Downlights in 50mm thick plasterboard

[Kel]

Morning all,
I’ve been off the tools for the last 20 years but have kept myself up to date with the regs and what’s going on in general, however I am struggling with this and wondered if any more up to date sparkles can help.
It is my own kitchen extension which has a vaulted ceiling, the ceiling joists have 11mm OSB on the underside and then a 12mm plasterboard with 25mm Celotex insulation. The plasterer is coming to fix and skim the plaster boards in the next couple of weeks and I intend fitting downlights into the ceiling.
What I can’t get my head around is how the downlights will fix into the plasterboard given the depth, which I am assuming will be too much for the spring fixings to hold correctly.
I have thought of cutting out a 100mm hole in the OSB at each downlight position, then when the plasterboard is fixed and skimmed, cutting out my normal size hole for the down light and removing the Celotex from around the hole so the springs will be clamping onto just the 12mm plasterboard.
Are there any downlights available that have a different fixing to the normal springs, or has anyone else had this problem and what did you do?
Thanks Kel

 

[telectrix]

i would do as you suggest. cut away the celotex (150mm hole). low depth downlights such as JCC V50 only need 50mm above.

 

[Baddegg]

As above and as you suggest...

 

[GeorgeCooke]

Building control won't like you removing the Celotex.

 

[telectrix]

Building control won't like you removing the Celotex.

Sod 'em. what they don't see won't hurt them.

 

[Baddegg]

Building control won't like you removing the Celotex.

Ask them how to do it then.....they won’t have a clue...

 

[telectrix]

was once asked to quote (labour only) to fit 320 downlights in a barn conversion. 6" celotex on top of all ceilings, boarded and skimmed. dog-rough 1" holes cut for cables which were on top of the celotex and below UFH. i quoted £7500. wonder why he didn't phone me back.

 

[Midwest]

Sod 'em. what they don't see won't hurt them.

Does tend to vary according to the inspector. One vaulted ceiling with down lights I did for a customer, we had to batten out out the vaulted ceiling to accommodate the down lights. Another wasn’t too concerned, as he said the fitted Velux windows would reduce the U value more than a cut out for a couple of down lights.

I tend to ask, cos it’s too late after chopping the Celotex.

 

[Kel]

Thanks all for the replies