My house was built in 1953 but flipped before I purchased it a few years ago. Parts of the house still have character from the 1950's and it is a compilation of three builds at different times. My basement is two different slabs and I also have a house above a crawlspace. The two basement sections are cinderblock walls that are not insulated on outside or in, one of the basement areas with it's own slab is considerably colder than the other area, almost 15 degrees different. I have HVAC system putting heat into my basement and crawlspace.
In one of my rooms above the basement on two exterior exposed walls I notice that a certain section of wall is extremely cold, with IR thermometer it was reading 40 degrees when the room was 68 and the outside temperature was ~0F. This part of the house is over the basement and it is a cinderblock wall that is uninsulated. The rim joists are also uninsulated, although I recently put batt insulation in the rim joists to try and help with the entire basement comfort. This is a long intro, but i'm trying to figure out how to diagnose whether or not I should work on the first floor or basement first.
Option 1. Start in first floor with wall. Pull off drywall from room, check that proper insulation is in place. I've pulled off a wall plate and examined insulation, there is some behind the drywall but it looks as if it was pressed down, so doesn't look perfect.
Option 2. Insulate basement rim joists. Area is behind an old finished area.
For insulating the rim joists I previously had a contractor use Thermax sheathing for my crawlspace and it has worked wonders. I was contemplating putting thermax sheathing in each rim joist against the external wall and then filling with batt insulation. Other option for rim joists is to contract someone to spray all of the rim joists or purchase the spray foam kit from menards.
I have a feeling that my uninsulated basement, cinderblock wall without any insulation outside or in, is causing most of my issues but wanted to make sure I wasn't missing something obvious.