But it will need a LOT of work, insulation, updated wiring, new roof, heating system, and potentially a foundation.

Any mold in attic will require new insulation.

In any case, storm windows over a well painted and caulked window trim can almost equal the r value of double paned windows and it matters little because glass is a terrible insulator whether single paned or triple w argon.

Some units are poorly insulated and the tenant downstairs can hear the tenant upstairs talking on the phone or watching TV.

If you sheetrock & insulate the ceiling and add appropriate recessed lighting, it adds a sense of completion that is currently lacking.

Kitchen, flooring, bathrooms, paint, insulation, drywall, electrical, plumbing, new heating system, windows, doors What was the outcome?

I am in full agreement that a positive cash flow is a very important goal, due to the fact it insulates you from gambling on appreciation, equity, and rent increases. 

Is it pipe insulation, boiler insulation, some kind of blanket insulation?

No allowance for insulation or other thermal improvements - such as upgraded windows.  

And having a third (or fourth) mortgage would insulate you from "calls" on your LOC.