Sound "isolation" (both in and out) is hard to do and expensive, and as noted, you can't remove 100% of "sound" in the environment, and for all practical (especially in a "home recording" sense) purposes, not going to make a difference in your final mix. Treat the room or arrange the space so
it is not the problem, and then just get good at workarounds if you want to get on with recording, vs. trying to get a "silent" recording space.
Sure, you can turn off your furnace, but you won't be able to use your recording space all the time. I have to turn off the A/C here if I want to do actual recording for a few months of the year, so I tend to do very little during those months, honestly. It has to be more planned and practiced than I am usually in a mood to do
. Fixing that problem would essentially mean building a new house, and that's not going to happen...
And outside noises intrude. There's a nearby, small airport, and it seems like the neighbors always have some lawn equipment or hobby woodworking project (one likes to grind metal!) going on. Keeping that noise out would also require a new house, or moving this one. But, if I am patient, there are gaps of relative quiet, and I just have to be able to wait out the small airplane that decides to buzz overhead right when I'm starting to record, or do a re-take if it does it at the very end of a take. Low-level noise stuff in the middle of tracks will often just disappear in the mix, or can be cleaned up, or a comp created from a re-take in some cases.