So you're saying that your drawing is wrong, and there is NOT EPDM spacers between each layer of sheet rock on this wall?
The only thing I'm aware of that might be called "exterior" sheet rock is the "greenboard" version - this is higher cost and mainly used for wetter areas, such as bathrooms and kitchens. I don't know of any gypsum product that's code acceptable for cladding on outside frame walls, because it's not that resistant to nail tear-out and won't give much "racking" strength (resistance to stud frames getting "non-parallel", or out of square)
If you only have one effective leaf and can hear loud conversation, your wall is probably around STC 35 or so -
Since the inner wall is already in place, you can't really use the cinder blocks in a "surface bonded block" mode, which would actually give more strength AND better sound proofing, but you can use them in a conventional, "buttered block" mode - this is better left to a pro, it's not real easy to get the mix right or maintain true over the length of the wall if you've not done a LOT of this. If you go with cinder block, you WILL need to sand-fill or you'll have quite a bit worse LF performance.
I would glue either rigid fiberglass or mineral wool to the outside of your existing sip, then build the block wall as high as you can before running into that overhang - at that point, you could trim 2x's (PT, and put the trimmed side toward the WOOD, NOT toward the blocks - the cut side should also be treated with Thompson's or equal) - these would go at the inside and outside edges of the blocks, topping off the sand fill in each cavity before the outer 2x gets placed -
So the order would be, build the wall, place the inner 2x PT "filler" on top of the inside edge of the block, getting it as close to size as will allow you to put it in place - then fill the gap with caulk if it's less than 3/8", if it's more than that you'll need foam "backer rod", also called "caulk saver", Home Depot usually carries it in either the caulk or weather stripping section - with the larger gap, caulk the foam rod thoroughly.
Once you have the inner edge of the block wall airtight, you can finish the sand fill of all cavities, level with the tops of the blocks - if you can, vibrate the wall all along its length to make sure the sand is settled as much as it will. Ask your locak tool rental place about a tool to do this.
The insulation I mentioned will also keep any mortar droppings from shorting out the air space between the sip and the blocks.
IF you want the best performance you can get from this definitely wierd conglomeration, you could drill holes every foot or so along the outer 2x board on top of the blocks, and fill the cavities with spray foam - this will keep that short section from becoming triple leaf. I'm not sure how much difference this would make, just that I would do this if I were building it.
Then, you can cover the whole thing with siding, stucco it, whatever - that part, unless you fur out and use siding or something, won't matter much one way or the other.
I hope you hurry up and finish this wierd thing, it's giving me a headache
And I'd like to know how it turns out... Steve
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