Replies

1. Expert Member
   Dana Dorsett | Feb 19, 2018 02:23pm | #1

   Cut 1" wide strips of foil faced polyiso and install them as spacers where the framing meets the stucco/lath for the full perimeter of each stud bay, and at the center of the bay between studs. Dabs of foam board construction adhesives should be good enough for mounting the strips. Then cut'n'cobble 1/2" half-inch foam board to fit between the studs installed tight to your spacer strips. Cut your panels 1/2" narrower than the space between studs, and use can-foam to seal the edges.

   You can then use compressed unfaced sound-abatement fiberglass or rock wool batts into the remaining 2.5" space, which will perform at about R10-R11. The half-inch polyiso is good for R3+, and the foil facer adjacent to the air gap is good for another ~R1, so it would be roughly the same installing R15 batts from a thermal performance point of view.

   From a moisture point of view the half-inch air gap is an adequate capillary break, and the foil facers are a true vapor barrier, so even when the sun is baking out any rain/dew moisture from the stucco, it stays on the exterior side of the assembly.

   If you want to bring it up to IRC 2015 code minimum performance, in addition to the cut'n'cobbled foam on the exterior side of the cavities, add half inch polyiso + half inch CDX Bonfiglioni- type edge strips to all the framing to make the cavites 3.5" deep, and use R15 rock wool in the cavity. See: http://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf

   If there is currently no weep screed venting the stucco it may be worth adding small screened vents (or at least a drill hole) at both the top and bottom of each stud bay to guarantee drying to the exterior via convection. This additional feature is more important if the roof overhangs are minimal or the location is prone to wind-driven rain wetting of the walls.

2. GBA Editor
   Martin Holladay | Feb 19, 2018 06:53pm | #2

   Phil,
   Here is a link to a relevant article: Insulating Walls in an Old House With No Sheathing.

3. 
   pbout | Feb 19, 2018 08:07pm | #3

   Dana, Martin,
   Thank you both for your responses!
   -Phil

4. 
   user-6184358 | Feb 19, 2018 08:16pm | #4

   A Correction on what a stucco weep screed is. It is placed at the bottom of the stucco to allow for & direct water to direct water to drain out of the stucco toward the exterior face. It is only placed at the bottom of the wall.
   It has nothing to do with ventilation.

5. Expert Member
   Dana Dorsett | Feb 20, 2018 11:50am | #5

   Weep screeds on stucco and weep holes on masonry walls have a dual function, venting is one of them.

   Yes, a guaranteed bulk water release path is a primary function, but convective venting/drying the cavity is another, especially when combined with a top vent to the cavity of similar or larger size. A venting area larger than typical weep screeds would be a good idea, particularly frequent direct wetting is an issue in your location.

   From https://www.mtidry.com/weep-screed:

   -----------------------

   It is critical that the drainage/weep holes be

   * numerous - less than 1 inch apart
   * located directly below the rainscreen drainage plane
   * large to ventilate the rainscreen air gap

   ------------------------

   See also: https://www.mtidry.com/uploads/Getting_to_the_Bottom_of_Moisture_Management.pdf

6. 
   user-6184358 | Feb 20, 2018 12:30pm | #6

   So, It seems a weep and weep screed per the document provided are not air vents (Figure 12). It is different in a rainscreen application.