[jsb5717]

I recently did something similar in ours. I opened the wall in the under-storage and found HUGE holes through the floor to the sealed underbelly for plumbing. Also found a couple large holes into the main living space for plumbing in addition to the air vents under the steps. Using both rigid and spray foam I sealed all of those leaving only the return vents under the steps. Also insulated the wall to the main living space. There were also gaps above the screw-on wall in the under-storage between the floor joists. Rigid foam and tape sealed those up.

Now the only air the furnace can breathe comes from the living space. The furnace is also even quieter now in the living space. Heading out again in a week. We'll see if it makes much of a difference.