I drape 2 large mylar sheets over my fluorescent fixtures for reflectivity. But moisture builds up and the plants tend to get powder mildew or some other sort of mildew.
What is the best way to vent my mylar hood?
You need a fan but don't install one if you have powdery mildew. It will just spread it to all the plants. You have to get rid of the mildew first. Try a 50/50 milk and water solution. Spray your plants every 5 days. If the mildew is not gone in one month you may have to throw the infected plants out or try a chemical solution.