A roll-up closure utilizing a flexible cover (18). The cover (18) is at least as wide as the portal (11) to be covered and is typically longer than the portal (11). Each lateral margin (21, 22) of the cover (18) has a strip of fastener material (27) affixed thereto, and a complementary strip (26) is affixed to the lateral margins (12, 13) of the structure defining the portal (11). A first end (19) of the cover (18) is affixed across a first margin (17) of the portal (11). The opposite end (23) is upturned and connected to a take-up roller (31) mounted above the first margin (17) of the portal (11). A transverse rod (34) is supported within a pocket formed by the upturned end (23). Activation of the roller (31) lengthens or shortens the effective length of the cover (18) and the mating fasteners (26, 27) are positioned to seal and unseal the cover (18) to the lateral margins (12, 13) of the portal (11). Additional elements can be included to promote formation of an adequate seal.