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Old 03-15-2014, 05:31 PM   #1
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adding vapour barrier


existing 1 1/2 story is poorly insulated upstairs. i plan on building another wall in the attic and adding an R20. but the original R12 has no vapour barrier. should i peel it out and wrap vapour barrier around the studs, or can i put the vapour between the two layers of insulation?

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Old 03-15-2014, 07:17 PM   #2
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adding vapour barrier


No the vapor barrier goes to the warm side. do not! add it now! or you will rot your roof out. Just add the insulation and call it good.

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Old 03-15-2014, 07:30 PM   #3
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adding vapour barrier


No the vapor barrier goes to the warm side. do not! add it now! or you will rot your roof out. Just add the insulation and call it good.
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Old 03-15-2014, 08:16 PM   #4
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adding vapour barrier


What wall? Are you talking about just putting more insulation in the attic?

Where is the home located?
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Old 03-16-2014, 02:56 PM   #5
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adding vapour barrier


the home is in southern ontario,
i know the vapour barrier is went for the warm side, so i was wondering if it would be ok to put vapour on the back of the existing r12, then build a new knee wall in the attic with an r20. so from inside the room you would have drywall, then r12, then vapour barrier, then r20, and then a tyvek. not really sure if removing the existing insulation and trying to install the vapour barrier around existing studs is worth it. it wont flat or very airtight to the top or bottom plates
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Old 03-20-2014, 12:04 AM   #6
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adding vapour barrier


There is a general 1/3- 2/3 rule that the vb should be in the warm 1/3 part of insulation to prevent condensation. You could do as planned, on the back of the studs/plates, then add 2/3 more insulation. "Vapor barrier, which also acts as an air barrier. The primary function of the vapor barrier is to ensure that moisture from the inside is prevented from entering the structure. The vapor barrier is normally mounted 1/3 inside the thermal insulation layer (seen from the indoor side) to protect it from being perforated." From, pp.2; http://www.google.com/url?sa=t&rct=j...,d.cGU&cad=rja

Rigid foam board is better (without any poly) on the existing wall attic side- may require ignition barrier- check with local AHJ, if looking for for best installation... it will control any cavity condensation- thickness based on location; http://www.buildingscience.com/docum...apor-retarders

Keep in mind, any gaps, rounded corners of insulation will give a reduction of R-value due to convective loops in cavity...

Gary

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