If you refer to the Ruuvi, then they gave the answer in the specs:
The environmental sensor breathes through an IP67 certified water-resistant vent sticker made by GORE.
However, your question is basically showing the crux of the whole thing: the sensor will break sooner or later if you’re exposing it as-is to an outdoor environment but protection is going to impact the measures. I believe @dk9uv gave a good indication how a suitable protection could work - together with the radiation shield.