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?
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Replies
Rob,
Q. "Why would one opt for the double stud wall design?"
A. It's easier to install windows and siding without the furring-strip hassle.
Martin,
Are you saying window installation will be easier in the double stud wall because there is structure at the perimeter of the building (as opposed to a structural wall covered with 3" of insulation)? That makes sense. As I proposed things, it seems that there would be the furring-strip hassle in both designs. Right?
In your opinion, is the advantage of avoiding the window installation hassles great enough to outweigh potential moisture issues?
Rob,
Both systems have fans. There is no right way to build a wall; you should build the wall you prefer.
That said, installing furring strips on top of mineral wool -- something I have never done -- is reportedly fussy work and somewhat of an acquired skill, because the mineral wool has too much "give."
Outsulation is a huge pain and very labor and resource intensive. The dangers of cold sheathing have been much discussed, rarely (if ever) shown conclusively.
I've done both and agree with Dr Lsiburek that exterior insulation is ideal since it keeps the structure warm. On the other hand, to build one takes carpenters willing to learn and pay attention to specific details they may have never heard of. The beauties of a double stud wall include: no foam necessary and no special carpentry skills necessary; anyone who can build a good house can build a double stud house.; standard details; standard labor. There are builders exploring varieties of Larsen Truss walls - typically a sheathed structural wall with an exterior truss to add more insulation; the best of both worlds.