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Discussion Starter #1
I have a rough-plank gable roof attic space over an old flat tar roof. There is a cement chimney along a now-enclosed wall of this space. I use it for storage. I know I need to insulate the access door, and am wondering if that is all I should do. If I insulate the rafters of this space, how would the heat from the masonry chimney(which I use all winter) affect the humidity---ie: would this create more of a problem to insulate the space? There are no pipes or vents going through to worry about, and the ceiling under the tar roof is insulated, although somewhat minimally. (The house was built in 1933).
 

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There should not be any heat from that chimney, or at least minimal heat. I am assuming that the chimney has a flue liner and is surrounded with block or brick??? But do not insulate the roof rafters, that will usually cause moisture build up on the underside fo the sheating and rot out the sheathing.
 

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Is the original flat roof insulated and now the floor of the new attic? If so wouldn't insulating above in the rafters will create a heat trap?
YOU COULD STILL INSPECT THE GABLE AREA TO SEE IF IT IS PROPERLY VENTED
 

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I am under the impression that you have a building that at one time had a flat roof and that some one built a gabled roof over it. If the flat roof is still intact, that is a problem. It is creating a vapor barrier at the wrong place, and a very effective one at that. You should slice that roof membrane every 3 inches or so if there is insulation somewhere under it. Your vapor retarder should be as close to the warm in winter space as possible. If it is constructed as I am picturing, you have the makings for a condensation issue.
 

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Discussion Starter #5
the should-i-insulate attic...

thanks for your replies----There is heat---the chimney acts as a heat sink all winter, and gives off warmth. It may not have a flue liner. The floor was a sealed roof, and has been insulated and vapor-barriered before putting sheetrock over the compressed sugar-cane stuff that took the place of sheetrock before the war.... The rafters are vented by the rough nature of the roof job---rough-cut wood planks that shrank as the wood dried. There is an asphalt-shingle roof over that. I'm beginning to think that I should just make sure this space is well insulated/isolated from the living space upstairs....The idea of cutting swaths in the floor kindof freaks me out--It has several layers of tar on it....
phew--this is still a conundrum. any other comments out there?
 

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The ventilation you need is not between the planks of the roof boards. It is to the outside. You should have ventilation at the eaves and the peak directly to the outside. With an old asphalt roof on top and a vapor barrier below, you have a sealed envelope on your floor. Any moisture inside of that envelope will migrate to the top and condense on the underside of the old built up roof and then drip back into the insulation, the deck boards and elsewhere. The only saving grace may be that the chimney is emitting enough heat into the attic space to keep the old flat roof above dew point. It is unlikely that it is doing that everywhere across the attic floor. The fact that you are heating all winter tells me that you live in a place where temperatures easily fall to a level where condensation is certain. In my opinion, that flat, built up roof needs to go and you need to vent your attic space. When those things are accomplished, you can add insulation without fear of creating any issues.
 

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Discussion Starter #7
thanks again for your comments....yes, i live where it usually rains all summer and rains or snows all winter. I'm afraid removing the old flat roof is not an option, at least not at this stage of my budget....It sounds like I have two options: insulate the attic roof with soffit and peak ventilation, sealing the attic space totally.....but then how does the heat from the chimney escape? There is a window that I could crack when I'm using the chimney....
The other probably most realistic option is to leave it in its present state and seal the space off from the living area. Then the chimney and the present more-than-ample ventilation can just continue to heat and cool as they are....
 

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You certainly DO NOT want to insulate the roof rafters. That will trap warm moist air in the attic space and lead to all kinds of condensation issues. Your insulation needs to be on the attic floor. Use a linoleum knife to slice the old flat roof about every three inches, so it can breath. Then you can add insulation on top of it without the fear of an intermediate vapor barrier creating condensation issues within your insulation system.
 

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Actually a lot of newer attics have the roof rafters insulated
They use rafter vents & ridge vents - just like a cathedral ceiling
Most are then finished w/sheetrock
Many of our friends who have bought newer houses have the attics finished in this manner
Proper ventialtion is key

Usually older houses are not sealed as tight as newer houses
I've seen a number of houses where a lower roof had a new higher roiof placed over it
My MIL's house has this in a few areas
In each case part of the lower roof was removed for ventilation
 

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Using the proper system of vents and insulation, I agree one could insulate between the rafters, but not with an old built up flat roof and insulation under it in the equation. You can't turn the attic into a thermos bottle, having a vapor retarder in the wrong place, without the risk of causing a condensation issue somewhere within the system. The attic either needs to become part of the heated living space or vented and thermally isolated from the rest of the structure.
 
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