I have several IoT systems installed in farm hoophouses (used to be called "greenhouses"). Each configuration is housed in a (mostly) waterproof plastic container -- about 12" x 18" x 8". The hoophouses are environmental trouble -- often high humidity and the daily temperature can range 50F to 120F. Processors are either Particle Photons or Raspberry Pi Zero-W and nearly all include relay boards. E.g.:
The boards are screwed down (loosly) to 1/4" plywood -- non-conductive, but I didn't like the back side of the boards:
So I decided to stick masking tape on the back of the boards. At least they ere easier to handle. Like so:
At least, in the case of the particular product I used and/or the temperature range, that tape was NOT an effective insulator. I've had a few "relay board failures". But today (belatedly) I proved to myself that it was the stupid masking tape. When I replaced the tape with the black electrical type my failing relay board was fixed.