Adding a drop ceiling under a cathedral ceiling ; insulate all the way up ? Do I need a vapor barrier at the roof ?


Ive put in a layin 2x4' tile ceiling under what was a cathedral ceiling for my 24x16' living room . The heat gain to the living room was cut dramatically last summer here in Florida . I have a space above the drop ceiling about 3.5' feet in the center tapering down as you move away from center . Is it absolutely imperative that I insulate (using batt insulation) the entire space above up to the underside of the roof ? And, must i put in a vapor barrier of some sort at the underside of the roof ? Your thoughts appreciated. Thanks.
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Normal commercial lay in ceilings have the insulation on the ceiling tile. You can buy it as 2x4 batts, faced or unfaced. Typical would be 6" faced with the VB against the tile (toward the conditioned space).
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Vapor barrier goes on the warm side of the framing, erto, it shoul dbe udner your 2x4' tile. No need to go all the way up with insulation, 9" should be enough.
dunno about that dead space you created, a professional might want to ventilate it somehow.
Harry K
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On Sat, 5 Dec 2009 07:20:31 -0800 (PST), " snipped-for-privacy@gmail.com"

Just bear in mind that high temps and moisture up there will shorten the life of roofing. Make sure there is air circulating from the eaves to the ridge to help keep roof temps down.
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On Dec 5, 11:46 am, snipped-for-privacy@dog.com wrote:

Thanks all for the responses. The space I have above this false ceiling is naturally quite small, so, can i go with a very small fan to just circulate the air within that space , or is it necessary to be vented outdoors ? TY.
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On Sat, 5 Dec 2009 13:29:27 -0800 (PST), " snipped-for-privacy@gmail.com"

You need cooler outside air entering at the lowest point and exiting at the highest point all the way from one end to the other.
A fan just blowing air around inside the space will do nothing except add the heat of the fan motor.
No fan should be necessary if the roof was constructed properly with soffit vents and a ridge vent.
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snipped-for-privacy@gmail.com wrote:

When you say 'cathedral ceiling', do you mean traditional T&G with exposed beams, and little or no insulation above, or a drywall cathedral with scissor trusses or something? I'm wanting to understand why the place got so hot on you. Is there a big window on sunward end and it was just greenhousing or something? Too late now, but I'm not sure you took the best approach to the problem. If it is a traditional exposed-wood cathedral with little or no insulation above, you have now created a superheated dead space that will radiate down into the living space all night, much like an insufficently ventilated ranch house attic. Without some sort of air exchange to outside, an insulated ceiling will buffer the heat at best. And like the others said, a hot roof will not last as long. At this point, you have an attic, and need to invent some way to vent it like one, in a way that won't make your drop ceiling rattle when the wind kicks up.
Methinks a re-roof on the cathedral part, with about 4 inches of high density foam above the roof deck, and some heavy tint on any big windows, might have worked better. Your problem is a common one for churches and 'california modern' flying-wing houses built back when energy was cheap.
Standard disclaimer- I'm no expert, but helped build a lot of cathedral ceiling houses as a kid. And I know first-hand what a difference improving the venting of the attic made on this northern ranch. You aren't soaked in sweat in 30 seconds when you go up there on a summer day now, and the ceiling isn't warm to the touch at midnight anymore.
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