Exterior Insulation technique with Rockwool – opinions please!
I am curious what folks think about the following technique for applying Rockwool comfort board outboard of sheathing:
After sheathing, nail ripped 2x6s (1.5″x2.75″) horizontally thru sheathing, to studs. Space the these nailers about 23.5″ edge to edge (or what makes a snug fit for an uncut piece of comfort board) up the wall. After insulation is installed, nail strapping to 2x nailers for siding (either one or two layers depending on siding type).
I am interested in this method because I have found that strapping Rockwool can be tedious in keeping it in plane and not compressing the Rockwool. One already needs a good amount of 2x at windows and doors (and corners sometimes) so as I can tell you can’t avoid them entirely. Also finding the studs with long, expensive screws can be tough. With this method, you have rock solid nailers at the right depth and only have thermal bridging where the horizontal 2x crosses the vertical stud.
Curious what folk think. Are there other downsides to this that I’m not seeing?
Thanks!
Replies
Try fiberglass z-girts for horizontal or vertical applications. This avoids thermal bridging and the expensive long screws. Otherwise, I recommend sloping the top of the ripped furring strips and wrapping them with G-tape, so moisture doesn't sit on top of them.
I think you have the right idea but you can do even better. The reason is that a horizontal piece of wood doesn't allow the ventilation from bottom to top that you are generally hoping for behind your sheathing. The ventilation is good to keep out insects and vertical means the strips can't collect water. What about ripping a sheet of Huber Zip R-9 and use VERTICAL 2 inch thick strips of that to separate the rock wool? I think a sheet is 2 inches thick. So you could probably get 20 strips of 2" x 2.25"w x 96"l with a single sheet. or use Huber Zip R-12 if your roxul Comfortboard is thicker than 1.5"
With the vertical strips you don't have thermal bridging (your zip strip has plenty of foam to prevent that) and you already have a nailer for your siding. No need to purchase any strapping at all. The Comfortboard when fully pressed in would have a nice air gap above it of 0.25 to 0.5 inches. The top half inch of the insulated Zip strip is a perfect wooden surface to nail your siding to.