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Old 12-03-2010, 09:21 PM   #16
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The After-roof roof project


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Not exactly prevent. But it keeps the water vapor inside from reaching the cool underside of the sheathing where condensation would occur. Open cell lets the vapor through.

Water coming the other way through the roof will find it's way in past either type foam.
ah i see what you are saying now...thanks for the input. what about if i drywall it up after installing the foam? that way the vapor will hit the ceiling/drywall and not travel through the foam

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Old 12-07-2010, 03:07 PM   #17
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The After-roof roof project


Drywall can be an air barrier though never a vapor barrier. ADA the drywall: http://www.buildingscience.com/docum...wall-approach/

Find your location: http://www.buildingscience.com/docum...ecommendations

“In Zones 1 through 3, open cell (vapor permeable) foam can be used successfully to enclose building spaces with moderate or dry interior humidities. In colder zones (Zone 4 and 5) open cell foam will require additional vapor control on the interior (in the form of spray applied vapor retarding paint). In cold zones (Zone 6 and higher) a closed cell foam is generally required. Dense cellulose can control convective loops, and provides significant amounts of safe storage capacity. However, in Zones 4 and higher, additional vapor diffusion and airleakage control (in the form of vapor retarding paint coatings and sealed drywall respectively) are required. In higher interior humidity spaces, closed cell foam or other roof designs are recommended.” http://www.buildingscience.com/docum...on?full_view=1

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Old 12-07-2010, 07:10 PM   #18
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The After-roof roof project


I thought it was best to use open cell under sheathing.I'm not a foamologist though
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Old 12-10-2010, 01:18 AM   #19
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The After-roof roof project


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Originally Posted by GBR in WA View Post
Drywall can be an air barrier though never a vapor barrier. ADA the drywall: http://www.buildingscience.com/docum...wall-approach/

Find your location: http://www.buildingscience.com/docum...ecommendations

In Zones 1 through 3, open cell (vapor permeable) foam can be used successfully to enclose building spaces with moderate or dry interior humidities. In colder zones (Zone 4 and 5) open cell foam will require additional vapor control on the interior (in the form of spray applied vapor retarding paint). In cold zones (Zone 6 and higher) a closed cell foam is generally required. Dense cellulose can control convective loops, and provides significant amounts of safe storage capacity. However, in Zones 4 and higher, additional vapor diffusion and airleakage control (in the form of vapor retarding paint coatings and sealed drywall respectively) are required. In higher interior humidity spaces, closed cell foam or other roof designs are recommended. http://www.buildingscience.com/docum...on?full_view=1

Gary
thanks for the link gary. so i do need more than the drywall. time to slap some more paint on!
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Old 12-15-2010, 10:43 AM   #20
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The After-roof roof project


In addition to the closed cell foam, I would also use 6 mil poly sheeting behind the drywall as an additional vapor barrier.
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Old 12-16-2010, 01:42 AM   #21
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thanks for the input-so you are against the open cell too? may i ask your reasoning?
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Old 12-16-2010, 07:55 AM   #22
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It's all about moisture. I think you must do what you can to prevent water vapor from migrating through to the cold underside of your decking, where it will condense. If allowed to get through, the bottom of the decking will be constantly wet and will deteriorate in short order. Freezing and thawing cycles of the condensed vapor could also compound problems. As seeyou mentioned, open cell foam, by it's nature, allows this.

On the other hand, closed cell foam, poly vapor barrier and well sealed drywall are all deterrents to this migration. The foam must completely fill the rafter bay between the decking and the drywall (i.e. no air spaces). Use of exhaust fans for cooking and bathing areas is also essential. With an unvented roof, some would even go as far as using an air-to-air heat exchanger to control excess humidity in the entire house, but i think that's a bit over the top.

P.S. What was your other thread--I would love to see the pics!
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Old 01-05-2011, 10:53 PM   #23
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It's all about moisture. I think you must do what you can to prevent water vapor from migrating through to the cold underside of your decking, where it will condense. If allowed to get through, the bottom of the decking will be constantly wet and will deteriorate in short order. Freezing and thawing cycles of the condensed vapor could also compound problems. As seeyou mentioned, open cell foam, by it's nature, allows this.

On the other hand, closed cell foam, poly vapor barrier and well sealed drywall are all deterrents to this migration. The foam must completely fill the rafter bay between the decking and the drywall (i.e. no air spaces). Use of exhaust fans for cooking and bathing areas is also essential. With an unvented roof, some would even go as far as using an air-to-air heat exchanger to control excess humidity in the entire house, but i think that's a bit over the top.

P.S. What was your other thread--I would love to see the pics!
well isnt the whole point of doing spray foam under the decking it to make teh attic part of the conditioned space? therefore whatever water vapor is floating about will be part of the entire house, not 'travel' through the room into the attic as they are now one whole continous space. does this make sense? or am i missing something crucial

i think GBR already gave me a good solution...i am going to use high quality paint on the drywall to prevent anything from passing through

Last edited by federer; 01-05-2011 at 10:56 PM.
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Old 01-05-2011, 10:55 PM   #24
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heres my original thread-lots of pics
My Flat roof project-help!
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Old 01-06-2011, 10:31 AM   #25
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well isnt the whole point of doing spray foam under the decking it to make teh attic part of the conditioned space? therefore whatever water vapor is floating about will be part of the entire house, not 'travel' through the room into the attic as they are now one whole continous space. does this make sense? or am i missing something crucial

i think GBR already gave me a good solution...i am going to use high quality paint on the drywall to prevent anything from passing through
Yes, that would be the point of doing the spray foam but, is your attic truly part of the conditioned space? i.e. does it have heater vents and cold air returns? I couldn't imagine that for storing Christmas decorations! Seems it would be a dead air space that collects moisture and give it no escape route.

From your pics, it looks like your ceiling is already insulated with foil faced fiberglass, and it is installed upside down! The foil facing is designed to be a vapor retarder and needs to be on the warm in winter side. As it is now, it will trap moisture within the fiberglass, reducing it's effectiveness and promote mold and decay.

IMHO, I would flip the foil insulation so it is installed correctly and add more unfaced insulation to level with the top of the joists. And cover that duct work too, it's got to be losing heat! Just don't block air passage to the soffits. Then cut in soffit vents and, ideally, add a ridge vent. If it's too late for a ridge vent, then gable vents would be a second, albeit somewhat less effective, option. They make solar powered gable vents now and one or two of those, properly sized, should work well with the soffit vents.

http://www.google.com/search?client=...UTF-8&oe=UTF-8

Nix the spray foam in the rafter bays, you don't need, or want, insulation or vapor barriers in both places. Probably save a ton of money too, even after buying the gable fans. As a bonus, this approach will leave the underside of the decking visible for detecting future leaks in the roof, addressing one of your previous concerns.

If possible, I would still add the 6 mil poly tho. It's cheap at less than a nickel per square foot, and more effective as a vapor barrier than the insulation's facing or paint. Remember, it too needs to be on the warm in winter side. Presuming the ceiling already has drywall, you can weave the poly up and down the joists as you flip the insulation.
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Old 01-07-2011, 01:21 AM   #26
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The After-roof roof project


thanks for the response man! now this is interesting...you are right the attic is not really part of the conditioned space. its just an crappy attic. the only problem is though since i am stuck with teh flat roof, the space between the ceiling and the rafter bay is like less than 2ft. so i really cant crawl up there and flip the insulation and add more. because originally i was going to do fiberglass spray, but due to the low height the roofers said it wouldnt be worth it, and thats why i went to the spray foam method.

now why did you say
Nix the spray foam in the rafter bays, you don't need, or want, insulation or vapor barriers in both places. ?? isnt that the whole point is to insulate under the decking in the rafter bays?

and where are you proposing to add the poly sheet? i dont quite understand what you mean by weaving it up and down the joists...sorry if i am missing something.
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Old 01-07-2011, 12:00 PM   #27
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thanks for the response man! now this is interesting...you are right the attic is not really part of the conditioned space. its just an crappy attic. the only problem is though since i am stuck with teh flat roof, the space between the ceiling and the rafter bay is like less than 2ft. so i really cant crawl up there and flip the insulation and add more. because originally i was going to do fiberglass spray, but due to the low height the roofers said it wouldnt be worth it, and thats why i went to the spray foam method.

now why did you say
Nix the spray foam in the rafter bays, you don't need, or want, insulation or vapor barriers in both places. ?? isnt that the whole point is to insulate under the decking in the rafter bays?

and where are you proposing to add the poly sheet? i dont quite understand what you mean by weaving it up and down the joists...sorry if i am missing something.
At that height, the attic sounds pretty useless. I just don't see any sense in spending the time and money to insulate an unusable space! I think you should concentrate your efforts where they will do the most good. Additionally, as the closed cell spray foam makes an excellent vapor barrier at rafter thickness, any moisture that migrates into the attic from the living spaces below, and some will no matter what you do, will be trapped in the unvented attic. It may not condense on the spray foam, but the rafters themselves, being a poor insulator, may become cold enough to cause condensation. The heat and humidity of the summer months could also turn your attic into a veritable sauna as well. As the humidity level rises so does the dew point. If the humidity level in the dead air space is high enough, you could experience condensation in the summer months!

The whole point is really to insulate your living spaces from your non-living spaces. If you are set on the spray foam, why not yank out the fiberglass and have them spray it in the joist bays, directly on top of the ceiling below, where it would do the most good? You'll still need to allow for ventilation with soffit vents and a ridge vent, or gable vents set as high as possible on the end walls. If you leave a dead air space you will collect moisture.

As far as the poly, ideally it would go directly behind the drywall. You could yank out the existing ceiling, insulate with proper thickness insulation oriented the correct way (insulate the duct work too), then staple the poly to the joists and re-drywall. Or, staple the poly to the existing ceiling and add a new layer of drywall over it using extra long screws. In any case, if you leave the fiberglass as is, go up there with a utility knife and slash the foil facing side to side every six to twelve inches to defeat the vapor retarder.

By weaving the poly you start at the top of the first joist, down the inside face of the joist, across the bottom of the bay (backside of ceiling), up and over the second joist and down the second joist and across the next bay and so forth... reinstalling the fiberglass as you go. In the pics, it looks as though the soffits are open. That should give you more access to work. If you don't use the attic, any additional insulation you may add can be higher than the joists.

Don't get me wrong, spray foam is a great product and perfect for walls and unvented cathedral ceilings, but rather expensive, about $10 per sq ft around here for my 4.25" thick application.

Sorry to be so long winded but I just hate to see someone spend extreme cash on a project and have problems with it down the road. Believe me, I've had my share of shoulda's, coulda's and woulda's!
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Old 01-15-2011, 03:38 PM   #28
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At that height, the attic sounds pretty useless. I just don't see any sense in spending the time and money to insulate an unusable space! I think you should concentrate your efforts where they will do the most good. Additionally, as the closed cell spray foam makes an excellent vapor barrier at rafter thickness, any moisture that migrates into the attic from the living spaces below, and some will no matter what you do, will be trapped in the unvented attic. It may not condense on the spray foam, but the rafters themselves, being a poor insulator, may become cold enough to cause condensation. The heat and humidity of the summer months could also turn your attic into a veritable sauna as well. As the humidity level rises so does the dew point. If the humidity level in the dead air space is high enough, you could experience condensation in the summer months!

The whole point is really to insulate your living spaces from your non-living spaces. If you are set on the spray foam, why not yank out the fiberglass and have them spray it in the joist bays, directly on top of the ceiling below, where it would do the most good? You'll still need to allow for ventilation with soffit vents and a ridge vent, or gable vents set as high as possible on the end walls. If you leave a dead air space you will collect moisture.

As far as the poly, ideally it would go directly behind the drywall. You could yank out the existing ceiling, insulate with proper thickness insulation oriented the correct way (insulate the duct work too), then staple the poly to the joists and re-drywall. Or, staple the poly to the existing ceiling and add a new layer of drywall over it using extra long screws. In any case, if you leave the fiberglass as is, go up there with a utility knife and slash the foil facing side to side every six to twelve inches to defeat the vapor retarder.

By weaving the poly you start at the top of the first joist, down the inside face of the joist, across the bottom of the bay (backside of ceiling), up and over the second joist and down the second joist and across the next bay and so forth... reinstalling the fiberglass as you go. In the pics, it looks as though the soffits are open. That should give you more access to work. If you don't use the attic, any additional insulation you may add can be higher than the joists.

Don't get me wrong, spray foam is a great product and perfect for walls and unvented cathedral ceilings, but rather expensive, about $10 per sq ft around here for my 4.25" thick application.

Sorry to be so long winded but I just hate to see someone spend extreme cash on a project and have problems with it down the road. Believe me, I've had my share of shoulda's, coulda's and woulda's!
good stuff! thanks for the detailed reply. i see what you are saying now. i dont think i explained myself as clear as i could have.

so although the attic space between the ceiling and the decking of the roof is only about 2ft, the actual attic is like 12ft or whatever because my house is originally a cape cod and the guy finished out the attic, plus he added a addition. so essentially i am living in the attic space. does this make sense?

so by doing the spray foam to the underside of the decking i am sealing it completely as part of the conditioned space. because as you pointed out the whole purpose is to seal it from living to none living space. in fact i did some demoing today so i got pics to illustrate.

also, i dont have any venting at all in my roof-i closed off all the soffit vents, and i dont have any box vents or ridge vents or anything. thats also another reason why i am doing spray foam-because i am doing the new style of unvented roof.

which also means i cant spray it on top of the ceiling.

NOW
my only issue is from the pic you can see there is about a good 18inches of space from the decking down to the framing for the current ceiling. so after the spray foam, which is only about 6inches, that will leave a 'dead space'. so how should i put the new ceiling up?
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Old 01-15-2011, 03:47 PM   #29
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Old 01-15-2011, 03:48 PM   #30
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