Inadvertent vapor barrier?
I live in MN and have a 12’x16′ porch addition that I’m about to insulate the vaulted ceiling. I’m calling the porch a 3-1/2 season porch since I’m installing electric base board heat which allows me to heat the room, but only when I intend to use the room, which may not be that often in the dead of winter. The scissor trusses are just 4/12 exterior pitch with a 2/12 vaulted ceiling and 7″ heel height at the wall line, so not a lot of room to meet R value requirements here.
My thought to insulate the ceiling was to install 2″ thick XPS foam boards cut to fit between the top chords of the truss with fiberglass batts under the rigid foam. The foam would be installed up the roof slope with a 1″ air gap beneath the roof sheathing to ventilate from the soffit vents. At the truss peak the foam would run flat so the vented air is free to travel along the ridge and to the ridge vents. The edges and seams of the ridged foam would be sealed with spray foam. Using the 2″ XPS foam gives me more R value in this small insulating space and would prevent the batt insulation from compromising the air ventilation space.
My local inspector will expect a minimum 4 mil vapor barrier on the warm side of the insulation. Am I creating a harmful double vapor barrier with this concept? Also, and I’m not sure if this is important, though the porch ceiling would be tightly sealed The vented ridge does run from the new gable wall to the new attic space created over my existing house and neither of those would be sealed. I thought I had a handle on this, but the more I research I do the more confused I become.
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The first piece of rigid foam that you describe, inserted between the rafters, is called a site-built ventilation baffle. You can use 2 inches of rigid foam if you want, but that much rigid foam raises concerns about a wrong-side vapor barrier, as you suspected. One solution is simply to use thin plywood instead, or even 1 inch EPS (which, unless faced with aluminum foil, is a vapor-permeable choice), or a commercially available ventilation baffle.
That said, moisture problems due to vapor-impermeable ventilation baffles are either rare or nonexistent, for the reasons explained in this article: Site-Built Ventilation Baffles for Roofs.
If you want to improve the R-value of the entire insulated roof assembly with rigid foam, the best way to do that is with a continuous layer of rigid foam on the interior side of the rafters. That's a much better approach than cutting the rigid foam into narrow rectangles and inserting the foam between the rafters, because it addresses thermal bridging through the rafters. For more information on this approach, see How to Build an Insulated Cathedral Ceiling.
If you don't end up installing interior rigid foam, but you end up following the approach described in your original post, you don't want to install interior polyethylene. It would be better to install a so-called "smart" vapor retarder with variable vapor permeance, like MemBrain. (Hopefully, your building inspector knows that building codes don't require interior vapor barriers -- just vapor retarders, in some climates.)
Thank you for responding to my question. I've spent the last few days trying to digest the information in your response as well as the two articles you had links to. I'm hoping you can help me a little further.
Thanks for the tip about the MemBrain vapor retarder. I had never heard of it before but with a little research I found that it's available locally. I plan to consult with my Building Inspector to make sure he's OK with my using it.
I understand why you feel the ridgid insulation would be more effective on the underside of the bottom chord of the truss. But I don't think this will work for me as I dropped my wall height to increase my heel height while matching fascia heights, and I'd prefer not to loss any more ceiling height.
So I'm back to the baffles,and though I'm a little wiser, I'm still trying to understand a few things. I now understand that using the 2" XPS makes the baffle a wrong side vapor barrier. But your response and the article also mentions that problems with a vapor impermeable baffles are either rare or nonexistant. Is there any way for me tell if my application, or factors such as my climate here in MN affects that belief, and whether I'd be safe using it? I'd still like to reap the benefit of the higher R value I'd get, but not at the cost of it rotting my trusses or roof sheathing. Would it make any difference if I used the 2" foam at the truss heel and up the slope until I reach 12" of insulating depth for the batt insulation? In my case that would be 4' at each side wall leaving the center 8' where I could use a vapor permeable baffle and batt insulation.
The other item I find puzzling is if vapor permeablity is desired at the baffle why is it important for the baffle to be air sealed? As long as the insulation isn't being wind washed what purpose does sealing the baffle serve? I would think leaving the baffle unsealedt would only help the vapor to reach the air vent.
In advance I'd like to thank you for any help or advise you can give me.
Q. "Would it make any difference if I used the 2 inch foam at the truss heel and up the slope until I reach 12 inches of insulating depth for the batt insulation?"
A. It sounds like you didn't specify raised-heel trusses, and you have insufficient height at your truss heels for full-depth insulation. This is a worrisome problem, and is much more serious than the question of the maximum permissible thickness of your ventilation baffle.
Using cut-and-cobble rigid foam, as you propose, at a truss heel that is too low for full-depth insulation almost always results in poor performance. The usual result is air leaks and enough thermal bridging through the framing to result in ice dams (assuming, of course, that you live somewhere that gets snow).
The heel of your truss is the area where insulation is most critical. It definitely isn't an area where you can skimp on insulation. But it sounds as if you'll have to.
Lowering the ceiling (to allow for a continuous layer of rigid foam on the interior side of the rafters) would certainly be the best option. The second-best option would be to install a site-built ventilation baffle and then fill the rest of the rafter bays with closed-cell spray foam, up till the point where you have enough room for adequate insulation to meet code.