Huisken and Ilmanen in [25] created the theory of weak solutions for inverse mean curvature flows (IMCF) of hypersurfaces on Riemannian manifolds, and proved successfully a Riemannian version of the Penrose inequality. In this paper we investigate and construct the sub-Riemannian version of the theory of weak solutions for inverse mean curvature flows of hypersurfaces in sub-Riemannian Heisenberg groups. We extend the weak solution theory in [25] to the first Heisenberg group and prove the existence, uniqueness and basic geometric properties of horizontal inverse mean curvature flows (HIMCF). By a Heisenberg dilation on HIMCF, we find a horizontal perimeter preserving flow (1.7) in the first Heisenberg group, and prove the existence and uniqueness of weak solutions to (1.7). Using this existential result, the present paper gives a positive answer to an open problem: Heintze-Karcher type inequality in the Heisenberg group. At the same time, this article also proves a Minkowski type formula in the first Heisenberg group.