Can I put vapour barrier over joists?

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  #1  
Old 02-23-03, 11:55 AM
Chute
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Can I put vapour barrier over joists?

I have purchased an older house between 50 and 100 years and Im planning to add blown insulation to the attic. The problem I am faced with is there is not a vapour barrier between the drywall and the attic joists. Therefore if I wanted to add a vapour barrier I would need to pull up the existing insulation and run the vapour barrier inside each joist as well as over each joist, keeping it as tight as possible to the surfaces. My question is would covering the joists cause any problems with moisture being trapped inside the wood? The climate I live in is a mixed climate with the winters hitting lows of -30c/-22F and in the summer highs of 30c/86f

Another question I have is how should I go about sealing off light fixtures to prevent any air from the house escaping into the attic? Mostly concerned with what material will not be susceptible to heat build up resulting in a fire.
Thanks for any input.
Ed
 
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Old 02-23-03, 03:37 PM
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http://www.eere.energy.gov/consumeri...riefs/bd4.html This is a Dept. of Energy brief on Vapor Diffuser Retarders (VDR) or if you prefer vapor barriers and Air Barriers.

It discusses the questions you are asking. If the insulation that is already down is the batt type, more than likely a vapor barrier is already there. If not, I would not recommend removing it just to add a vapor barrier. If you read the brief, several layers of paint qualifies as a vapor barrier. You could also use a vapor barrier paint on the ceiling below the attic, if you like.

The brief gives less importance to vapor barriers than the air barrier. This has to do with what you have concerns about with light fixtures leaking air into the attic. The most common product used to seal these areas is spray foam, which you can buy usually at any home improvement store.
 
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Old 02-24-03, 03:07 PM
Chute
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Thanks

Thanks for the info, I will take a look at that site.
 
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