Parallel chord truss: insulation, ventilation, no eve access due to intersecting roof valley.
Hello, we are building a pretty good cabin in zone 7 of Eastern North Dakota. The basic specs are 2X6 stud walls, integrated WRB and 1.5″ exterior polyiso. The main great room has a parallel chord 12:12 vault. Each wing off this has a 6:12 conventional roof truss. The roof will be hidden fastener steel.
If I use a ventilated chase between joists, the first two joists with not have any access to fresh air (no eve access). The remaining chases will have convection, but it would be hot attic air.
I am leaning towards an insulated, unventilated cavity, with a smart vapor membrane, but am looking to tips and convincing arguments to any better solution.
– Shawn
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Given your setup and climate zone, going with an insulated, unventilated roof assembly using a smart vapor retarder is a solid choice—especially since proper ventilation in those first few joist bays is difficult. Just make sure the cavity insulation is dense-packed and that your air sealing is tight. The smart membrane will allow the assembly to dry inward if needed, which is ideal for your cold climate. Just avoid any interior vapor barriers like poly, and ensure exterior layers stay vapor-open or allow controlled drying. This setup minimizes moisture risk and simplifies detailing.
Filovaj,
What you are suggesting is risky and doesn't meet any building codes I'm familiar with.
- You can not use dense packed insulation on un-vented roofs outside climate zones 1, 2, or 3 without impermeable insulation above.
- In heating dominated climates (like zone 6) the vapour-drive is to the outside, so you can not rely on inward drying to keep the roof safe.
Shawn,
Which joists don't have access at the eaves?
The whole roof looks easy to vent and insulate as long as joists are deep enough. There is no real downside to pulling ventilation air for the vaulted portion of the roof from the trussed area.
The vent baffle of the joist bays closest to the lakeside and roadside elevations will be obscured by 16-20” of insulation in the attic of the traditional trusses.
Can you build those baffles from the inside, before the drywall lids go up? When the upper floor ceiling is open, you will have access to the extremes of those joist bays from the interior, or at least I think you will based on the drawings you posted.
The other option is to go with closed cell spray foam in the problematic rafter/joist bays to create a safe, unvented assembly in those areas. You don't have to spray foam the entire roof in this case, but you do need to ensure that the spray foam doesn't obstruct the ventillation channels of any adjacent rafter bays.
Bill
Shawn,
I was assuming that the heels on your trusses are as high as the blown insulation on the ceiling, so you don't end up with less R-value along the exterior walls. If not I'd strap the 12/12 roof with 2"x3"s @ 24" oc.
I'm with Malcom, I don't see any issues here. For those rafter bays stop the baffles above the insulation level so they can use the attic space as vent intake.
Also if a couple of truss bays don't have intake vents, it is not the end of the world. Trusses are pretty open, you'll have flow from the neighboring truss.
If you are still worried, you can strap out your trusses with horizontal 2x2s before the roof deck goes on. These now provide a horizontal path for airflow to connect up all the rafter bays.
>"If you are still worried, you can strap out your trusses with horizontal 2x2s before the roof deck goes on. These now provide a horizontal path for airflow to connect up all the rafter bays."
This is actually pretty commonly done where the soffit is all open inside but there are only periodic vents. While I prefer continuous soffit vents, the periodic venting works fine when the NFA is high enough for the roof being vented.
For the OP: I also don't really see an issue venting things here. You should be able to get access to where you need to go as long as you do it in the proper order during construction, and it's fine to vent some of the roof from the attic space as long as you allow for the additional venting area in your NFA calculations so that you size the venting correctly. Basically this means the attic area needs enough soffit vent area to cover the attic AND the extra roof area that is essentially vented "through" the attic space. This should be pretty easy to manage.
BTW, try to get a bit more intake vent area than exhaust (ridge) vent area so that you don't keep your attic at a slight negative pressure.
Bill
I think I was over estimating how many of the joist cavities would be obscured. I wasn’t thinking about having a 12 to 16 inch heel on the common rafters.
The metal roof manufacturer does not recommend hidden fastener snap block roofing on strapping. I would end up with a double roof type configuration if I with this route.
The strapping would go directly over the trusses before the roof deck, so you don't have a 2nd roof deck.
I've done this when working with I-joists which have no airflow path sideways. With parallel chord trusses, this is not an issue as they are not solid, so the strapping isn't adding much.