and other similar hardware stores.PS. Please keep in mind that foam is flammable, thus, dangerous, unless you fireproof it. Then, it becomes more expensive.