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Vapor Diffusion Ports

Mike_Fritz | Posted in Expert Exchange Q&A on

I live in Aiken, South Carolina.  I’m building a shop connected to my existing garage and over it.  Imagine adding a two car garage space to an existing three car garage, using the addition down stairs for heavy tools, and having a 60 foot long upstairs with cathedral cealing for lighter tools and hand tool work = dream shop.  The big question is what approach I should take for the insulation plan.  First constraint is I can’t put rigid insulation on top of the roof sheathing.  It would mess up the roof line with the remainder of the existing connected house, and the wife/CFO won’t allow that.

The existing garage has active soffit vents and a long ridge vent, and 2×6 rafters.  Plan A is to extend the rafters to create a 2×12 space then keep 2″open for an air channel up against the roof sheathing, then 2″ rigid foam boards cut in 14″ width and edges spray foamed for a tight air seal, then 2 layers of 3.5″ R15 Rockwool for 11″ and R40 total.  At the 9 foot cealing height I would bring this same set of layers across, leaving a small hot space above this triangular insulation envelope which would extend all the way down the rafters to the floor.

Plan B that my contractor and his insulation sub would like me to do is spray foam open cell into the existing rafter bays.  They would add 2x2s to create a 2×8 space and fill that just up to the edge so they don’t need to saw off any excess foam.  The foam would be applied directly to the sheathing to convert the space into an unvented attic.  They made no mention of a vapor diffusion port, so if we went this route I would request that modification to the existing active ridge vent.  They argued that 7 inches of OC would be just as effective as the other approach.

Assuming both plans cost about the same, but plan A involves an extensive amount of my labor.  What should I do?  This is a shop, not traditional living space.

Small detail if we went plan B, could we use the existing active ridge vent as is, or why do we need to add a vapor permeable layer?

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Replies

  1. James Howison | | #1

    Check this out and see if it applies (I think you are probably CZ3)

    https://www.buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned

    If it does then it's something like: you convert your ridge vent to a vapor diffusion port, close the soffit vents, remove/notch/lower any blocking across the bays, and fill the whole rafter space with rockwool (or other air permeable fluffy stuff) (but read the article for process). Seems easier than the cut/cobble you discuss.

  2. Mike_Fritz | | #2

    Didn't realize the cut and cobble approach had an actual name, and a bad rap.

    So if I went with the open cell against the roof sheathing, which also has many critics, but maybe is the lesser of two evils in my case. The big question might be to reduce the possibility of failed roof sheathing down the road would you a) provide an AC supply and return vent in that area, b) create a vapor diffusion port, or c) do both? Trying to figure out what will do the best job to reduce my possibility of a major roof sheathing failure.

    Thanks for your advice...

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