Installing recessed lighting in a Celotex insulated flat roof

I'm planning to install recessed lighting (GU - 50W, 8 of them) in the ceiling of a flat roof extension (the ceiling is 12'x18' approx, 200mm joists). The ceiling is to be insulated with Celotex (140mm), but I'm told by Celotex that you should not install recessed lights in a ceiling insulated with Celotex ! The main reasons they give are that it compromises the U value of the roof, since you have to cut holes in the celotex to accomodate the recessed fitting.
I looked at fire-caps to sit over the recessed lights - they help with heat loss and noise, and of course stop potential fires getting into the ceiling too quickly - but again the fire-cap people suggest not to put recessed lights into a Celotex insulated ceiling.
Help ! I see plenty of programs/articles where people have recessed lights in flat roof ceilings - how do they do it ? Do they use Celotex ? How big are the holes they cut in the Celotex ? Do they use something else to insulate the ceiling ? Do you use fire-caps ?
Any ideas much appreciated.
Steve...
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     snipped-for-privacy@pobox.com (SteveRoche) writes:

Well, it's rather a strange thing you seem to be doing. On one hand, trying to insulate your house to conserve energy, and on the other hand using such inefficient lighting you've got loads of waste energy to dump through your house insulation ;-).

Some of them burn down their houses -- seriously. Celotex may not burn well itself, but in a fire I suspect it will give off loads of cyanide, pretty much guaranteeing you won't get out. I would want a good fireproof/heatproof barrier between any cellotex and possible ignition sources. I wouldn't dream of putting recessed 50W spotlamps anywhere near the stuff. If you do, at least make sure you use GU10's, (aluminised) and not GZ10's (dichroic -- cool beam -- which will dump even more heat into the ceiling). I would have suggested low voltage, but I don't think aluminised LV reflector lamps are available any more; I only see dichroic ones. Personally, I wouldn't dream of using large numbers of downlights for general lighting. They are more appropriate to task and accent lighting.
--
Andrew Gabriel

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Andrew Gabriel wrote:

Well, I have done similar. here are some real world facts.
Celotex DOES burn, but does not sustain the reaction. I.e. its not exothermic. It DOES produce a lot of nasty fumes. Cyanide is not one of them tho. I know cuase I have whoffed cyanide in teh chem lab. However I would not care to fill my lungsd with the smoke that coes of celotex when you stick it in a fire. It needs to be almost red hot to do it mind.
I have loads of 50W LV's for general lighting in my ceilings. Some are near celotex. The rest are near rockwool. Proividd you leave a few inches around the,m and make sure they are not near combustble material (right alonsgide joists) and ensure that no combustile materail *thatch, birds nets etc) can drop onto them, they are fine. The best solution I heard here was put an inverted flowerpot over the top Lets face it they have PVC covered wire feeding them, and that burns well...so its no big deal.
I jave mine manly in rockwool, with the insulation pushed back, and the joists covered with chipboard. The chip is warm but not hot above them -0 6" from ceiling.

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I've heard this flowerpot approach before - just a normal plastic flowerpot with plenty of holes in the top ? What size hole do you cut ? 10inch diameter ?
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SteveRoche wrote:

Ceramic flowerpot.
With the hole it comes with.
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What size would you recommend? What about plasterboard? Could you make a plasterboard box. If so how big should it be?
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How many you put in is not the issue - its more to do with HOW you put them in.
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