Dual vapor barrier within wall assembly

[Mr. H.]

I have a home where the walls are constructed as follows from the inside out: gypsum board, plastic sheeting, 2x4 studs with blown in cellulose insulation, asphalt impregnated fiberboard exterior sheathing with Masonite siding. The homeowner wants to remove the Masonite siding, repair any damaged fiberboard sheathing, add a layer of OSB sheathing with a moisture barrier, and install new vinyl siding. The addition of the OSB sheathing is because he doesn't like the idea that someone can literally cut their way into his home with a box cutter which I don't blame him. If we remove the fiberboard sheathing then replacement of the the insulation would be required as well as the blown in cellulose is retained inside the wall cavity by the fiberboard sheathing and the plastic sheeting on the inside of the wall. I am worried that if we add the OSB sheathing with an additional moisture barrier on the exterior of the wall assembly we could get moisture issues within the wall cavities but I also don't like the idea of not adding a moisture barrier behind vinyl siding either. The homeowner resides in Charlotte, NC which experiences relatively mild winters and warm humid summers. I was curious to everyone's thought on whether the new proposed wall assembly consisting of gypsum, plastic sheeting, 2x4 studs with blown in cellulose, fiberboard sheathing, OSB sheathing, moisture barrier, and vinyl siding would present any potential for trapping moisture in the wall cavity? If so what if anything could be done to mitigate the potential for this to occur?

 

[Enable]

Oh fun! I got you.

Good on you for recognizing the problems with locking vapor into the stud cavity. Here is a primer on vapor retarders that is highly recommended but you are correct that you do not want a class 1 or class 2 vapor retarder on both sides of an assembly.

If you have the option of moving the vapor control layer to the exterior face that is what I would recommend for the climate you're in. Charlotte appears to be a cooling dominated climate and in cooling dominated environments it should be on the exterior (since the vapor drive is primarily from exterior to interior). That would entail eliminating the poly sheeting from the inside in its entirety.

If you cannot move the vapor control layer to the exterior and have it only on the exterior (either because the project doesn't entail re-doing the entire wall, it would be annoying with the blown-in insulation that you have, or because local building codes have ripped off Canadian building science and misapplied it...happens a lot) then leave the vapor control layer as is, and use a vapor permeable membrane on the OSB such as Blueskin VP160.

Blueskin VP160 will act as your water resisting barrier but allow the passage of vapor via diffusion. To mitigate issues of your vapor control being poorly placed, specify the Blueskin to be sealed appropriately at all transitions (and laps) with BES925 to make it your air barrier as well. Less chance of hot humid air getting into the cavity that way.

 

[Enable]

By the way if you don't want a true WRB because it's a home and not a commercial job, then you can always use typical house-wrap. A combination of OSB + house-wrap + vinyl siding is considered vapor semi-permeable (just please do not do something stupid like seal the joints...vinyl is actually a class 1 vapor retarder but in the case of siding air movement allows transport of vapor through the joints so the assembly is dubbed semi-permeable. If the vapor must pass via diffusion alone the assembly will act as a vapor barrier).

Still leaves you with your vapor retarder on the wrong side of the assembly but that would be no different than what is there now.

 

[Mr. H.]

Removing the vapor barrier on the inside portion of the wall is cost prohibitive so that is not really an option. Sounds like the Blueskin VP160 is a good way to go. Do you see any issues with installing the OSB overtop the existing fiberboard sheathing?

 

[phamENG]

Don't expect a quantifiable increase in LFRS strength or stiffness. And make sure you increase the fastener length by the thickness of the old sheathing to get minimum penitration for wind suction.

 

[Enable]

Pfftt phamENG we are trying to have a building science conversation over here don't ruin it with your structural wisdom just because this is a structural sub-forum! But yeah, listen to phamENG he's right about the fasteners.

I would have few concerns with the OSB overtop the fiberboard provided that the contractor does a good job on the WRB. Asphalt impregnated fiberboard sheathing is vapor permeable so even if the assembly gets nominally wet (and it will get wet) it can dry via diffusion since the OSB / VP160 are vapor semi-permeable as well. If the WRB leaks like a civ you would have a problem.

In light of the above, I'm not sure what protocols for vinyl install are in your neck of the woods but I would surely strap it overtop the OSB / WRB to back vent it. And have the strapping sealed with BES925 either at fastener locations or more likely - since they'll simply not do that and tell you they did - have them seal the entire back. This way all penetrations through the WRB are sealed (it's supposed to be self-healing(ish) like its Blueskin SA big brother but those are moderate lies) and vapor can diffuse to the strapping plane and be taken away via air movement through joints in the vinyl.