Out-sulation vs Double Stud Walls
I was reading some old GBA discussions about double-stud wall designs and the added difficulty of keeping the sheathing dry (relative to wall designs that place appropriate amounts of insulation over the sheathing). This prompts me to wonder why the double-stud approach has its adherents when an out-sulation approach can be used to achieve the similar R-values and protect against thermal bridging while dispensing with the uncertainty regarding moisture. Besides being of general interest to me, I am hoping the responses will help me decide the ‘best’ way to proceed for a future project.
To make the discussion a little more specific (with the hope of achieving an apples-to apples comparison of the two design approaches), assume two walls with the following configuration:
(1) drywall, 2×6 wall filled with mineral wool (MW), plywood, WRB (here or outside of the out-sulation), 3″ MW (for zone 6), strapping (rainscreen), siding.
(2) drywall, 2×4 MW-filled wall placed at edge of band joist which is set back 5.5″ from the front edge of the foundation, 2″ MW, structural 2×4 wall filled with MW, plywood, WRB, strapping, siding.
I have tried to pick two designs that are similar in performance (R-value, protection against thermal bridging), although tweaking (adjusting wall thicknesses a bit in the double stud wall, framing patterns, etc.) here and there could, no doubt, improve upon this. That said, why would one opt for the double stud wall design?