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Old 03-15-2011, 08:42 AM   #1
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Old paper-backed insulation = Vapour Barrier?


From what I've heard, on old paper-backed insulation (R8?) the paper acts as a vapour barrier. Here's my dilemma:

I am redoing an old house but I don't want to tear apart the attic because the current insulation (paper-back+blown) is below floor boards in about 1/2 the space.

In the other parts, I had to pull up boards to drop wires and vents. I would like to use a thick vapour barrier rated paint on the ceilings to ensure this barrier is continuous.

Will the paper-backed insulation cause a problem due to a "double-vapour barrrier" or is this not an efficient vapour barrier in the first place?


On another note, when I blow more insulation into the attic when I am done, I know to install the channel vents, but do you also put a small barrier to prevent the blown insulation from falling onto the soffit around the channel vent?

Thank you.

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Old 03-15-2011, 11:01 AM   #2
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Old paper-backed insulation = Vapour Barrier?


If the paper is laminated w/ asphalt, it has a perm of 0.3 (Class I vapor retarder), but it is leaky. VR paint has a perm of about 0.45. I don't think you'll have a double vapor barrier problem because of the proximity of each, but I could be wrong. Both should be at the same temp, w/ only sheet rock between them, so they should not condense. Yes, block the blown from escaping.

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Old 03-15-2011, 01:39 PM   #3
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Old paper-backed insulation = Vapour Barrier?


I think that is what it is. It is paper like a brown paper bag, with the bottom of the lower paper black (doesn't really increase the thickness at all).

Thanks for the help.
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Old 03-15-2011, 08:38 PM   #4
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Old paper-backed insulation = Vapour Barrier?


Read the % solids content on the v.b. paints, not all are the same; http://www.mde.com/publications/Jan07News.pdf

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