Exterior mineral wool insulation: Batts vs. denser boards
I am planning to add exterior mineral wool insulation over the exterior of my double wythe brick walls. The typical choice is high density boards such Roxul comfortbatt 80. They are substantially more expensive than the medium density mineral wool batts that typically go into stud bays on the interior such as Roxul Comfortbatt. I think the price difference is factor 3 at least on a $ per R per sqf basis. Hence my idea: Attach a 2 by 4 on the flat with Tapcons to the brick. Screw 3 inch thick rigid foam strips on top of the lumber, and then another 2 by 4 on the flat screwed through the foam into the other 2 by 4. This now has a depth of 5.5 inches. I can create exterior bays at the appropriate spacing of 16 or 24 inches like that. Fill these bays with cheap mineral batts of 5.5 inches depth such as Roxul comfortbatt for R 23. Put housewrap or a better membrane like Solitex Mento 1000 over the entire plane and then screw furring strips to create a rainscreen gap and attach siding. Here are my questions:
1. The batts would only be friction fit into the exterior bays, no furring strips compress them snug against the brick wall as in the standard approach with high density boards. Would you be concerned that this lack of tight fit against the brick may reduce the R value somehow?
2. Has anyone done something similar in the past and has any experience?
3. Is housewrap on the exterior good enough or would you use a super airtight membrane like Mento 1000?
4. Do I need to apply a liquid water barrier to the brick before adding insulation? I do not think so, because my membrane put on the exterior over the mineral wool will serve as my air and water barrier and will be integrated with window flashings as best as possible. ( I am not reinstalling windows to do this best practice, just too much work)
5. Anyone think this is nuts? Seems like labor is about the same as using 3 layers of high density boards for same r value of 23. With the high density boards I still need a 2 by 4 on the flat screwed to the brick, filled with 1.5 inch thick board then 2 layers of 2 inches thick Comfortboard over that fastened with furring strips through screwed into the 2 by 4. I could not find a way to fasten 5.5 inches of mineral wool (of whatever density) directly into brick. This would require 8.5 inch long Tapcons ( assuming 2 by 2 furring strips on top of the wool) which I could not find.
Thanks for any advice that might be offered, I appreciate this.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part