Jim
I’ve been meaning to reply to your query for days now and just haven’t had the time, but thought I’d make it in that I have some thoughts and techniques which fit your project.
I used to build super-insulated stick framed homes back in the 80’s and most of it comes from that experience, but have also adapted some of these techniques to do built up enclosures for timber frames.
After you lay bare the walls, strip the kraft paper off the glass, fill any voids. If you are rewireing ( more on an additional reason why you might want to later ) and are making a mess of the glass you might consider replacing it with the extra dense 3 ½ ” batts R-13 sometimes labeled as “full wall” Buy this from an insulation installation company, they can install for less than you can buy it from your local supply. Do not expect good work though, the installers are usually paid piece work, have them stuff the full and easy to cut bays and do the fussier things yourself and you all make out.
After that apply 2 X 4’s on the flat horizontally 24” oc, between these lay in EPS panels 20 ½ “ X 1 ½ “ The beadboard stuff – Buy this from a foam plastics maunufacturer not an insulation supply, it will be cheaper and they can custom cut it to 20 ½ “ and save you the trouble of ripping down 24’s and do not be tempted by the additional R’s in extruded Dow board, the difference in price is vast and so is the payback.
Over that layer apply ½ “ rigid foil faced polyiso, shiny side in, logos out, foil tape the joints, this is your vapor barrier. Do not be tempted to increase the thickness of the this layer beyond 1” There is a “creep” issue with cantilevered fasteners and you just don’t need to this system works by eliminating thermal bridges with multiple thermal breaks, And also eliminating the convection current common to stud bays and the resultant heat they lose.
Now apply 2 X 3’s vertically, also on the flat – 16” OC using quality screws.Don’t forget to add a block betwixt these at the bottom for nailbase for you mopboards Run your wiring here in this reflectant dead air space, with no penetration through your rigid vapor barrier, this requires 1 ½ “ deep junction boxes with reducer plates.
Drywall over these.
Done right, adding some gaskets and beds of caulking at the subfloor and your air exchanges will be down in the SIP range. A range where you need add a heat exchanger to your mechanicals to remove pollutants and moisture.
Out of interest what are you adding to the top plates to distribute the point loads imparted by those timber trusses ?
Keep looking for answers and, Be your best
