Outdoor enclosed letter boards are specially designed to be used out of doors and have been modified from tradition letter boards to be able to withstand the elements and harsh conditions when used outdoors. These changeable letter boards or directory boards have special silicone seals and venting holes designed to release excess heat. The grooved material used to hold the white plastic letters in place is made with vinyl vs. felt. Felt material would not stand up outdoors and would mold, mildew and eventually rot out.