GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Confirmation – sheathing air gap needed for insulating existing wall?

sam_l | Posted in Energy Efficiency and Durability on

Hi, I’m insulating a few triangular exterior walls as part of my “devils triangle” retrofits. 1922 house, zone 5B, almost 6B, northern CO. Relatively small interior fixes so not involving AHJ for the moment, but want to do it right. Have to go in from the inside – outside is too big of a project right now and has historic district issues. This is a conditioned utility space without electrical wiring or windows on the exterior wall.

Exterior wall is:
Old but solid stucco (painted many times, including last year)
Metal Lath
Old but intact felt (probably original)
1 x horizontal pine sheathing
2×4 16OC bays (true 2″ x 4″)
1/4″ fiberboard (doin’ nuthin’)

Plan is to fill the bays with mineral wool batts, then do 2″ of continuous foil-faced polyiso on the inside. Fur out with 1x4s, then 1/2″ drywall. That gets me to ~R26 without much complexity.

My questions are:
– Do I need to leave a gap against the existing sheathing? I see that is recommended for old walls where there is no housewrap, but is the original felt good enough?
– Do I need a smart vapor barrier like Intello or Membrain over the mineral wool? Or does the taped foil faced polyiso perform that task? Winter indoor RH is ~25% at best, no humidification.
– Anything else dumb I’m missing?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. FrankD | | #1

    I don't see any real benefit to leaving a 1/2" gap unless you also drill some holes at the bottom of each stud bay so that any water that leaks in can drain back outside. And unlike fiberglass, mineral wool is hydrophobic, so water will drain down through it anyway.

    The foil facing is a very good vapor barrier, so you don't need to add any other. If you tape the seams and seal the edges, it will also be an effective air barrier.

    However, if you are concerned about water leaking into the wall, you might want to allow for drying to the interior. The foil facing will prevent that. Instead, you could use fiber-faced polyiso or unfaced EPS or GPS, which will allow some drying. Then add Membrain and seal that for the air barrier.

    1. sam_l | | #2

      Thanks, appreciate the gut check. I'm not seeing any indication that water gets into these walls normally - they are under a decent overhang and don't have any windows or other questionably flashed penetrations. I think any moisture getting in would be very minor and could hopefully dry to the outside still.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |