A weatherproof sealing apparatus (10) to seal the opening between upper (200) and lower (100) window sashes in the open position; wherein, the sealing apparatus (10) includes a support member (13) having a generally cruciform cross-section and a resilient sealing block (20) attached to, and projecting outwardly from, the support member (13) to compressively engage the space between the upper (200) and lower (100) window sashes.