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Community and Q&A

When to Use a Vapor Barrier

callumshill | Posted in General Questions on

Here is yet another post asking about vapor barriers. This is for a full remodel in Zone 4C(Vancouver island) The roof is torch down with 6” of EPS foam over a peel and stick vapor barrier on ½”ply. Then 10” of dense pack cellulose. The upper walls are 5” (2×4 with 2×2 horizontal strapping) of cellulose then ½” ply with taped seams and Siga Majvest. That’s covered with 3” of Comfortboard 80. The ground floor was framed with 2×8 so that’s 8” of cellulose plus the 3” of CB80. The stem walls are 8” thick with 1.5 of foam on the inside and 3” of CB80 on the outside. Lastly I’ll have the walls completed with ½” drywall.
With all that being said do I need a vapor barrier or ‘smart’ barrier or can I just use the dense pack mesh?
Thanks in advance.

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  1. Expert Member


    Practically, all your assemblies will work just fine without a vapour-barrier. However from a code perspective the BCBC wants a vapour-barrier installed "sufficiently close to the warm side of the insulation to prevent condensation at design conditions." So if you are getting things inspected you may be required to either use a smart vapour-barrier, or a vapour-barrier paint.

    With the unusually high insulation levels you are proposing to use in zone 4C, your house should be both very comfortable and very, very cheap to heat.

  2. callumshill | | #2

    Hi Malcom, that's been the plan. I've lived in too many places where I was cold. I've seen where either you or Akos had weighed in on the topic so I wanted to see your options were. It's a reno so I'm not sure if my inspector will want to see a barrier. I'll be sure to check with him before I get to that stage.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      There are all sorts of metrics discussed here for what levels of insulation makes sense, which are very useful. But if you are comfortable going a bit beyond what's usually done, there is no downside.

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