Weird black geometry pops up when rendering translucent material

Hello, I am rendering a scene where the facade of the building is made with translucent material (ie polycarbonate). However, a few weird black geometries keep popping up at some parts of the facade (not all). The facade is close to the exterior wall it attaches to but there is still a 20cm gap. I tried with different lighting the problem persists (currently under vray dome light with HDRI and panel lighting for interior). so just wondering how should I fix this problem? Thank you