Wall Insulation details with “Outsulation” – exterior rigid insulation panels
In Zone 5, 2″x 6″ wood stick framing, I chose to adopt the outsulation concept of applying 1.5″ rigid XPS on the exterior of the 5/8″ plywood sheathing (min thickness to avoid dew point on interior of sheathing). Tyvek first on the exterior side of the plywood followed by the XPS, caulked and taped at all seams. Final siding will be stucco and stone veneer after use of Tyvek drainwrap on top of XPS and lastly Keene driwall under scratch coat. My plan was to use the fluffy pink stuff in the bays to insulate the walls. With the low perm of the exterior Fiberglass insulation, the wall needs to dry to the interior. This is critical with a stucco/stone veneer exterior. Wall must have adequate drying ability, despite the Keene driwall ventilation plane over the Tyvek drainwrap to facilitate water exit from wall assembly.
Had 2 separate insulation contractors come in and seemed puzzled by the wall assembly details. One from a very large local insulation firm, after walking around for 20 mins said he did not want the job, expressed concern for his warranty liability and left. Other is also concerned with the warranty on his work and seems not willing to get me a proposal. Exterior rigid insulation is not common in my area, can’t imagine these guys have never done a job with exterior rigid insulation. The wall drying ability of the wall is one of the main questions that was discussed with both contractors. The use of kraft faced or unfaced fiberglass insulation seems controversial.
My understanding is kraft faced insulation should be used to retard vapor infiltration INTO the wall from the interior. Does the same vapor retarder pose a substantial risk for the wall to dry to the interior and thus should NOT be used?
Thanks
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Replies
Sal,
Kraft-faced fiberglass insulation is fine. (The kraft facing is a "smart" vapor retarder with variable vapor permeance.) It's also possible to use unfaced fiberglass batts with vapor-retarder paint on the drywall.
Sorry to hear that your local insulation contractors are ignorant. I hope you can find some insulation contractors who know a bit more about home performance and building science than the ones you've talked to so far.
By the way: If it's not too late, you might consider substituting 1.5-inch-thick polyiso for the XPS. Polyiso is manufactured with a blowing agent that is much more environmentally friendly than the blowing agent used to manufacture XPS. For more information on this issue, see Choosing Rigid Foam.
Sal,
You want to install the drywall in an airtight fashion. Perhaps someone will comment on whether adding MemBrain would be a good idea.
Sorry you are having issues with finding a contractor. Where in Zone 5 are you located?
Sal,
You are appear to be using a belt and suspenders approach, which in the world of moisture management is a good thing. My one recommendation is clarifying where your drainage plane is - in other words, which of these layers of housewrap or taped insulation will you be flashing to? The second layer of tyvek confuses things. Looking at your layers I would put the layers as sheathing - drainwrap wrb - rigid insulation with taped seams - rainscreen air gap - lath & stucco, and then flash to the drainwrap wrb. We know that there's a chance water could become trapped between the sheathing and rigid insulation so having some drainage capacity there is valuable (such as drainwrap). Ensure that the terminations have appropriate drainage capacity for your rainscreen product (a weep screed with large slots to drain the rainscreen).
In the attached image the Sure Cavity/Gravity Cavity layer is what creates the rainscreen air gap. It is from https://www.mtidry.com/hyperspecs/stucco-rigid-insulation
I assumed moisture can get between ANY one of the layers. Thus I installed Z- flashings above all windows, sealed with quality polyurethane sealant, directly to the exterior surface of the plywood. Once the windows were installed - openings were picture framed with pressure treated sleeper 2"x4"s, all window openings were flashed twice. Once between the Tyvek and the opening BEFORE the sleepers with blueskin, seoncd AFTER the sleeper picture frame with Grace Vycor, wrapped before and after with a plump bead of sealant applied to the perimeter of the window before setting them. I then installed a counter aluminum flashing strip above the window nailer and under the Z-flashing edge, thus in a water shedding fashion. From surface of plywood to the exterior. A 41" soffit overhang (exposed rafter tail timbers) helps keep the walls dry in almost all types of rainfall, adding additional water protection. The slotted weep screed at the bottom, and 3/8" opening at the top for air circulation into the ventilation plane goes without saying. Sure Cavity came in at about $1.25 a sq ft., Keene driwall is about 0.68 a sq ft. Decision made. I reccd the Kene driwall,. good product, excellent drying potential, better than the Sure Cavity, at a great value. Thanks for the nice graphic.