Hi, I'm trying to do destruction for this simple house I modeled in Rhino3D.
It's exported as obj.

I get this bad result after the Voronoi fracture.
How can I fix it? Should I be modeling it in Houdini?

Before fracturing, make sure the geometry is solid, with no open holes in the mesh. You can use a Group SOP > Group Unshared Edges, then PolyFill to close any holes. Use Fuse SOP in case you have disjoint vertices.

That just looks like overlapping geo... sure you are not viewing 2 objs?