flat roof insulation

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  #1  
Old 12-23-05, 06:33 PM
mike5432
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flat roof insulation

I have an old house with a flat tar and gravel roof. I found it cold this winter and when I poked some holes in the ceiling, found there was not a piece of insulation. I want to put insulation in the ceiling, but I'm not sure on the venting requirements for a flat roof. I dont have any overhang over the exterior walls. Do I have to vent? or can I just fill the joist cavities with insulation and put a vapor barrior on? I plan to rip the ceiling down as the plaster is all cracked and was damaged from the previous owners, so I will have full access to the ceiling. Any help on this would be greatly appreciated.
 
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  #2  
Old 12-23-05, 07:05 PM
Join Date: Mar 2005
Location: USA
Posts: 6,126
flat roof insulation

If you are going to rip down the ceiling and replace it, you could install r2"rigid foam before the sheetrock. I would also go with a vapor barrier on the warm side. Only R10 - R14, but no infoltration and not thermal short circuits thru the steds like fiberglass, to it will perform similatr to fiberglass insulated ceiling with higher theoretical R-value.

This will not interfere with your present level of ventilation.

You will lose 2" of headroom, but will have a very level ceiling.

Dick
 
  #3  
Old 12-23-05, 07:19 PM
mike5432
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Just to clarify, your saying just put 2"rigid foam insulation with a vapor barrier and then sheetrock overtop of that? I dont have to put any insulation between the joists or install any vents? I just want to make sure so I don't mess it up and end up having to do it again. I live in ontario so it gets pretty cold and I want to make sure that I have the least heat loss possible.
Thanks for the quick reply,
Mike
 
  #4  
Old 12-23-05, 07:30 PM
Join Date: Mar 2005
Location: USA
Posts: 6,126
flat roof insulation

I was assuming you didn't want to file the void between the joists with insulation for fear of stopping what ventilation you currently have.

You can put in fiberglas, but I would not fill it completely and would leave an inch or so open above the insulation.

Since you are in Ontario, you may not have room for enough insulation to satisfy you. There is nothing wrong with using both methods together. In that case I would put the insulation on the bottom of the foam (just before installing the rock).

If you can find some way to ventilate the area above the insulation, I would certainly do it - your roof will last longer too. It seems you can never have too much insulation.


Dick
 
  #5  
Old 12-23-05, 07:40 PM
mike5432
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I currently have no ventilation that I can tell, and the only way I can think of to add some would be to drill holes out the sides of the house and I don't think that will work very well. If I install a good vapour barrier do you think I could skip the ventilation? And sorry to ask so many questions but if I combine the methods of foam and fiberglass, would I put the fiberglass between the joists then the foam on the bottom of the joists then vapour barrier then sheetrock?
Thanks again,
Mike
 
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