Replies

1. 
   freyr_design | Apr 17, 2024 03:53pm | #1

   This is a pretty typical assembly, and if installed dry shouldn’t have issues. That being said, you could install a vapor diffusion vent on outer plywood layer that could allow drying of the polyiso and plywood. This would consist of ripping back the plywood at peak a couple inches and installing a high perm peel stick, over 30 I would suggest, over gap, then using a ridge vent. I would probably do this if it were my roof, though I’m not sure it would be needed, just a little extra peace of mind.

1. Expert Member
   MALCOLM TAYLOR | Apr 17, 2024 05:21pm | #2

   freyr_design,

   The trade off in adding a diffusion port is that while it may provide an outlet for excess moisture that gets into the foam, it also makes that more likely to happen by interrupting both the underlayment and roofing.

   My feeling is: in for a penny, in for a pound. It's either an un-vented roof with the weaknesses and benefits that brings, or a vented one.

1. 
   freyr_design | Apr 17, 2024 06:43pm | #3

   I disagree. There is a lot of evidence that this works very well. This has been used to fix sip roof ridge rot failures as well dehumidify entire attic spaces. It is not really like interrupting your underlayment as you will still have a membrane that will shed bulk water. This provides insurance while negating the downsides of simply having a ridge vent. Especially if the climate of the op, I don’t think there is really a downside.

1. Expert Member
   MALCOLM TAYLOR | Apr 17, 2024 07:02pm | #4

   freyr-design,

   I miss-read the stack up and thought the I&WS was directly under the roofing, which I didn't see the point of interrupting. You are right, there seems little downside to adding the vapour-diffusion port.

2. Expert Member
   Akos | Apr 17, 2024 08:02pm | #5

   For similar roofs with exterior insulation and metal roofing, I've used permeable underlayment for the top layer. This allows for a bit of drying capacity in case anything gets into the foam layer. The cost premium of permeable roofing underlayment is pretty minimal. There is not much point for the peel and stick on the top layer except where your local codes require it.

3. 
   DamionL | Apr 18, 2024 04:03pm | #6

   Freyer_design, I think this is good peace of mind, especially since I do have a metal ridge beam, so hopefully any condensation that may occur due to the steel beam will diffuse out.

   To implement the diffusion port I will do the following:

   1. Keep a 2 inch opening (approx 1/150 of 1900sqft roof) where the plywood meets at the ridge on both the structural sheathing and the upper layer plywood sheathing
   2. Install vaproshield slopesheild SA approx 24 inches wide over the ridge and over the I&W underlayment on both the structural sheathing and upper layer plywood sheathing. Overlap by 4"

   I still plan to run the insulation tight at the ridge and spray foam any gaps where the insulation meets at the ridge, but the polyiso states it has permeability of <1 perm (polyiso with fiber facer) and no matter how well you spray foam the gaps at the ridge, I'm sure there will be some inevitable gaps.

   Does this seem like a good idea or should I just put the vapor diffusion port at the upper layer of plywood sheathing and keep the vapor barrier at the structural roofdeck?

   Akos, I agree but I would just be concerned about having warranty issues down the line and having the roof manufacturer blame my choice of underlayment vs their preferred brand. Although I'm sure at the ridge I should have no issues if I run the vapor diffusion port.

1. 
   freyr_design | Apr 18, 2024 05:18pm | #7

   I would not do a diffuse port on your structural sheathing layer, this will introduce interior moisture into your assembly with a poor drying path if you continue your polyiso over it. Just do it on exterior layer sheathing and keep your ice and water continuous on the lower.