Air/water sealing vapor diffusion port
What options do we have to build this at this point to maintain the fire resistance and seal the attic space from water and air (which is a requirement of a VPD as I understand)?
1 Should we have the roofer apply a vapor permeable roofing membrane on the outside of the Densglass over the ridge and seal it to the rest of the roofing membrane with flashing tape (see marked up detail)? Any specific product recommendations for the membrane and flashing tape?
2. If we use an ember resistant continuous ridge vent (such as Vulcan vent) with a vapor permeable membrane this should give us the fire resistance. We could remove the Densglass but I wonder if it can be left in place? If both the membrane and the Densglass Gold are 20 perms – will there be enough vapor permeability if both the Densglass and the roofing membrane are both on top of each other?
3) For the roof over the garage, we will still need to have an HVAC vent into the attic space to make the vapor diffusion port work despite the garage below not being conditioned.
4) Does anyone have any recommendations for companies in the San Diego area which might have experience with building ventilated attics with vapor diffusion ports in this manner and may be able to consult with us/our GC?
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Your diffusion port looks a bit small, but a photo is hard to gauge. I would just double check your NFA calc is correct. You cannot just leave it open to air and install a Vulcan vent, this is mixing two different strategies and will not work for a number of reasons. Georgia pacific sells a liquid flash for densglass called dens defy. This is what I would use.
Appreciate the reply. My thinking was to use the Vulcan vent (for ember protection which is what the Densglass offers over other vapor permeable materials), remove (or leave) the Densglass, and then seal a vapor permeable membrane over the 4 inches in the center of the ridge.
Here are the calcs of the diffusion port. Let me know if there is a miscalculation somewhere.
I think the Vulcan vent solution will be way more expensive. NFA seems good, I would just go with a liquid flash if it were me.
You also don’t technically need a Vulcan vent for your ridge vent though, wui allows for 1/8” metal mesh for ridge vents. I think your dens solution will be the most fire resistant and affordable.
Your first detail seemed fine, but I suppose the execution was the problem? One needs to make sure the VDP is airtight (and water tight) which you already mention. At this point, I would just wrap a vapor open membrane taped from the sheathing, up and over the existing densglass and then taped to the sheathing on the opposite side. It will be plenty vapor open, especially if you use a product like Siga Majvest or Delta Dry (there are plenty of other options, but these are upwards of 50 perms +). I would then attach all the 2x in the drawings over the top of this. No need for vulcan vents, although for a VDP I don't see that it would harm anything besides your wallet. I'm also not sure it would do anything from a fire perspective since the roof isn't vented. You may be aware, but good info is here from BSC: https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
Thank you for looking at it. Removing the 2x in the drawing will be difficult and very labor intensive since they are nailed into the trusses from the outside of the roof and into the sheathing from the underside of the roof.
If you look closely (hard to tell in the photo), but in the current condition the 2x are currently nailed into the trusses and covered with the roof membrane.
Is there a way to do what you mention without removing the 2x and just attaching the vapor open membrane to what is already in place now?
It looks like the roof membrane is mechanically attached, yes? If one was to just attached a vapor open membrane over the top of this, one could get minor air leakage. That being said, one has to weigh the pros/cons and consider the spectrum of risk. I get that it is labor intensive and expensive to re-do. It is likely fine and if you do a nice job with the airtightness of the home and have a good HVAC system installed chances are it will be fine.