An integer Heffter array $H(m,n;s;k)$ is an $m\times n$ partially filled array whose entries are the elements of a subset $Ω\subset \mathbb{Z}$ such that $\{Ω,-Ω\}$ is a partition of the set $\{1,2,\ldots,2nk\}$ and such that the following conditions are satisfied: each row contains $s$ filled cells, each column contains $k$ filled cells, the elements in every row and column add up to $0$. It was conjectured by Dan Archdeacon that an integer $\H(m,n;s;k)$ exists if and only if $ms=nk$, $3\leq s \leq n$, $3\leq k\leq m$ and $nk\equiv 0,3\pmod 4$. In this paper, we provide new constructions of these objects that allow us to prove the validity of Archdeacon's conjecture in each admissible case, except when $k=3,5$ and $s\not \equiv 0\pmod 4$ is such that $\gcd(s,k)=1$.