Clarify insulation

[Ed209]

Posted yesterday re floating Celotex floor and was advised concrete,50mm insulation, polythene barrier, then chipboard floor, Could someone please explain the reason and how it works.
Next scenario I want to 100mm Celotex my garage roof between the rafters. So do I put Celotex tight up against the existing board that the Roofing felt is on the top of, I will be left with approximately 120mm gap between the joints after I put on the plaster board ceiling which I can the sink my led down lights in to, If so far this is correct and I have silver foiled taped any gaps and joins n Celotex to the sides of the rafters do I still need to put a moisture barrier in before fixing the plaster board?
Will I get moisture build up in the void between Plaster board and Celotex as it could come through cut outs for down lights.
Sorry if this is all a bit anal but trying to get my head around insulation and moisture build up
Paul


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[MikeG.]

In your roof/ ceiling, the insulation should be hard down on top of your ceiling, leaving airflow above it. If you have airflow above then there is no real need for a vapour barrier, but if you were to want to fit one then foil backed plasterboard is fine, or, in a workshop/ garage you might consider OSB in the place of plasterboard, in which case it is virtually vapour impermeable anyway. Frankly if you have so much room in the depth of your rafters I would consider the much cheaper alternative of mineral wool ("fibreglass") at double the depth. It really is much easier to fit as well.

No-one reading this should apply the same advice to a house. An intermittently occupied and heated space is a very different thing.

 

[Ed209]

If I use mineral wool do I still need a 50mm air gap above it or just pack it in ?


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[MikeG.]

You should keep a 50mm gap.

 

[Ed209]

Think I have it now
50mm gap above 75mm Celotex then I will have a gap of approximately 40mm then the vcl and plasterboard, Which I can then sink my low depth LED lights in to, according to Kingfisher site this is acceptable method when you need to put downlighting in but I am open to comments regarding the suitability of this


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[Ed209]

What I did
Put Roofing batons full length on rafters for the 75mm Celotex to but up to leaving a 50mm air gap above, same full lengths under Celotex to hold it up, I have left the Celotex approximately 40mm short from butting up against the wall to provide air flow, the other end is open in to the other half of my garage.
I have the plasterboard ceiling 12mm that is going to be skimmed and painted with Bedec paint along with the 18mm ply walls. This is leaving a 40mm void between the top of the plasterboard and bottom of Celotex, That is enough room for the LED down-lights this void will also be vented.
Don’t know if this is correct time will tell.
I know it’s not 100% airtight but that could be a good thing, Us humans do need to be able to breath.
Ps I am going to put some vents in ceiling to allow airflow above it.


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