Several product manufacturers offer CMU admixture products that make the resulting CMUs less absorptive of water; these CMUs are used in conjunction with a companion mortar admixture to reduce the amount of water absorbed by the wall. The product we are familiar with is Grace Construction Product's DryBlock.
Most designers will use a "belt and suspenders" approach on a single wythe wall and combine the water resistive admixtures with a careful application of an elastomeric coating on the exterior. These are mostly acrylic coatings that have some crack bridging and flexibility properties. Cementitious coatings and silicone and silane penetrating treatments are also available.
There are some bottom of wall products around that attempt to address any infiltrated water in single wythe construction by directing it to the exterior; these can be flashing type materials or special CMU shapes.
Use a breathing type interior finish on the wall, so that trapped moisture can find its way out without causing material failures or mold formation.
The addition of the insulation inserts into the cores of the CMUs as mentioned above will do nothing for moisture management and little for thermal insulation. Not having continuous insulation across a wall assembly is like leaving every other window open. Your client wouldn't be considering this type of construction if thermal performance was important to them.
Last but not least, send a pleasant letter to your client explaining that while you will do your best to specify their chosen wall according to standards of care, it will not perform as well as the wall you designed for them and to not call you when it doesn't.
Good luck!
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Philip Kabza AIA
Partner and Dir Technical Services
SpecGuy
Charlotte NC
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