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Exterior insulation- retrofit-foam board with Rockwool comfortboard 80

PJ109 | Posted in Energy Efficiency and Durability on

Hi, looking for some guidance


We are completing an energy retrofit in climate Zone 7A, our climate is relatively dry in Alberta.


The house is from 2000, ACH level is 5.44 2×6 wall, r20 fiberglass batt, with an interior vapor barrier, and exterior sheeting is OSB.


The plan is to leave the existing interior vapor barrier/cavity insulation, tape/caulk the OSB joints, WRB is Siga Majvest 200 tape joints/seams, the window bucks are outtie covered by fast flash with exterior insulation. The exterior insulation is where I require help.


The wall assembly is a little unconventional, I have roughly 60- 4×8 foam board on hand which should be utilized, and a large roll of Siga 200 waiting to be used.


Looking for feedback, I was thinking of having 2 layers of insulation on the exterior, the first layer would be 1.5″ of comfort board 80 R6, followed by 2″ EPS foam board R10 (it has a reflective film on both sides), 1/2″/3/4″ ripped plywood for a drainage plane covered with vinyl siding.


I do like the fact that the foam will provide a better base for the strapping. Is 1/2″ strapping stable enough or should 3/4″ be used? 


Does this assembly allow enough drying potential if condensation/bulk water were to occur? Or would the foam prevent it from outward drying?


Has this wall detail even been completed with success? 


What are my unforeseen issues and concerns that may occur?


After reading the article “Mind the gap” it suggests having a drainable “crinkled” WRB to provide a gap if exterior foam is used with a wall assembly that can not dry to the interior, comfort board 80 seems to allow for a small drainage plane on either side as it’s slightly ribbed and it’s permeable.  Although the article refers to spray foam application, I would assume the same procedure would apply with an interior vapor barrier??


Upon reading “Calculating the minimum thickness of rigid foam sheeting” it suggest having enough insulation on the exterior to avoid condensation when using exterior foam.


Since I live in a cold climate, I believe I would require at least R15 of foam to keep the wall assembly warm enough to avoid the risk of condensation. 


Does the physics still apply if you’re using foam and comfortboard?  I realize it doesn’t apply if I was only using comfortboard due to its permeability.


Also, do the rules apply to faced EPS?


I don’t want to risk relying just upon a drainable “crinkled” WRB with an interior vapor barrier, hence the reasoning for the comfort board 80. 


Although the assembly seems crazy, using the materials on hand would save me roughly $6k-$7k+/-.


I truly appreciate your guidance, time, and feedback. 

GBA Prime

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