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Community and Q&A

Air Sealing the Ceiling Junction

Jim Pierce | Posted in Energy Efficiency and Durability on

I am currently in the process of building my own house and high airtightness is a key goal.   The house is of a conventional design with trusses over 2×6 walls and, in most areas, an insulated, vented, unconditioned attic area.  My primary air seal boundary is a tight rimjoist area, taped and sealed OSB on the walls, and the interior drywall at the ceiling.  Assuming I execute correctly, I think that the latter, the ceiling drywall junctions are the weak points in my plan.  I saw an online video where a builder attached a 12″ wide strips of 3/4″ subfloor material between the top plate and the trusses, extending into the house interior.  He then had a continuous glue or sealing surface to attach the ceiling drywall.  This overhang wood would be required for all exterior and interior walls and also require furring out the trusses with 3/4″ material for a matching ceiling plane.  Effectively, it raises the overall house height by 3/4″.  Finally though maybe not specifically called out in the video I saw, I think the trusses were connected to the top plate with Simpson truss screws, in this case going through the top plate and the 3/4″ board and into the truss.

This sounds like an excellent technique to ensure a positive and long lasting air seal along the ceiling boundary or the wall to ceiling air barrier transition.  Does anyone have experience doing this?  Was it deemed successful?  Any pitfalls or learnings?  Other than the extra effort, particularly furring out all the ceilings which is not usually done in my Colorado build area, one of my concerns was the structural integrity of the plate to truss connection since there would be less screw penetration into the truss heel due to the 3/4″ wood in the middle.  However, my structural engineer is satisfied with 2″ of screw penetration into the truss and that is still achieved.  

So again, any experience?  Thanks in advance.

Some other info:
  – raised heeled trusses with exterior overhang 
  – 2″ exterior rigid foam from basement footing to roof soffit
  – R-21 fiberglass in 2×6 walls
  – Climate zone 5

Jim

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Replies

  1. Expert Member
    Akos | | #1

    I've done something similar but instead of plywood, use a wide piece of flex flashing tape (3M 8067 is great for this). Remove only half the backing of the tape to adhere to the top plate and sheathing, leave the other half until your ceiling vapor barrier is installed. Once the VB is in, remove the backing and adhere to the vapor barrier. Same thing for interior walls.

    Pretty simple to install, doesn't effect standard build workflow and no strapping needed. I would also install blocking between the trusses so you can caulk under your drywall around the perimeter as your secondary air barrier.

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