I suggest mylar increases your best use of the "all expensive light" produced from your grow lamps up to 20% vs 5-8% of your white paint. (As measured by PAR readings compared to other materials or nothing at all)
Mylar has other advantages that paint will never have. Such as, Light control, moisture / humidity control by keeping the moisture in/out, if you have a properly sealed cabinet or room you will also aid in keeping out unwanted pests or non-filtered air (assuming you have a proper filtered air intake)
Maybe a few % points helping your lights deliver all their juice to the plant canopy isn't your thing, you might think meh who cares, generally you could be right, the light loss, the humidity controlling factors all depend on a lot more than just a material (Mylar/paint/foil/reflectix etc) however, all of these things work together to create a controlled environment.
Its more than just a siding, its a collective event that you want to work towards.
I suggest mylar properly attached around a custom PVC or EMT pipe frame within a space can be the best solution because you can start to dial in all aspects of the growing cycle. If you leave the walls painted, the paint can eventually take in the moisture, pulling it into the drywall or walls, creating chances for mold and mildew, attracting pests, odors, and adding generally a lot of grief to your grow.
If you build a space, think of as if you want to keep it forever, and if that's true whats the best way to make the area repairable with as least impact to the original space as possible while controlling all of the variables possible?
Do you control vairables and dial every detail into focus by...
A) slapping paint and forgetting about it? No, You get 1-4 grows if that and you begin to add hazard to your cannabis.
B) spending more time, effort and money in proper materials ? Ding! Winner Winner Chicken Dinner.
Mylar tightened by proper fastening and adhering to a strict sense of professionalism in your environment makes it possible to pull 4 harvests a year never mind running it for years on end before replacing a section of mylar.
Think about this more long term, more about dialed in perfection rather than the here and now.
Use the best you can afford, 600D mylar or higher grade using a diamond diffused pattern (Rather than just FOIL or Panda Wrap) and purchase a canvas ring grommet punch tool and punch some rings into the corners so you can draw them tightly around the posts so you can avoid using staples throughout the mylar. the more holes you punch into this sucker the less sealed it is.
A proper mylar will take a grommet really well and if properly done will offer many more options such as ability to add/remove/replace panels for maintenance reasons than trying to rip down entire sheets stapled all around the room.
Lastly, the FOIL TAPE while sounds good, looks good for a while, and does the job does degrade over time, peels and other issues should really only be used in sparing conditions and only if properly adhered.
Heavy duty Contact cement properly applied and cured should be a better friend, but not for "sticking" the mylar directly to walls, i dont support that because of the damage to the walls and all the glue, yikes... that much contact cement can get expensive.
If you dont use hole punches or poles or any other methods you can consider slats, small 1"'-2" wide 1/4" thick slats and adhere those to the wall, typically around the top and bottom of the mylar positions, then make a sandwich with the mylar between another wood slat and secure every 16" on center, properly done that mylar is never coming down by accident, no glue required, no moisture issues, no staple issues, done like dinner.
Good luck
I can't decide if I want to paint it white, or if I should hang Mylar reflective Inside.. Just wondering what everyone's thought's are on the subject...
