Need thoughtful input on first grow room

First time grow room here..

I've been planning for about a month. Today I've got things rolling, however, I'm in need of some real thoughtful insight on this project.

There are two issues I am facing that may need some rethinking.

1. I'm questioning if I should use the foil faced foam sheathing I have for insulation on the inside of my grow room. The dimensions of the room are 39 inches deep, 30 inches wide and 5' 8'' tall. My plan was to use this insulation then cover it with mylar. The real question is. Should I still use the sheathing in combo with the mylar or simply attach the mylar directly to my OSB.

2. Ventilation is going to be a primary concern. I'm not wanting the whole house to stink. I'm debating if I should trade a little bit of height for better ventilation options. specifically I'm considering lowering the overall height of the room by about 4 inches. I'm simply worried if height during flowering will result in issues, as I'm planning a 400 MH setup *already have the bulb(s)* *ordering ballast next week *. With this extra 4 inches shaved off, I'm looking at a total height of only 5'4''. This, however, would give me the room needed to ensure good ventilation running for pipes and proper intake into the enclosure. Should I be worried about losing this extra 4inches of head room effecting the growth for the benefit of the ventilation options?

don't lower the ceiling, you want as much room as possible. get some fans in there.

mylar to osb imo...really no need to insul a small cab..u can put thew fan on the out side of the box and use a small filter..5'8 should be ok i have less then 4 feet in my cab..just make sure u LST or grow a indica...

No reason to worry about water and moisture with using just the mylar?

would posting pictures entice some other replies? I'd really like somewhat of a good discussion on this .. then simply being told what to do.

I appreciate the help however fellas