Hulstrunk’s 100 Year Envelope
Eric Colbeck
| Posted in Green Building Techniques on
I am building a single story addition with a vented cathedral ceiling. I will be using double stud walls, parallel chord trusses and dense pack cellulose. I would like to avoid the use of foam.
I am wondering if any builders are using Bill Hulstrunk’s 100-year envelope design.
It appears to have the advantages of an exterior air barrier (where possible) and an interior air barrier to accommodate for the roof venting. Does anybody out there have thoughts on this design?
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BTW
Name = Eric Colbeck.
Changed my screen name but still shows up as the robotic sounding USER-6882359.
Top plate spanning double stud walls (and sealing inside top edge of walls) could be 3/4" ply (instead of 5/8"). These would meet fire codes, and reduce the chance of fire spread from inside walls to attic. Minor cost upgrade.
Seems risky to use cellulose below grade. Might increase chances of mold developing in the walls. What's your climate? If its a humid climate, especially summer, might need to keep dehumidifying the basement level, since the interior air would be humid. Planning on a bathroom or laundry in the basement?
Robert,
I agree with the upgrade to 3/4 and I plan on doing that. I'm only looking at the parts of the plan above grade. Below grade in my situation is an unconditioned crawl space with ICF walls.
I'm most curious about the transition from an exterior to an interior air barrier at the top of the studs. Is it practical?
As long as the ceiling drywall air barrier is air sealed to the interior wall plywood air barrier, you have a continuous air barrier.
Its unclear from the drawing how far down the wall the plywood extends. I would not continue the plywood all the way to the floor (as the drawing seems to indicate), but terminate the plywood where it meets with the ceiling drywall. That would require you to either leave tape underneath the bottom edge of the plywood (tucked under where it meets studs) to later be sealed to the ceiling drywall edge, or seal the ceiling drywall edge to the wall plywood from above, inside the attic space. Do you have a specific detail in mind? Best to plan these details before construction, so you don't end up with a difficult task during construction (or forget to do it during construction).
I'd seal all the nails or screws that poke through the plywood top plate as well as the plywood on the interior side of the wall (facing the attic space). Same for the exterior plywood sheathing. The drawing says sheathing edges sealed, but fasteners that poke through the sheathing plywood could be sealed too. Zip sheathing claims that their product will seal any fasteners through the sheathing automatically. Instead, could use a smart vapor barrier on one of the stud walls (interior side of the exterior studs, or exterior side of the interior studs). It is difficult to dense-pack cellulose if the area to be filled becomes too large. 12" deep x 8' high x 10' max length (for fire code) would be too large but I'm not an expert on dense pack. Have you discussed this with an insulation subcontractor?
Double wall construction would require that you have 3/4" ply or some firecode blocking every 10' or less of wall length (to avoid large interior spaces that fire might spread, as well as mice). Spanning plywood across the interior of the wall probably best to do at door or window locations where already you are blocking across much of the interior of the double stud wall with window or door wall bucks.