There is much discussion over which heaters are waterproof and/or weatherproof and if you read the Heat Outdoors description of the 'IP' ratings you will see that there are specific values given to each product for the level of water and solid particles that can get in. In theory an IP65 rated heater is completely secure against dust and water and can function outdoors in all weather all year. Many of the products that have an IP65 rating are surprising in that they have no physical barrier between the heating lamp and the air around, with just a simple wire mesh or grill to prevent anything getting in. Many questions have been asked as to how this can possibly claim an IP65 rating, the answer they will give is this "The essential electronics are sealed in a separate compartment and protected against moisture and dust etc. The lamp element itself is exposed to the elements, however, is fully functional and will even happily operate under water".
Now comes the argument of does IP65 mean the unit is hermetically sealed in total or only the parts that might be affected are sealed? To further confuse matters some manufacturers rely upon a system of self certification where as others send their products away to independent laboratories in order to gain their IP65 certificated status.
Undercover heaters are often used in outdoor environments and will usually be perfectly fine as long as they are not subjected to a direct deluge of water. Many systems fitted to parasols do not need to be fully waterproof as the parasol itself offers significant protection against the elements and they can usually be easily taken down and stored indoors when not in use.