What are the negative aspects of using Tyvek on a double wall system instead of plywood sheathing?
I am detailing the exterior walls to be a double wall, its sequence as follows from interior to exterior: gypsum board on structural 2×6 with rockwool insulation, 1/2″ plywood sheathing, continuous 10 mil vapor/air barrier, 11 7/8″ TJI joist with dese packed cellulose insulation, 1/2″ plywood sheathing, WRB membrane, 1×3 rainscreen furring strip and hardi-plank siding. I have read Thorsten Chlupp’s wall detail that uses Tyvek thus eliminating the use of the 1/2″ plywood and WRB membrane. I would like to go down this path because of the economy of materials but am not sure about using only Tyvek at that location. Any guidance would help.
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Do it, data log it, report back.
Read above. Please specify climate.
Karen,
One of the unknowns about this type of wall assembly is the long-term effect of the "pumping" action of wind on the Tyvek. Wind will pressurize the windward side of the house; when the wind shifts, the same side of the house can be depressurized. This back-and-forth pressurization and depressurization can pull the housewrap, making it balloon out between the studs, and flap back and forth.
Over many years, what will the effect of this pumping action be on the housewrap fabric or the fasteners that hold it? Researchers aren't quite sure.
Karen,
I'm not sure which detail of Thorsten's you are refering to...
If you haven't already, have a look at his ARCTIC detail as applied in the Sunrise Home.
On my own home, I chose to omit exterior sheathing, with Typar fastened directly to the exterior studs.
I will not be including a rainscreen detail, however.
I don't think that, all things considered, the added complexity of a rainscreen is justified in my case.
Also, by fastening the siding directly over the Typar, I'm not too concerned about the "pumping action" Martin refered to - this might be something you may wish to consider too if your climate is not-so-wet.
You might also consider deleting the poly air/vapour control membrane and using an adhesive tape like Siga or Proclima to tape the seams of the first layer of plywood.
Do I understand your wall correctly? Is it really 18+ inches deep? What is your target r value? 2x6 +1/2" sheathing inner wall and 11 7/8" TJ outer wall insulated with cellulose? WHY the truss joists? The webs are thermal bridges reducing the effectiveness of the cellulose. What's wrong with a 2x4 outer wall spaced away from the sheathing on the inner wall? If the inner wall's sheathing provides all the required shear strength why not sheath the outer wall with 3/4" fiberboard then housewrap. Not using the truss joist but 2x4s instead will allow a thinner wall with the same or greater r value. Is a 2x6 inner wall justified or needed structurally ? Couldn't it be 2x4 as well? For economy of materials using Truss Joists in a wall is really hard to justify!