DIYbanter - View Single Post - Adding a drop ceiling under a cathedral ceiling ; insulate all theway up ? Do I need a vapor barrier at the roof ?

aemeijers

wrote:
On Dec 5, 11:46 am, wrote:
On Sat, 5 Dec 2009 07:20:31 -0800 (PST), "

wrote:
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
culasort at the underside of the roof ? Your thoughts appreciated.
Thanks.
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.

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.

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.

--
aem sends...