Thermally evaporated stoichiometric films of red HgI2 show preferred orientation growth with either (102) or (002) orientation. The as grown films shows a change from one preferred orientation to another depending on their thickness, open-air heat-treatment and in-situ heat treatment of films. The in-situ heat-treatment of thermally evaporated stoichiometric films of red HgI2 with preferred growth of (102) orientation shows a gradual linear decrease in film thickness with in-situ heat-treatment temperature. On in-situ heat-treatment above 80°C, it is found that HgI2 films become thinner than 900nm, which are otherwise difficult to grow due to high vapor pressure of HgI2. For these films the preferred orientation also changed from (102) to (002). The optical band gap (Eg) also found to increase linearly with in-situ heat-treatment temperature. It is interesting to note that in-situ heat-treated films having (002) orientation had higher values of optical band gap than (102) orientation films. On combining these results with those of as grown and open-air heat-treated red HgI2 films reported in the literature, it is evident that there exists an optical band gap demarcation around 2.15eV for red HgI2 thin films depending on their preferred orientation growth. Films with (102) orientation are found to have optical band gap less than 2.15eV and those with (002) orientation are found to have optical band gap more than 2.15eV. This is irrespective of the physical mean of obtaining the preferred orientation. The preferred orientation can be achieved by either physical means such as growing films with higher thickness, heat-treating them for short duration in open air or heat-treating them in-situ.