Plaster as vapor barrier


Old 08-23-09, 10:50 AM
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Join Date: Jul 2008
Location: canada
Posts: 345
Plaster as vapor barrier

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My old house [ Ontario Canada] has a flat roof and no attic, only a small space about 18''tall and no access to it.
The ceiling has some kind of 1'' fiberboard about 1/2'' cement and 1/2 plaster but no insulation[see pic]

Im doing some repairs and electrical work,I have open some holes in the ceiling and have limited access to this space.

I like to add some insulation where ever possible and wondering if the plaster on the ceiling makes a good enough vapor barrier to install some rock wool or fiberglass without condensation problems
Blown in ins is not an option for now
Thank you
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Old 08-24-09, 08:21 AM
Join Date: Nov 2001
Location: USA
Posts: 1,873
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

This website discusses vapor diffusion retarders and it states that all paint retards moisture flow. The several layers of paint on your ceiling clearly qualifies as a vapor barrier. The only concern would be penetrations like the electrical work your doing. As well as other penetrations in the ceiling like plumbing, heating pipes, cables and wires.
Old 08-24-09, 09:51 PM
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Location: canada
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Thank you for reply and link.
Old 08-25-09, 06:15 PM
airman.1994's Avatar
Join Date: Dec 2005
Location: VA
Posts: 5,632
Paint and plaster do not have a high enough perm rating to be a VB

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