The Reflective Abilities of popular grow room materials:

Mylar 90 - 98%

Plastic material, prone to creasing, can be reused if well maintained

Panda Film 85-90%

Plastic material, prone to creasing, can be reused if well maintained

Flat White paint 60 - 90%

Paint, typically needs to be reapplied. Look for anti-fungal properties

Semi-Gloss paint 70-80%

Paint, easy to apply and maintain. Look for anti-fungal properties