I am new to indoor growing but this is my experience. I started off lining the inside of my grow area with the dull side of foil and it didn't work out very well for me due to overheating, so if you are going to choose this option I'd make sure you have some adequate ventilation. When I decided to start again and try to do things a bit more properly, I lined with matt white and found that this assisted in keeping temps down a bit. I believe foil can be bad because if it is not properly applied (ie, not perfectly flat) the wrinkles can cause hot spots within your grow area which can concentrate heat on plants and burn them. From researching mylar I also believe that if you are going to use this, you have to ensure you apply it so that it is perfectly flat in order to get the optimum relectivity from it. If you're going to go with matt white it would probably be best to paint the area or use some kind of plastic lining as opposed to paper which could create a potential fire hazard with strong lighting and encourage mould/fungus. I stress that I am no expert and am just sharing my experiences with this issue.
