based on similar questions on mesh volume, volume of a mesh can be calculated by following equation:
volume = ((vec1 x vec2) . vec3) /6
where vec1, vec2, and vec3 are the vectors from origin to a mesh triangle, and x is cross product and . is dot product.
and for the volume of the whole mesh, that should be calculated for all triangles.
Afterwards, the area of the triangle can be calculated by the following equation:
area = || (vec2 - vec1) x (vec3 - vec1)|| /2
where vec1, vec2, and vec3 is the same as previous equation.
calculating volume and area, the height can be obtained by following equation:
height = 3*volume/area
However, I am not sure how to perform final calculation to obtain the wall thickness when I have all the heights of triangles (which are the vectors from origin perpendicular to triangles).