Flat Roof - any benefit to adding insulation on top

Most of my house (located in Toronto, Canada) has a flat roof with a reasonable run off. It was built in the 20's and the roof is BUR with various layers added over the years. Some recent damage got me looking at the roof in preparation for winter. When I peeled back the membrane (looked like old ungranulated torch on), I noticed all the fiber board (cover up) boards were soaking wet. My question: is it worthwhile taking off all the wet fibreboard and replacing it with 1 or 2" polyiso and then recovering? Or should I just replace the fibreboard and recover with granulated torch on or roll on?
Thanks in anticipation.
Jimbo
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Why not look at foam roofing, thats the best way to insulate, at R 7" you can really reduce heating if you add as much as you can. But if there is no parapit wall it would be too ugly. I would think using foamboard under roofing there would be unequal expansion- contraction.
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Is this spray on foam? Can it be applied to the outer roof surface? Does it need a membrane on top or is it waterproof?
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Jimbo wrote:

How much, if any, insulation is on the underside of the roof? I'd do whatever it took to get to at least a minimum recommended R value for your climate.
Obviously, anything that's currently wet needs replacing at minimum and if there's wet where you've looked there's good chance there's damage under that to be repaired as well.
--
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The membrane is alligatored, the cover up fiberboard below that is very wet. Below that is gravel and below that is the original tar and paper which is dry. This is the layer I'll have to get to before putting anything over it, I figure. Cleaning down to that will take a lot of work and a dumpster to get rid of the stuff.
As for insulation in the ceilings: the original ceiling is lath and plaster and there is blown rockwool above that, maybe 4 to 6 inches. The roof void is not properly ventilated but clearly there are gaps around the edges where the roof rests on the brick walls.
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Roofs are expensive and need to be done just once. If the base is still usable, then merely covering it with fibre glass will give you an easily repairable roof good for forty years or more. This roof will be good enough to walk on and will not be effected by any ice dams that may form. 2. Any insulation needs to be laid on top of your lathes and plaster ceiling, where it will give you maximum saving in heat, and therefore money. Sprayed foam will find its way into every odd shape and hole. It is a tight fit, no holes equal best heat saving. Also you can stand things on foam without it deforming and loosing its insulation properties, five inches should do the trick. Leave the loft cold, unless you live in it. Perry
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I've contacted a number of well known roofing companies around Toronto asking for estimates to repair the existing roof. From what I'm reading here (and eleswhere), the best bang for my buck would be to make good the existing roof by overllaying it with membrane and then put more insulation into the attic space?
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One more piece of information I have received which may be useful: because the house is so old and the living space is not sealed, a warm roof approach was suggested. This entails sealing the envelope everywhere possible with caulk and foam, and adding insulation (up to R50) on top of the the roof itself. By doing this, heat within the house is retained (preventing ice dams, etc) in winter and solar gain in summer is also reduced.
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A roof covered with spray on foam is good in areas that are very hot in the summer. In cold areas it doesn't work as the target has to be keeping the heat in the living area of the home - there is no financial point in letting the loft get warm.
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In my case, the loft is an unvented space with some insulation that is 8" high at the shallow end and about 12" high at the other end. My thought was to include this in the living space.
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