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Old 03-13-2013, 10:29 PM   #1
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Vapor barrier or not on sloped ceiling wall


I have a sloped ceiling wall (picture attached) that has roughly 5 inches of clearance for insulation. I am taking down the drywall to add baffles down to the sofit. It is currently packed with insulation. I will then add fiberglass bats over top the the baffles. My question is do I put a plastic vapor barrier up even with the low amount of insulation above it? I will be putting the vapor barrier in-between the drywall and rafter boards.

Note the rest of the attic ceiling will have no vapor barrier, just the slanted ceiling walls.

I am located in WI
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Old 03-13-2013, 11:26 PM   #2
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Vapor barrier or not on sloped ceiling wall


Scott please go back and add your location to you profile, not just mention it in your post. Just go to quick links.
Five inches would be about 1/2 of the insulation called for in your area if you go with batts.
May want to concider have it spray foamed.
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Old 03-14-2013, 08:51 AM   #3
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Vapor barrier or not on sloped ceiling wall


When you say overtop the baffles, you are meaning that the baffles will be tight to the underside of the roof deck and you will be adding insulation to the drywall floor (i.e. warm side ceiling) up to the underside?

Joe is right that while that might be proper in terms of a venting standpoint, it will not give you the thermal performance you are looking for out of that assembly.

As long as the venting is preserved, you condensation and moisture issues should be nil, but it will still be ice cold.
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Old 03-14-2013, 09:03 AM   #4
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Vapor barrier or not on sloped ceiling wall


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When you say overtop the baffles, you are meaning that the baffles will be tight to the underside of the roof deck and you will be adding insulation to the drywall floor (i.e. warm side ceiling) up to the underside?
Yes the baffles will be tight to the roof deck. I'm not real concerned with the insulation value since what I will be adding will be an improvement on what is already there. The house is warm now. I am just correcting the venting issues to remedy some mold growth in the attic. My main question is should I use a plastic vapor barrier?
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Old 03-14-2013, 09:13 AM   #5
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Vapor barrier or not on sloped ceiling wall


The baffles are unnecessary. you need to have a "hot" roof.

as far as insulation I would be using closed cell which would act as a vapor barrier and give you max insulation value.
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Old 03-14-2013, 09:16 AM   #6
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Vapor barrier or not on sloped ceiling wall


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The baffles are unnecessary. you need to have a "hot" roof.

as far as insulation I would be using closed cell which would act as a vapor barrier and give you max insulation value.
How would the attic be ventilated if there are no baffles to the soffit? The reason the attic has mold growth is because someone plugged the path to the soffits.
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Old 03-14-2013, 09:25 AM   #7
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Vapor barrier or not on sloped ceiling wall


If you sprayed the roof deck in that section like framer mentioned, you could install some shingle over vents or some vent hats above where the spray foam stops.

If you aren't worried about the R-value there, you are on the right track.
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Old 03-14-2013, 09:25 AM   #8
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Vapor barrier or not on sloped ceiling wall


In terms of vapor barrier, I don't think you need it. Just make sure the drywall and top plates are sealed up well.
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Old 03-14-2013, 09:59 AM   #9
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Vapor barrier or not on sloped ceiling wall


if the thermal performance isn't an issue, can you elaborate a bit more on the mold issues you are having. The fiberglass should allow circulation if there is no pressure boundary sealing off the top or bottom of the bay. Very good possibility you need to airseal or add a ventilation source rather then address this roof slope.
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Old 03-19-2013, 12:56 AM   #10
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Vapor barrier or not on sloped ceiling wall


The continuous baffles will supply any new/existing soffit vent air to the upper attic. Use asphalt paper-faced f.g. or "MemBrain" for your Zone 6, 7 required vapor barrier;

"In addition to an air barrier at the ceiling line, a Class II vapor retarder (see sidebar) should be installed in Climate Zones 6 or higher (see Map 1).
Class I vapor retarders (i.e. vapor barriers – see sidebar) can be installed in vented attic assemblies in Climate Zones 6 or higher (see Map 1) but should be avoided in other climate zones as top side condensation can occur in summer months during air conditioning periods.
No interior attic assembly side vapor control is required or recommended in climate zones other than Climate Zones 6 or higher (see Map 1) for vented attic assemblies (note the distinction, this is not the case for unvented attic assemblies as will be discussed later). With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation." From; http://www.buildingscience.com/docum...on?full_view=1


IMO, the f.g. won't have much, nor need/want, air pressure to move any diffused moisture coming through the drywall ceiling. Block the lower end of the f.g.(sloped ceiling) at the knee wall to prevent any air movement through it. Your heat/moisture will flow up to the level top attic ceiling line anyway, I doubt diffusion at the slope. Diffusion is minimal compared to air leaks;



ADA the drywall when installing, caulk the joints before abutting against air/moisture through the paper tape at the ceiling/wall planes; http://www.buildingscience.com/docum...wall-approach/


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