Insulation, Radiant and Vapor Barriers - should I fill or cap the airspaces in block construction

My house is stucco over concrete block with interior plaster over furring, so I don't know if there is anything I can do about insulating the walls. Recently I was in the attic and noticed a few places where I can see down inside the block walls. Is there something I should do about that to help heat retention or does the block need to 'breathe'? Will trying to blow insulation down there help or will it just get soggy and moldy; impossible to remove? Can I cap them somehow to maybe cut drafts or will that just build up moisture? I'm in Boston and temperature in the teens the last few nights have really made me wish I did something to improve heating efficiency last fall.

Block needs to breathe.

Use a breathable injectable foam in the furring space, installed from the attic. It will amaze you.

And realize that cold air is only coming in through the block walls if hot air is going out of your house somewhere. So air-seal your ceiling too.

Great idea! I don't know why it didn't hit me to try to spray something down between the furring strips from above. If I can see electrical cables coming up into the attic and contemplate running more down, why didn't I think of that? I've been worried about the idea of patching holes at the top and bottom of every stud cavity to get foam injected from the interior.

Still, do you have any pointers or brand names on finding the equipment and material, or someone to come in and do it (I'm in the Boston area, if anyone has local info)? Are there brand or product names I can start with? I guess insulators are busy this year, since I haven't been able to get anyone. My local home center doesn't have spray equipment or supplies for rent, only blow in cellulose. I found one Canadian company, I forget their product name, from a link from Ask This Old House, but their online dealer list was outdated and those who still provided it weren't interested.

With proper construction practices, you should not be able to look down into the cores of the block for many reasons.

It would be good to close the cores with another 2x next to the sill plate you have now. No real need to go overboard aith sealing and caulk. Just do what you can to prevent air circulation up, down and out of the cores into the cold attic..

Dick

Your stucco will be relying on the block cores to keep both the stucco and block dry. Don't block off the top or bottom of the CMUs.