Fiberglass placement (air spaces?)

Is it recommended to fill the entire "joist-space" with fiberglass, or is it recommended to leave an airgap. I've read Rod's book, but it seems both things are done in his examples (or I have misunderstood the drawings). Which of the alternatives in the attached drawing is recommended? In case this is construction dependent: I'm building a room-in-room construction with drywall + GG + drywall + GG + drywall in the ceiling.

depends on the space available. if you can put a layer on the upper floor and on the lower ceiling and leave an air gap, that's probably ideal. if not, then filling the space is the next best option to ensure both the upper floor and lower ceiling are dampened. just don't pack it tight.

gullfo wrote:

depends on the space available. if you can put a layer on the upper floor and on the lower ceiling and leave an air gap, that's probably ideal. if not, then filling the space is the next best option to ensure both the upper floor and lower ceiling are dampened. just don't pack it tight.

Ok, so you're actually dampening the leafs with the insulation? That's useful info. Thanks! Today I don't have enough fiberglass to fill the space, so I think I'll add a few cm to get it filled (but not packed tight). Thanks again!

Rune, you can always count on Glenn for good advice........ you really want to damp both surfaces if at all possible - and as long as you don't over pack (thus creating a bridge between the leaves) then you'll be just fine filling the entire cavity....... Rod

Thanks guys! I'm a little unsure where the limit is for over packing though... My air space is about 24 cm high. From my old ceiling I have 20 cm Rockwool insulation. Would adding a 5 cm layer be considered as over packing (25 cm insulation for 24 cm air space)?