Natural gas to Propane change over help

There are online but requirement calculators. The better the r value of building the easier it is to heat. Open rafters vs drop ceiling?
As for the conversion....i bought a "Big Maxx" propane furnace. I bought the NG conversion kit at the time, right off shelf for an extra $30.
You HAVE TO VENT, when you heat with gas/flammable liquid.