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Old 12-14-2007, 11:12 AM   #1
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Unventilated Flat Roof


I've got the simplest and cheap flat roof system - 2x8 wood rafters 16" o.c. frame grid, 7/16 OSB decking, ISO board layer - varies from nil to 3", roof membrane. This is a new reroof - total tearoff to the rafters. Sure wish I could redo that reroof.

All joist/rafter cavities are unvented - no practical way to change that.
All cavities have batts - the tearoff beat them up too with roof chunks.
Some cavities have a flimsy plastic VB on the bottom & some don't.
Mostly dropped ceilings. Is this a "perfect storm" for condensation under the deck?

During the tearoff, I noticed lots of rotted decking right atop large areas where the batts looked clean. Leaks, it seems, usually add some darkish color to the batts - especially when not detected right away. This must mean that condensation was what destroyed my old deck and not just leaks. Now, I fear this new reroof is going to be a repeat performance within 5 years if no corrective measures are taken.

I'm tempted to clean and stuff the cavities with faced batts and lay sheetrock with 2+ coats of oil paint for a VB and seal around all cracks to stop air leaks. Rather not use plastic VB because it will catch water if I ever do get a leak later and make the leak take longer to detect.
I have real doubts about doing it this way. Even with no leaks, some moisture will eventually get into a batt filled cavity and then have no way to get out - and then rot the deck in 5 yrs.

Closed cell spray foam would solve this but creates a new problem - when the membrane does eventually leak, the foam will make it take alot longer to detect the leak. Open cell foam lets leaks thru but that it might require some venting or a heavy, $$ layer is needed to stop moisture from hitting the deck.

The best way to fix this seems to be to redo the reroof - toss out all batts, leave the cavities empty and beef up the ISO layer on top of the deck - maybe 4.5 or even 6 inches. Cold & snowy here.
The contractor would not be too happy about that.

Any other way to do this without sacrifice down the road?

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Old 12-16-2007, 02:33 AM   #2
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Unventilated Flat Roof


you need the vapor barrier attached to your attic/cathedral space insulation to prevent condensation,any additional batts should be unfaced w raftermate or an airspace between the insulation and the decking,sometimes you don`t have enough insulkation and it causes condensation,sometimes too much can do it because of no breathing,you mau be able to vent soffits,and fabricate a vent at the wall,closed cell foam is great stuff,but you really should post some pics regarding the setup ,roof pitch/condition etc. as well as where you live and avg. weather conditions for that area so we can tell you better-you need different things in different areas---I personally would prefer the insulation in the ceiling cavity as well as the insulation on the roof,w/the insulation on the roof only,you move the condensation point to the wood sheathing(osb) which will rot out quickly as a result

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Old 12-16-2007, 06:15 PM   #3
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Unventilated Flat Roof


How hard would building a pitched regular roof over this stuff be ?? In OUR area flat roofs are a CONSTANT headache,pitched shingled roofs are good for 20-30 years!! I had a flat roofed building I got MAD at and threw a pickup load of lumber up there and just banged away till it was 4-12 or better. The OLD flat roof had soffet vents on each side,,,does yours?? You would be ALOT better off with a regular roof than a FLAT one any day!!! MY whole roof leans back the degree of slope but who cares and who can see that???
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Old 12-16-2007, 10:51 PM   #4
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Unventilated Flat Roof


that wasn`t the mans question there Don,most shingle roofs last longer than flat granted,but you can also have an IB roofing system warranted for the life of the owner for the flat even if it ponds water,most diy`ers don`t have the knowhow to build another pitch on top of theirs,I would like to see pics of yours though !
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Old 12-18-2007, 09:01 AM   #5
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Unventilated Flat Roof


justdon - yup, pitched is better than flat but I already made the wrong decision and reroofed with a supposedly "premium" type of flat roof. A flat roof can be made right, methinks, but it comes down to designing it right before construction, not after. No soffits & no venting here.
The only way to vent would be to use those silver dollar shaped vents and drill holes through the structure on each side of the joist cavity. The problem is there are a few rooms to go through before reaching the outside. The air would to travel through a series of cavities all with those small inlet/oulet vents and get too restricted to do anything worthwhile. <see pic>
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Old 12-18-2007, 09:25 AM   #6
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Unventilated Flat Roof


roof god - this flat, unvented roof is located in SE Michigan,. Zone 5, 7000 hdd. See prev post with pic and reason why putting in vents is next to impossible. I kind of like your idea of a combo warm deck/cold deck roof with rigid above and vented batts below the deck. I wish I could do that.

In a warm deck (ISO on top, nothing below deck) where would the moisture be coming from that condenses on top of the deck? The interior? I wonder how much the OSB breathes vapor. Maybe sealing the OSB might help.

I'm about ready to sandbag this project - stuff batts, seal and paint. Its not allowed by code but I'm amazed how many contractors told me to do it this way.
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Old 12-18-2007, 10:03 AM   #7
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Hey A Warning For You,your Pic Shows Mold At The Joists,that`s A Big Problem That Has To Be Dealt With 1st,major Potential For Health Problems There,yes The Heat Comes From The Interior,lwithout A Vapor Barrier In Place It Would Condense On The Plywood,lwhich Is Probably What`s Creating That Yellow Growth Of Mold-condensation Occurs Where A Warm Front Of Air Meets A Cold Front
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Old 12-18-2007, 10:34 AM   #8
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Unventilated Flat Roof


that yellow growth is actually bits of old fibreglass insulation. There used to be a room size thin layer of fibreglass between the joists and the deck - the whole structure had this fibreglass "gasket" between it and the deck.
The contractor left it there when he redecked and now I'm pulling what I can.

There have been lots of mold problems here before reroofing - the whole old deck was a mold pile or completely gone - asphalt layer directly on joists in some spots. The only reason it didnt cave in was that there were three old roofs put on before I tore them all off. The thickness of the old roofs sort of became the decking.

Latest best way to build an unvented flat roof: 1. roof membrane 2.thick ISO substrate 3. deck/joists with no insulation below 4. VB - maybe just drywall with a few coats of oil paint and seal all air leaks. This method can only be done when reroofing.

All other post reroofing remedies fall way short of above standard and that includes spray foam under the deck. Spray foam, though it has minuses, is the only insulation that can be used in an unvented rafter assembly.
With batts, even if well sealed, some moisture will eventually find its way in there and then never get out.

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