semi cathedral ceilings

I am putting up insulation and I have a knee wall of sorts in my attic. There will be insulation on the partial cathedral ceiling. My question is on the backside of the kneewall... on the opposite side of the vapor barrier... this will be the cold side but will also be an attic. Can I cover the insulation with something so that it isn't hanging loose? plywood? firring strips or straps?
Inside the attic
http://www.geocities.com/guitarage41/knee_wall_cath_attic.JPG
inside the living area or the semi cathedral ceiling
http://www.geocities.com/guitarage41/semi_cathedral.JPG
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M wrote:

Tyvek.
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tyvek won't act as a double vapor barrier in this case?
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M wrote:

It shouldn't - it's not a vapor barrier. In fact, in an attic space you may want something with a lower perm rating so less vapor is driven into the wall from the attic side. Tyvek is ~60. There are other products down in the 5 - 10 perm range. (Products with a perm rating <1 are considered vapor barriers. 4mil poly is rated at .8 perm)
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