Your garden environment must be clean. Very, very clean. Most diseases and insect problems come from a less-then optimal clean. You might get bugs and mildew, but a clean environment will minimize these problems.
If your walls are concrete, such as a basement, and are less than ten years old or unfinished, you will need to cover them with a plastic barrier called panda cloth. Older concrete walls have probably been painted many times. If so, it is still a good idea to repaint the walls with FLAT finish paint in the purest white you can find. While it seems to make sense that glossy paint would reflect light better than flat paint, it is not true. Gloss in a paint is created by adding a shellac or other substance that will dry on the surface of the paint, rendering it shiny. The surface is actually yellow in color, greatly reducing reflectability.
Grow tents are also useful if you must have your garden in another part of the house. Ventilation is easier, and the size of your light may be reduced. Figure you will need about 1 square foot per clone, and four square feet for bloom. 12 plants in veg will only take up a 3 x 4 space, while 12 plants in bloom will require 48 square feet, or a 7×7 foot room.
If there is no way to enclose your smaller veg area, a grow tent for your mother plants and clones is very practical, since there is no light leakage to interrupt your bloom cycle.
Sheetrock or plaster can be painted with a flat white ceiling paint or covered. Covering with panda cloth makes the walls and floor easy to sanitize. It is also waterproof. Always cover your floors with plastic. Sheetrock can be covered, or painted with a flat “super white” latex paint such as ceiling paint. If a single room must be partitioned off to provide a veg area and a bloom area, make sure the veg area is only about 1/4 the size of your bloom area. This is especially good for seed growers, since they can move ready plants into the bloom area as soon as harvest is finished.
Mylar is a very shiny plastic that imitates mirror. It is very difficult to install, because any crease or dimple will cause a hot spot, something equivalent to holding a magnifying glass over an ant. Mylar will also lose its shininess if it gets wet. It is thin and creases easily. Some highly experienced growers use it to great success, but, for most of us, mylar makes fancy wrapping paper and balloons.
Using mirrors is wrong. The light must pass through a layer of glass and then bounce off the silver backing, and return through the glass. Lots of lost lumens.
Use metal tape to seal seams, since it is highly reflective, waterproof, and does not leave a sticky residue when removed. Build your own with 2x4s and panda cloth with an adhesive zipper for a door. Amazing professionally built grow huts are available, which can solve many problems if your grow room has to share a space.
Make sure you have easy access to water and a drain. Keep your nutrients, tools, and other items organized and clean. Too many failures have been the result of messy growers.
Figure 35 – 50 watts per square foot of growing space for your bloom room. Use T5 fixtures in the veg room. A 600watt gives great coverage and penetration to a 4’x4’ area. Allowing four square feet for a normal plant, this allows four plants per 600watt. Weirdly shaped rooms, like a hallway or closet, require some tweaking needs to be low, or if heat with large wattage lamps is impractical, use a light mover. This will cover more space. Details of this aspect will be covered in the Lighting section.
Light MUST NOT leak into your room. To check, turn a light on inside the grow room, close it up, and turn the outside lights off. Fix leaks with metal tape. Then go inside the grow room after turning on the outside light and turn the inside light off. Cover any leaks with metal tape.
Before you set up any equipment, wipe down your walls, floor, and ceiling with a professional greenhouse cleaner like Physan. If you are organic, use 10% bleach or an organic disinfectant found at any health food store.
Keep pets, kids, and visitors out of your room. It is always nice to show off your juicy tomatoes in February, but each visit runs the risk of contamination by bugs or disease. Make sure you do not enter the room in dirty clothes, especially if you have been outside. (One grower I know showers and then works in his garden in the nude. Extreme, but no bug problems!)