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Community and Q&A

Two vapour retarders back to back

Erik Olofsson | Posted in General Questions on

Hi everyone,
The plan on our project was to use our 5/8″ t+g osb as the air+vapour retarder. it has acoustic sealant on all panel edges and where it touches all framing members. the bc building code does not explicitly state panels can be used as the vapour retarder but it does allow for vapour retarding paint. that said, the inspector reallllllly doesn’t like the idea of not using poly. the ceiling has an uninsulated 2×8 service core with the t+g on the exterior side. we plan to insulate to about r80 with fibreglass or cellulose.

Can I lay down some 6-mil poly on the exterior side of the OSB to appease the inspector so he won’t give me any grief about using the vapour retarding paint everywhere else? (i.e. the poly would lay flat on the OSB between and in direct contact with both the OSB and the attic insulation…

thanks again..

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I lay down some 6-mil poly on the exterior side of the OSB to appease the inspector so he won't give me any grief about using the vapour retarding paint everywhere else?"

    A. Yes, but the attitude of the building inspector is unfortunate. OSB is a "smart retarder" with variable permeance -- exactly what you want in this application.

    The poly is a dumb vapor barrier with a fixed (very low) permeance, and it will be on the wrong side of your insulation during the summer. If you house is air conditioned, it would be better to have the OSB than the OSB plus poly.

    That said -- sometimes we have to do stupid things, especially when the person ordering us to do the stupid thing is a figure of authority backed up by the laws of the land. In most cases, the poly won't cause any problems in this location.

  2. Erik Olofsson | | #2

    thanks martin. the project is in northern bc, there is no air conditioning and about 5600 heat degree days. the most current edition of the code code is dated 2006 and the previous codebook was dated 1998. i was using the older edition. unfortunately, and hopefully some gba readers can prove me wrong, the 1998 edition allows for four different 'vapour barrier materials': poly, membrane type vb other than poly, vb paint, and panel products. it appears the 2006 edition only allows three, with the panel type products absent from the list....

  3. Erik Olofsson | | #3

    martin, i spend hours reading gba content (just ask my girlfriend) but i'm still not clear on one thing and i hope you can take a minute and explain it to me like i'm a 6 year old: why are 'smart' vapour retarders a good thing? why is it good that a VR becomes more vapour open the wetter it gets?

  4. GBA Editor
    Martin Holladay | | #4

    In general, the sheathing on most conventional wood-framed walls takes on moisture during the winter. Fortunately, the sheathing usually dries out quickly in the spring. Some types of siding -- especially stucco -- retard drying to the exterior, however. During the spring or summer, if the sun comes out and heats up the siding after a rainstorm, the sun will drive the moisture from the damp siding and sheathing inward.

    If this moisture hits poly -- especially cold poly in an air conditioned house -- the moisture will condense and drip down to the bottom plate. Now the whole wall assembly is damp, but it can't dry to the interior.

    If the wall has a "smart retarder" on the interior, the smart retarder will take on moisture under these circumstances. Once it's damp, the material's permeance rises, and it allows vapor to diffuse to the interior of the home. That way the wall can dry to the interior.

  5. Expert Member
    Dana Dorsett | | #5

    Poly on the exterior of an OSB sheathed house in northern B.C. is a recipe for mold and rot no matter how air-tight the interior. If you MUST use poly per order of the code officials, it needs to be on the interior.

    Smart vapor retarders work, but they're not nearly as cheap as poly which is why they don't get used as often. They are not a complete panacea- the high humidity in a bathroom with a shower in a cold climate would let significantly more moisture to find it's way to the sheathing than vapor-retardent paints would.

  6. Erik Olofsson | | #6

    thanks for your reply, dana. are you suggesting vapour would pass more easily through the osb but not the poly? why would this be an issue if the poly is on the warm (in winter) side of the insulation?

  7. Expert Member
    Malcolm Taylor | | #7

    Erik, here is a link to the changes that occurred when the 2006 code was adopted. There is a section on vapour Barrier Materials.

    Vapour Barrier Materials (
    Vapour barriers are now defined as those materials with a vapour permeance of less than 60 ng/(Pa• s • m2), changed from 45 ng/(Pa• s • m2) in the previous code. Six mil polyethylene (with a permeance of about 3 ng/(Pa• s • m2) is not the only acceptable vapour barrier as a variety of materials meet this criteria, including:
    • extruded polystyrene (generally greater than one inch (25mm) thick, check product specifications)
    • vapour barrier paint about 26 ng/(Pa• s • m2)
    • foil-faced polyurethane which has a
    negligible vapour permeance, and
    • materials such as plywood and OSB which
    are also low permeance, but there is an exception for these materials because of proven performance — plywood permeance increases with moisture content so the assembly is self-correcting (see
    Membranes other than polyethylene must con- form to Canadian General Standards Board (CGSB) Standard 51.33, and polyethylene used as a vapour barrier must conform to CGSB 51.34 (polyethylene standard). Where paints applied on drywall are used as the vapour barrier, the permeance of the coating must be determined in accordance with CGSB-1.501-M

  8. GBA Editor
    Martin Holladay | | #8

    It looks to me as if Dana misunderstood. He thought that the OSB was on the exterior side of the insulation, when in fact your OSB is an air barrier installed on the interior side of the insulation (Katrin-Klingenberg-style).

  9. Expert Member
    Dana Dorsett | | #9

    Yes, I thought the OSB was (far more common) exterior sheathing, not an interior surface.

  10. Floris Keverling Buisman | | #10

    short term high humidity in a bathrooms will not effect the performance of a smart/intelligent vapor retarders in winter conditions - especially not membranes like INTELLO that I import.

    However, if there is an indoor pool, or other uses with permanent high humidity then yes, you need a class I fixed vapor retarder on the interior - and the exterior of the assembly needs to be sufficiently vapor open in that case.

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