Every day you operate your indoor cannabis grow room, you’re forking out money on electricity to run your grow lights. Light is, after all, an essential nutrient for your plants. You want every watt of electricity that goes into your grow lights to generate light energy that reaches leaves, so they can engage in life-sustaining photosynthesis to fuel plant growth.
For this reason, reflectivity is a big deal in grow rooms.
Part of the reflectivity game begins with the type of reflector hood you pair with a high-intensity discharge (HID) grow light.
Growers using light-emitting diodes (LED) grow lights don’t have to worry about this, in part because the diodes are encased in a material that functions as a reflector.
The wall reflectivity of your cannabis grow room has a massive impact on how much light reaches your plants’ leaves, and how much light is wasted. Light emanates downward and outward from LED grow lights and HID reflectors. Approximately 20–40 percent of it hits grow-room walls. You want as much of that light as possible to bounce back from the walls and onto your plants.
One way to ensure this is to make sure your grow space adequately fits the number of plants you’re growing. Light intensity drops off with distance, so the further the plants on the outside edge of your garden are from the walls, the less light will reflect onto them.
Make sure that no plants are touching the walls by keeping a gap of 3–4 inches between the walls and the plants to allow for airflow.
Wall Wisdom: How To Make Your Grow Room Reflective
One important choice growers must make is what they put on their walls to positively impact light reflectivity. The main choices are a coat of white paint, or an adhesive sheeted material.
Flat White Paint
Paint can sometimes be the easiest choice from a logistics perspective. Once you’ve painted the grow room, you don’t need to do anything else to the walls, and there should be no need to place additional material on top of the painted surface.
The smartest paint choice is a high reflectivity flat white interior paint that’s designed to repel molds, fungi and mildews, and has little if any off-gassing volatiles in it.
When properly applied, flat white paint reflects 75–85 percent of the light that reaches it. It’s even better if walls and ceilings are flat surfaced, rather than having an orange-peel, textured or bumpy finish. Using flat white paint on a flat wall, you get uniform bounce-back of light — but there can be as much as a 25 percent loss due to the paint’s reflectivity limits.
And white-paint reflectivity ratio isn’t as high as with Mylar or Panda Film.
On the other hand, with paint you don’t have to worry about wrinkling, and there’s nothing to mount to or concerns of the material falling off the wall. Plus, a painted wall is easy to clean.
Mylar Reflective Sheets
This loss of reflectivity is why many growers avoid the paint option in favor of using materials that attach to walls and offer higher reflectivity. Of these, the most reflective is Mylar, a thin, opaque material that looks like a sheet of silver.
Admittedly, it is tricky to attach Mylar to walls, and especially difficult to fix it to ceilings, because it’s a thin material that can easily rip, wrinkle or scratch. Mylar should be applied with the same amount of care as one would apply a window-tinting film on a car. The material must be absolutely flush to the wall, with no bends or wrinkles.
Any wrinkles in any kind of reflective material will create hot spots and dark spots. A hot spot is a concentration of light energy, while a dark spot is a deficit of light energy. When reflective materials are wrinkled, bent, or otherwise not flush to wall surfaces, they don’t reflect light evenly.
Mylar and general reflective sheeting materials must be attached securely so they don’t catch fan air and ripple with air movement. Many growers use Velcro to attach 2 Mil Mylar to walls (always use the thickest Mylar you can find). I use white Gorilla Tape, and as little of it as possible, which works well, unless you have inferior humidity control, which will cause your interior walls to sweat and the tape to loosen.
As with all silver-sheet reflective materials (including Foylon metallic sheeting), Mylar reflects heat radiation, not just light radiation. This isn’t desirable, and it’s one of many reasons why you want aeration fans blowing through your plant canopy whenever grow lights are on, and also a climate control system with enough capacity to remove excess grow-room heat.
Mylar is fragile and difficult to work with. If you foliar spray your plants and drops of water get on your Mylar-covered walls, or if the air in your grow room isn’t well filtered, the surface reflectivity may be dimmed by droplets or dust, and it’s hard to clean without tearing or leaving streaks.
While reflective sheets can prove burdensome to mount flush to walls, they do offer the highest reflectivity of any material, and as an added and unexpected benefit, they give you trippy visuals, much like a carnival trick mirror. (I confess to many evenings seeing cosmic meaning in the patterns of reflections that shimmered in my Mylar as it shifted in the fan breeze — after inhaling a pure sativa, of course.)
The most popular reflectivity material is white poly plastic, popularized under the brand Panda Film. I use the thickest version of this plastic sheeting I can find. It has a highly reflective white coating on one side, and black coating on the other.
Panda Film reflects 75–89 percent of the light that hits it and is much more rugged than even the thickest Mylar. It’s easy to clean and hang, and is cost-effective and versatile in ways many growers don’t yet even know about.
Indeed, I’ve used Panda Film as movable grow-room walls by affixing the sheets to the ceiling as a way of framing my grow space, with lead weights on the bottom to keep the sheets from blowing in the fan wind. The plastic served as my grow-room walls, and I could sculpt its placement to precisely match my plant spacing, so the leaves always got maximum bounce-back of light. It’s like a custom-sizable grow tent without a frame.
Dimpled German Aluminum
We’ve seen grow ops with walls covered in dimpled German aluminum, the same material used in the highest-quality grow-light reflectors. It was an expensive option, but the reflectivity was about 95 percent, and the material was easier to clean and more durable than Panda, Mylar or paint. It used to be that you could buy big plates of dimpled aluminum at a grow shop, but we haven’t been able to find it in recent times.
Clearly, each of these reflectivity options has its own advantages and disadvantages.
Bad Choices For Grow-Room Walls: Mirrors And Aluminum Foil
Growers have been known to use other reflective materials, but none are worth the effort and can be unsafe. Mirrors, for example, are a very bad choice, as they absorb light, and reflectivity can be as low as 50 percent. Also, they break.
Aluminum foil is another bad choice. It offers less than 50 percent reflectivity, has a tendency to wrinkle and bend, is difficult to mount and clean, it can cut you, and worst of all, it conducts electricity.
Some growers use thermal blankets, infrared blockers and similar materials, if those materials have a highly reflective coating on at least one side. Infrared blockers are particularly useful if you suspect police and/or thieves are using infrared detection devices to scan dwellings in your area, looking for the telltale heat signatures of grow lights.
In some prohibitionist states, police routinely fly aerial patrols using manned aircraft or drones that have infrared radar to see thermal data through walls and roofs. Police may use this data to convince a judge to sign a search warrant.
Thieves use thermal detection drones and handheld devices to gather the same kind of data, which they then use to identify grow ops they can later rob.
Infrared blocking material is only useful on ceilings and for external walls to discourage infrared scanning. Otherwise, it’s not an ideal reflective material in an indoor grow room because it traps heat, is expensive, thick, and tough to mount.
So, you now know the good and the bad about cannabis grow-room reflective materials. Mylar and dimpled German aluminum are the top picks, while Panda Film is a close second, followed by flat white paint at a distant third. The bottom line is this: Get as much light onto your indoor marijuana plants as possible, so every penny you pay for grow-light electricity generates the fattest buds possible.