# Vapor barrier needed?



## jklingel (Dec 21, 2008)

A vapor barrier is not required in Canada, but use one if you must. Talk to whoever is going to inspect it. I would use the airtight drywall approach with vapor retarder paint and not put in a vb of polyethlyene. If whoever pays the bills wants a vb and has the cash, or the inspector insists, apparently Membrain is a good product and less susceptible to the problems of poly vb's. I'd also mention putting better insulation in the walls while you are at it. Roxul or cellulose.


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## bcbud3 (Jan 22, 2010)

A correction to my opening description. Under the exterior siding there is actually tar paper not the cheap plastic stuff. No one will be inspecting the final wall except me. I am just trying to prevent drafts and not cause any rot to my siding. What do you mean by the vapor retarder paint approach? Do you mean throw some roxul in, slap board over it and paint it finsihed? Side question, why is it bad to use 2x6 insulation in a 2x4 wall?


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## jklingel (Dec 21, 2008)

bcbud3 said:


> A correction to my opening description. Under the exterior siding there is actually tar paper... •• Felt paper is good stuff. Is there a gap (rain screen) between the siding and the sheathing?) The gap is very helpful.
> 
> I am just trying to prevent drafts and not cause any rot to my siding. What do you mean by the vapor retarder paint approach? •• Google "airtight drywall approach". It requires detailed air sealing at all penetrations, edges of drywall, etc, and a vapor retarding paint. Your best bet to prevent problems is to air seal. Vapor diffusion is minor. Vapor traveling w/ leaks is major. Greenbuildingadvisor.com and/or buildingscience.com have tons of info on all this.
> 
> ...


 See after the bullets.


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## bcbud3 (Jan 22, 2010)

The felt paper is stapled to the studs and the cedar siding nailed directly over top.


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## jklingel (Dec 21, 2008)

I did not see any mention of sheathing. None? So the insulation will rest against the felt paper? Hmmm. That sounds a bit risky, but if you aren't going to take the siding off and it has worked so far, then so be it.


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## bcbud3 (Jan 22, 2010)

Correct, there is no sheathing. I don't know why. There was no rot or any wet insulation. I just don't want to introduce any problems


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## bcbud3 (Jan 22, 2010)

anyone have an idea or opinion? Should i put a plastic vapor barrier?


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## jklingel (Dec 21, 2008)

again, no. use membrain.


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## Gary in WA (Mar 11, 2009)

Don't trust Jk, eh? LOL..... He is correct. Without sheathing your cavities are wind-washed, fiberglass is next to useless. Because of the ventilated cavity, a poly v.b. will work with f.g. batts, but the R-value would be way, way less than stated. Back-prime the lap siding (with cavity open from inside) as it wicks water, be sure the bottom drip edge is painted as well. Use Roxul against wind-washing as stated. Here are test results on our area if any doubt, remember you are ventilated w. no sheathing to stop mpoisture and make mold: http://www.ewpa.com/Archive/2006/aug/Paper_296.pdf

The polyamide-6 (smart vapor retarder- MemBrain) in a test: http://eec.ucdavis.edu/ACEEE/2008/data/papers/1_8.pdf

http://www.bestofbuildingscience.com/pdf/Brainy%20membrane%20HEM_23-4_p37-40.pdf

Gary


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## bcbud3 (Jan 22, 2010)

Not that i don't trust, i just value more than one opinion. What do you mean by back prime the lap-siding (i'm not familiar with the terminology being used). Thanks


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