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Venting (or not) and insulting 6:12 parallel chord truss roof

planesandteeth | Posted in General Questions on

My first post a couple months ago was about an unvented cathedral roof with exterior insulation Ive been planning.  Plans are changing because of the span.  Well, my builder now wants to use parallel chord truss with a depth of 24″ to span the 41′ room.  (6:12 pitch, single ridge in the center of roof.  Standing seam metal roof.  CZ 4a.)  My questions:  I was originally going to go unvented with another I-joist roof design, but should I now vent this parallel chord roof design?  Im not sure how to insulate in the truss cavities now.  Spray foam, fiberglass batts, other?  It seems blown in products will sag and aren’t good for steep roofs like 6:12.  How deep do I need to go?  I think its obvious going the full 24″ depth in insulation would be overkill in CZ 4a.  I can avoid thermal bridging if the insulation is deep enough to cover the top chord and possibly avoid the expensive exterior Hunter Panel foam board I was planning.  I just need to know if this should be vented in some way, and what type and how deep the insulation should be.  I dont want condensation issues down the road either.  Thanks in advance.

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  1. Expert Member
    Akos | | #1

    24" is not that much for loose fill. Definitely more than code but should not cost all that much extra.

    My suggestion would be to staple up vent baffles from soffit to ridge and make sure the seams are sealed to keep insulation out. Frame up a mini attic at the peak, doesn't have to be large, say 4' tall at the ridge. Loose fill the whole thing with extra insulation in the mini attic. This extra insulation can now make up for any settling in the rest of the roof.

  2. planesandteeth | | #2

    What if I did 2-3" of closed cell foam directly under the roof deck, then another 10" of open cell foam on top of that? No venting. There would be about 12" of "dead" space until the backside of the drywall, but is that an issue?

    1. Expert Member
      Akos | | #3

      That should work, in your zone that is around 2.5" of ccSPF plus 8" of ocSPF. The gap between the open cell and the drywall is not a problem. If you do go with this type of roof make sure the spray foam is carried from the bottom of the roof deck all the way to the wall top plates bellow. You want to seal the roof from the soffit area completely.

      Spray foam is something you design out of new builds. Never mind the enviromental footprint, the cost is very high. I would try to design for a vented roof unless there is really no other way. About the only reason to go for an unvented roof is if you are in area with a lot of major storms, much less chance of water damage from wind driven rain with an unvented roof.

  3. Expert Member


    I'd put in vent baffles, and stuff as much batt insulation underneath as makes sense in your climate.

    1. planesandteeth | | #5

      Thanks folks. Ill approach my builder with these solutions. Filling a 24" deep parallel chord truss does seem challenging to fill with batts, and deciding to vent or not. If I'm understanding correctly- Vent roof when using batts or fill, no vent required when using ccSF/ocSF combo? Builder preferred no venting based on the style of roof. The entire roof (all cathedral with air conditioned attic), literally 94' x 43' is going to be built (vented or unvented) and insulated the same exact way. Thats ALOT of insulation!

      1. Expert Member
        Deleted | | #6


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