Based on my reading of the data sheets the SHT-11 and the SHT-21 have different interfaces. For the earlier products (SHT-1x) Sensiron used their own Sensibus (?) interface, but for later products (SHT-2x, SHT-3x) they used standard I2C. You might look at the SHT-75 which I understand is a mounted version of the SHT-11 but please read the data sheets very carefully.