caulk gap in block wall

Got a concrete block wallabout 20' high that had another wall built next to it to enlarge the building. There is about a 1/2 inch gap between them. The original caulk has seen better days and is letting water in. What is best to seal this straight 20ft verticle 1/2inch gap between 2 concrete blcok walls? I thought I might use sawzall to remove old caulk. I don't suppose anyone makes foam that I could spray in there that would waterproof this gap. Maybe spray foam to close up the gap within 1/2 inch of the outside then fill the last 1/2inch with a caulk? Any ideas. TIA
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The normal way to deal with this situation would be to remove the original caulk, preferably to the CMU surfaces. Install the proper sized backer rod in the joint. Caulk with urethane caulk. NP1, Tremco are good brands. Plan on tooling the joint for a uniform appearance.
(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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Thanks Dan I can get this backer rod at a bldg supply? Will the caulk want to fall out of a large gap like this one, 1/2 inch orso? How would I keep it in till it dries?
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Yes, any good commercial contractor store will have it. It is quite inexpensive. You need to buy the diameter that makes a good friction fit in the joint. It is better to force a slightly too large one in rather than something too small. You want to push it below the finish surface so that the caulk depth will be 1/4 to 1/2 inch deep.

I don't know what your previous caulk experience is, but you will have no problem with NP1 or equivalent falling out of the crack. Caulk should never make a 3 point contact, hence the backer rod. The caulk should have an intimate bond on each side, none in the center. You will probably not be able to run a finish bead from the gun, plan on tooling the joint. It is better to have too little caulk and need to add some rather than the opposite.
(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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