Before and After Air Seal and Insulate the Rim Joist in Cabin John, MD.
What you see here is the bottom of a 6' sliding glass door. A big local company installed it but cut out the floor to do so. They actually caused a huge problem where there was none before! It's what can happen when building science is not used correctly. The solution was to air seal and insulate the rim joist eliminating the huge gaps the sliding glass door left in the envelope.
We find this almost everywhere. A bathroom exhaust vent routed out of the soffit vent... Turns out a soffit is for air intake! Yup the black stuff is mold. Every time we run across this (all the time) we run the vent to the gable and cut a new hole for it. The vent must rise 1/4" per foot and be positively vented to exterior...every time....