Any reason to do vapor vent with 2″ exterior continuous (CZ2)
Hip roof in Austin TX. Converting an unconditioned attic, with ridge vents but no soffit vents) to a unvented, conditioned attic. Roof top to bottom will be:
1. Standing seam metal
2. 2″ polyiso (hoping for recycled, we’ll see)
3. Air barrier (probably peel and stick)
4. Existing sheathing (1940s 1x4s)
5. Batt insulation in rafters (probably two layers)
Bird-blocking at the rafters under the sheathing will be air-sealed inside (foam gun) and outside (caulk).
Is there any reason to include a vapor vent here? It wouldn’t be code required (because the ~R10 of the exterior continuous is high enough percentage), but on the other hand it is pretty simple to do (and the metal ridge cap already vents, so it is just the sheathing cuts and taping in a high perm strip).
Useful?
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Do you mean a vapor diffusion vent? Regular ridge vents are also primarily there to remove water vapor but the details are different.
In either case I don't see a need for one in your situation.
James,
It's a trade-off between what is perhaps a bit more resiliency, against creating a hole on your exterior insulation layer. I'd opt for the insulation.
Thanks. Yes, I meant vapor diffusion vents.
Thinking was mostly humidity removal from the attic space than worrying about condensation (since the exterior insulation will keep sheathing warm enough). But I guess that conditioning up there should take care of that (and it's not living space anyway, just storage and equipment space).
And yes, Malcolm, you would lose the 2" of insulation along that "cut" in the insulation. I don't think any vapor permeable insulation would be permeable enough for above a vapor diffusion port? Although TimberHP lists their batt insulation as 46 perm-inch (although I have no idea how to calculate what the perms of 2" would be, anyone know?)
Anyway, thinking about this more there are two points to a vapor diffussion port:
1. Avoiding condensation and adsorption of moisture into sheathing at the ridge. Not relevant here as sheathing will be above dewpoint.
2. Passive dehumidification. Certainly needed in this climate zone, but dehu provided mechanically and likely not worth interrupting the exterior continuous insulation.
James,
I don't know much about vapour-diffusion ports, but my understanding is they are designed to remove moisture in permeable insulation that accumulates at the peak. That's a different ask than trying to dehumidify an attic, with a considerably larger volume of both air and moisture. I'd be interested in people's opinions on whether they could be effective doing that.
Calculating permeance at different thicknesses involves calculus I have long forgotten, but a very rough rule of thumb is that if you double the thickness you cut the permeance in half. So very roughly, 2" of TimberHP batts is probably around 23 perms.
That's vapor-open but still probably not enough for vapor diffusion ports. I haven't paid close attention to requirements for them since they don't work in my climate zone, but I recall seeing Joe Lstiburek, Kohta Ueno or both present on the topic and stressed the need for highly vapor-open material, as in ±200 perms.
Thanks! Looks like 20 perms is the number: https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
I have an uninsulated shed built with one here and the passive dehumidification is actually sort of amazing.
But for a continuous exterior insulation application doesn’t seem worthwhile adding.
Does your whole-home dehumidifier you mention have a connection to the attic space with returns or supply ducts? The conditioned attic space needs to be conditioned.
Interesting that it's only 20 perms. I know when they started presenting on the topic 8-10 years ago they wanted super-high perms but testing must have shown that it wasn't necessary.
I was checking out TimberHP's new builders guide: https://www.timberhp.com/guide-for-builders (developed in collab with Joe Lstiburek).
I noticed that they do show a vapor diffusion port, when they have air permeable with exterior CI (in this case timber board).
Attached pic is on Page 56 of the PDF.
Also curious to me that they include a 1/6" inch "Drainage Gap" between the continuous exterior insulation and the roof decking. Odd to see that "in the middle" of the insulation stack.
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