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2 questions about SilveRboard xs graphite as cathedral vent… Can I perforate more on my own? Leave edges leaky?

BuildingFun | Posted in General Questions on

Excuse my other thread on cathedral ventilations, I’ve decided to move forward with using SilveRboard XS graphite on this job and have two quick questions related specifically to this SilveRboard XS graphite (micro perforated foil on both sides) 1.25″ insulation board….

1. I’m hoping I wont have condensation on the interior side of this foam board which is being used as baffles, but want a back up plan in case some moisture does accumilate.  This SilveRboard xs is micro perforated and claims a perm rating of roughly 2 at 1.25″ roughly R5.   Is there any downside to adding extra perforations on both foil sides to promote slightly better moister transfer?  I have a manual perforating roller I could run on both sides quickly to give more area for moisture to pass through.  Bad idea?

2. Since this SilveRboard foam board is only being used as a air channel/insulation baffle, is it safe to assume there is no point in fully air sealing it to the ceiling joists?  I cant imagine anyone else on the planet would even consider fully air sealing insulation baffling, but if anyone did it, it would be someone on this site!  haha.  I figure leaving it a little “leaky” between the board and the joists would simply allow any moisture to escape quickly and easily, especially this high in the roof assembly.  Thoughts?  My inner drywall layer will be impeccably air sealed.
Thanks for your time.

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  1. Expert Member
    DCcontrarian | | #1

    The problem you're fighting in a roof is moist air from the interior of the house getting up against the sheathing when it's cold and condensing, which leads to rot and mold. There are three ways to fight this:

    One way is to put insulation over the sheathing so it doesn't get cold.

    One way is to put a thick enough layer of impermeable insulation on the bottom of the sheathing so that it's not cold enough on the bottom of the insulation for moisture to condense. Since it's really important that this layer be impermeable it has to be spray foam.

    One way is to allow air to circulate between the insulation and the sheathing and carry the moisture away.

    I think you're trying to do the third method. I say "I think" because sometimes people try to do the second method using foam board insulation. In that case you want the insulation to be tightly sealed. Even then it's not a recommended method because it's hard to get a reliable enough seal.

    In the third method you want moisture to exit the insulation freely, so sealing the foam board seems like a bad idea. If the board is 2 perms that's pretty permeable, I don't actually know how permeable it needs to be.

    In all cases, the tighter you can make the ceiling drywall, the better.

  2. BuildingFun | | #2

    Thanks for the reply!

    The ceiling drywall is completely air tight.
    There is R30 rockwool above the drywall and under the SilveRboard baffling. I'm concerned about condensation where the rockwool meets the SilveRboard and just want to ensure it can escape outward. I'm not sure the perm rating of 2(ish) is enough to allow moisture to escape quickly, which is why I'm leaning towards NOT air ceiling the baffling. I can't see a reason to seal the baffles to the ceiling joists other than slightly minimizing the very minimal potential for windwash of the top of the R30 rockwool...which quite frankly, I could care less about as long as I dont have to worry about moisture. I'm just wondering if leaving the foamboard baffles unsealed can somehow increase moisture in my ceiling assembly, but I just don't see how it can. I suppose a little cold air leaking past the baffle and hitting the warm rockwool could cause some condensation? Even then, it seems like it would rapidly dry outward during the warmer day temps? Maybe I'm missing something?

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