My home is a bit over 100 years old, wood frame aluminum siding house, that was not very well kept up by the previous (30+ year) owner. We've lived in it for two years, and are looking to improve the insulation in the home both to reduce heating costs and reduce in-home variation of temperature.
We have three bedrooms on the second floor and living space on the first floor. Modern windows (2-pane). We're in the upper midwest, so very cold for 4 months or so. 2 story, single family home. Brand new very high efficiency furnace with ductwork that goes up the middle of the house to the attic and back down into the bedrooms, and two-zone (each floor separately) controls.
Last winter we rolled one layer of fiberglass insulation in the attic (it prior had blown-in junk that was basically useless), and that seemed to improve things some, but it's still quite cold in much of the house even when the thermostat thinks it's 70. The walls are definitely cold, so we assume there is little to no insulation between the aluminum siding and the (mostly) plaster walls, and the couple of times we've had to dig into the plaster (for wiring) we haven't seen any evidence of any sort of insulation.
What is the next low-hanging fruit in terms of providing the most bang for the buck? We've considered foam in the walls, but that seems very expensive for the R-values we would get. We could just insulate the second floor walls, but we're unsure if the cold first floor would make it too ineffective. We're also not sure if we could realistically foam the whole floor without doing a huge amount of re-drywalling/re-plastering. Would it be easier/cheaper to go at it from the siding side instead (and would that work)?