Construction Design-Concreteless Slab on Grade-Roof

The concreteless slab on grade home has one level.  I knew the heating system for this home was going to contain ductwork, along with other ducts for bath fans, a dryer and a kitchen range hood.  I also knew I wanted all those ducts to remain inside the conditioned space of the home.  We will get to how that was accomplished in a bit.  First, lets talk about some of the other problems with the roof that needed to be worked out.

First issue was how to extend the exterior wall air control layer to inside the home.  The walls were discussed in a earlier post, read that here.  We used Zip sheathing as the main wall air control layer, to extend that to the interior ceiling, we used a 12 inch wide tape which attached to the zip, extended up and over the top plate, and was attached to the backside of the ceiling air/vapor control layer.  Pro Clima’s Intello was the choice for those barriers.  A couple things I really like about Intello, it’s durable, works well stopping air, and is vapor variable, meaning it can change the rate at which moisture moves through the product depending on the moisture level in the wall or ceiling.  One problem we had, the tape did not seal correctly to the backside of the Intello.  This forced us to use additional tape to seal the interior vapor control layer to the ceiling air/vapor control layer.  I’m sure this created a partial penalty for our blower door number.  If I had to do it again, I would choose polyethylene sheeting for the ceiling.  This approach works well in my Northern climate, but may be a good choice for a mixed or hot climate.

Tape extending the air control layer from the Zip Sheathing to the interior air/vapor control layer.

With the tape in place over the top plate, we then could install the trusses.  There were a few different types of trusses used.  There were scissor trusses, which had a 6/12 exterior pitch and a 3/12 interior pitch.  We had a common flat ceiling truss installed in the garage.  Lastly we used a special type of truss called a plenum truss to accomidate the ductwork and other mechanical systems for the home.

A plenum truss includes a notch in the framing to keep ductwork and other mechanical systems within the conditioned space of the home.

The Intello air/vapor control layer was installed throughout the ceiling and around the notch in the plenum truss.  This was all completed before any inteior walls were constructed allowing for a continuous air/vapor barrier with very few obsticles that needed to be cut around.  We then installed 1/2 OSB through the notch in the truss to protect the Intello and allow for fasteners to be installed to hold ductwork and other systems in place.  The notch was eventually filled in to achieve a flat ceiling.

Ductwork was kept inside the building envelope.

The next challenge for the ceiling was to move all the electrical penetrations that would typically be installed in the unconditioned attic.  Changing the electrical location to inside the conditioned space of the home would further reduce our air leakage rate.  I had to borrow a method used in New England to accomplish this requirement.  We strapped the entire ceiling using 2 x 4 lumber which gave us an 1 1/2 inch space to run wires and install electrical boxes.

Ceiling strapping with 2 x 4 lumber. Also notice the lone interior bearing point for the roof, the metal post.
All the electrical wiring and boxes were able to remain inside the building envelope.

The ceiling was finished with drywall.  The insulation contractor used closed cell spray foam to seal the few penetrations through the ceiling and out the roof, these penetrations included a pipe for radon and a couple plumbing vents.  There was also one bath fan that was outside the plenum truss notch that was sealed.  R-70 blown fiberglass was used to insulated the ventilated attic.

Finished roof. Notice the vented rain screen.

The air sealing details and well above code insulation will keep this home comfortable for years to come.  Up next, I’ll talk about the mechanical systems selected for this home.  Stay tuned.

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