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Removing the interior wall will not give you access to the cavity above and beyond the top plate. Removing ceiling will expose the joist parallell to the sill plate, which also blocks access to the cavity. You would need to bore holes, that do not impinge upon the outer thirds of the joist, to gain access. The same is true if you remove siding except you will also be boring through the sheathing. Either way you do it, save the bores to glue back in with caulking to plug the holes. On a wall perpendicular to the sill plate, the ceiling is the easiest access- no boring required.
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The only way to test l can think of would be to fill a plastic bag with 1 cuft of air, drill a hole thru the joist, tightly fit a pipe to the hole, use a plastic bag to push the 1 cuft air into the cavity, and see how long it takes. Multiply to get cuft/min. This is VERY approximate as changing the air pressure, by squeezing, changes the air transfer rate.
General C&ontractor
35 years of troubleshooting construction, remodel, component & material failures. What to do next