I am thinking of getting my attic insulated,my house was built in 1975,at that time 2 inches of fibreglas was put between joists.Later on I added extra insulation that came in 4 x2 foot slabs.this is also 2 inches thick.Recently I have noticed that there are black marks on my ceilings where the joists are positioned.this is very noticable above the room where I have a solid fuel stove for heating.I have checked the insulation in attic and it seems to have "shrunk" in thickness ,I am thinking of taking it all out and redoing the entire thing again.Has anybody got any ideas about the marks on ceilings?.Also some advice on what product to use and depth needed.All help is appreciated.Thanks