residence is slab on grade 2000 sq ft
attic has 4 inches of fiberglass insulation
no vapor barrier in attic between fiberglass and ceiling sheetrock attached to rafters
will be adding 8" to 10" of blown in cellulose, should vapor barrier be added between existing 4" fiberglass and drywall attached to the rafters, before blowing the cellulose in?
or should no vapor barrier be used?
In a case like this you may be better off adding a vapor barrier on the ceiling. Apply a paint with a low-perm rating. Talk to any paint dealer.
A lot depends too on how cold it gets where you are. If there hasn't been an issue with condensation, it may not be a big factor...
Jim
Where do you live? It makes a difference on where your vapor barrier goes. If you are someplace where you run the heat more than the A/C you want the barrier on the house side. That is where the warm moist air comes from, the attic is cold. If you are in Florida you reverse that. The warm moist air is in the attic most of the time and the house is cold. Water forms in the insulation if you don't have the barrier on the top. It will turn your cellulose into grey mud that eventually just rots to clumps of dust. Want some, I have an attic full. I will mail you some.
Makes sense. Here where our climate is probably similar but somewhat colder than say New England, the rule is that a vapour barrier should be on the warm side of the wall; e.g. plastic sheet under the wall and ceiling plasterboard. And at most not more than one third (33.3%) of the way through the wall from the warm side. But that is for a 'cool' maritime climate where, without a vapour barrier, the danger is house moisture permeating out through the wall and condensing into water within the insulation making it wet, ineffective and with potential for mould and rot!
after reading a lot on this debate, i am thinking as a amateur that no vapor barrier would be better for let say ohio. and maybe every moderate climate area. of course i could be very wrong.
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