Better Strategy: Add more HVAC Registers or fully insulate walls?

Question

I have a fairly cold basement, two rooms specifically where there's about a 6 degrees C difference from set point. Basement is fully finished, built in 2007 and back then, half insulated walls were a thing and can confirm, they only insulated the top half of the basement walls.

I just insulated the floors (previous vinyl on concrete direct). I've noticed a big improvement but still not comfortable.

Two ideas come to mind next:

• For the exterior walls, remove the drywall and fill with rockwool (maybe I can get away with not tearing it all down)
• Add / optimize / modify air registers (keeps air moving, but the half insulated walls feel like a bigger problem)

Two Observations

• Heating from HVAC is in ceiling, returns are at floor level. I noticed in some rooms the registers are not even properly attached or sealed for the returns (its return grill and then open cavity)
• When the heat pump runs instead of the gas furnace, the basement seems way more comfortable (switches to gas when below freezing). I don't know if this is due to the heat pump running longer or just in less extreme temps...could be a red herring.
• Dehumidifier helps improve comfort even in the low humidity winter months

What feels like the better item to address? Is there something else I'm missing?

Thanks!

Answer 1

You'll pay for insulation once, you'll pay your heating bill every month, so I'd focus on insulation. The dehumidifier is likely running a lot with low humidity already, and its waste heat is probably what's contributing to the feeling that its operation is improving comfort.

In regards to the open ducts you're seeing, in many houses the wall cavities are used for returns with no gap sealing inside the walls, so that may not be an issue at all.

I think you're on the right track when it comes to improved comfort with your heat pump running. Since it's running a lot longer, the air is getting circulated more, so the cooler basement air has a longer time to be pulled into your return ducts. Although we have a natural gas furnace, we live in a ranch and the lower level is always cooler than our main level. So we have our furnace fan running at low speed 24x7 to help keep air circulating between the floors (via the furnace fan running all the time. It doesn't keep things perfectly even, but between that and partially closing some upstairs ducts, it's pretty close to equal temps between floors on most days.