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
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...
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.
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
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!
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