Insulation - am I breaking any rules?

[turnitup]

This may be a glaringly obvious question/answer, but I have not yet found a thread that specifically states the obvious, so here we go with a scenario ...

Single garage, measuring 20ft long x 11ft wide x 8ft high --- concrete block walls and concrete floor --- will be building the standard decoupled timber frame as close to the exisiting walls as possible with 2 x 5/8" layers Gypsum all round.

Ceiling and roof construction comprises 6x2 joists, with a "flat roof" (slight pitch) 18mm OSB deck on top of the joists and 2 layers of 5/8" Gypsum fitted to the underside of the OSB (between the joists) to beef it up --- this essentially forms the outer leaf.

Inner leaf ceiling will be 2 x 5/8" Gypsum fitted to a new set of decoupled joists resting on the studwall frames --- thereby leaving an air cavity/space in between those two ceiling/roof leafs of approximately 6" - pretty much the space between the original joists.

For reasons that I won't go into here to avoid an unecessarily long post (I will explain if required by anyone), I can only put a MAXIMUM of 3" of insulation into that 6" space.

My questions are:

1. Is there anything wrong with this?
2. Is it mainly a thermal compromise, or is resonance and/or vibration an issue?
3. If there are any problems, how much of an issue will it be?
4. If it is OK - would it be better to have all 3" lying directly on top of the inner ceiling Gypsum sheets with the 3" air gap above that, or rather 2" lying on top, a 3" air gap and then try to attach 1" to the underside of the beefed up OSB roof deck above if I am making sense?

Thanks for any answers.

Steve

[John Sayers]

3" is ample insulation in the cavity.

[rod gervais]

directly on top of the ceiling is fine.

[turnitup]

Awesome - thanks John and Rod.

Steve