Indoor Garden Design and Build

Planning your Indoor Garden

We're going to take you through a simple 4 x 600W HID garden and a large 6 X 1000W garden using non air-cooled reflectors and basic ventilation.

In this article we are going to discuss setting up two high intensity indoor gardens in the same space using:

4 x 600W HID Lights

4 x 600W High Intensity Indoor Garden


6 x 1000W HID Lights:

6 x 1000W High Intensity Indoor Garden

Lighting and ventilation go hand in hand. The more grow lights you have, the more ventilation you will require-otherwise conditions will quickly become too warm in your indoor garden.

There are many types of indoor garden-it all really depends on what you want to do with it. For instance, propagating seedlings and cuttings is one of the easiest applications because only a relatively small amount of space and light is required. On the other hand, if you want to grow light loving plants through their full cycle (from seedling or cutting through to harvest) using just artificial light (grow lights) this can be more challenging-but certainly not insurmountable!

The key issue for high light intensity indoor gardeners is dealing with the excess heat generated by the grow lights. It's essential that you keep temperatures in your indoor garden within the optimal range for plant metabolism-typically between 73 and 80 °F (23 and 27 °C) for many commonly grown plant species-for maximum growth and bloom.

High light intensity indoor gardens require a lot of light combined with a high-powered ventilation system to keep the growing environment from becoming too warm.

Evaluating Your Space

So - where to start? The first thing to do is to measure the total space you have available. In our example we have dimensions of:

Empty Room

30 ft (9.1m) x 18 ft (5.5m) x 8.2 ft (2.5m)   (Length x Width x Height)

Don't be tempted to fill all the space you have available, especially if you are a relative beginner. It's a smarter move to create a smaller room within the larger room.

In our example below, we will section off a space for the garden itself:

Partitioned Room

20ft (6.1m) x 12ft (3.7m) x 8.2ft (2.5m) (Length x Width x Height)

The remaining portion (10ft x 18ft) can be used to store your nutrient reservoir and other electronic equipment (e.g. ballasts) which are best kept out of the garden.

By creating this "room within a room" you gain better control of the environment. The best way of doing this is to create a wooden framework and partition walls. You can also line your partitions with reflective sheeting if you wish.

Highly Recommended

Before you reach for the wood, saw, hammer and nails-you might want to check out purpose-build, professional indoor grow tents first. Boy can they make your life easier! Grow tents are ready-to-assemble "grow rooms" that come in many sizes-modular kits are available too. One of the principal advantages of grow tents (other than the fact that no tools are required!) is that they make hanging lights, fans and filters child's play-they also make your grow room portable, and there's no need to drill into existing walls or ceilings. 


High intensity discharge (HID) are widely available at indoor gardening / hydroponics retailers. They are competitively priced and produce great results in a properly environmentally controlled indoor garden. There are two main types of lamps used in HID systems: High Pressure Sodium (HPS) and Metal Halide (MH).

There are various wattages: 1,500W, 1000W (most common in North America), 600W (most common in Europe) 400W and 250W. Each size light is suitable for a defined amount of floor space:

Effective illumination for plant growth from various wattage HID grow lights

1500W = 5-6 ft squared (1.5-1.8m squared)

1000W = 4-5 ft squared (1.2-1.55m squared)

600W = 3.3 - 4 ft squared (1-1.2m squared)

400W = 2.5 - 3.3 ft squared (0.75-1m squared)

250W = 2 - 1.5 ft squared (0.6m-0.45 squared)

The more powerful the light you choose, the greater the minimum distance from the plant canopy. If you have a low ceiling you should consider using lower wattage lights. The distance between the light (horizontally mounted lamp) and the canopy that most growers follow are:

1500W = 35-45 inches (89-114 cm)

1000W = 39-31 inches (100cm-80 cm)

600W = 31-24 inches (80-60 cm)

400W = 24-16 inches (60-40 cm)

250W = 12-8 inches (30-20 cm)

Growers using parabolic reflectors with vertically mounted lamps or air-cooled reflectors can position their grow lights closer to their plants as there is less direct radiant heat.

So the floor space available in the partitioned garden area is 20ft (6.1m) x 12ft (3.7m).

Now, as we already mentioned, you could try and squeeze as many lights as possible into this area, but as well as being productive, you need to make your garden easy and comfortable to work in. Don't underestimate how much additional space you will need to access your plants to make maintenance and inspections easy. Approximately 2ft (0.66m) around your plants is a good working area. Elderly or disabled growers may opt for considerably more space than this. In our first example we're going to use 4 x 600W lights.

If you want to make life more tricky for yourself, you could expand the growing area and squeeze in 6 x 1000W lights. In order to make this room work you would need to choose a growing system or technique that allows you to move the plants to gain access around the garden-perhaps growing in pots or movable beds.


Ventilation in your indoor garden is basically concerned with two things: the removal of hot waste air (i.e. (CO2 depleted) and the input of fresh cooler air. Hot waste air is removed actively using an inline fan, AKA the extractor fan. Fresh cooler air can either be drawn in passively through vents or pushed in actively using another inline fan AKA the intake fan.

Now we know the size of the room, and the amount if light being used, we can now work out the ventilation requirements. In North America most inline fans are rated in Cubic Feet per Minute (CFM), whereas in Europe they are usually rated in cubic meters per hour (m3/hr) (CMH).

Five sizes of inline ventilation fans. These fans are used by indoor gardeners to bring air in and out of their gardens.

How to Calculate The Size of Your Extractor Fan

Here is a popular and straightforward method to work out the size of the extractor fan you need.

First, you need to work out the volume of air contained in your active growing space. What's the active growing space? It's the floor space that's actually occupied by the plants (length x width) multiplied by the ceiling height.

  • Required extractor fan size in CFM= Volume of active growing space (ft) x 1.25
  • Required extractor fan size in CMH= (Volume of active growing space (m) x 60) x 1.25
  • In our example with 4 x 600W lights this is 8ft (2.4m) x 8ft (2.4m) x 8.2ft (2.5m), which gives the volume of the active growing area of 524.8 cubic ft (14.9m3)

Why is the volume of your growing area important? Because you need to change all the air contained in it once every minute!

This is a fairly standard rule of thumb for the majority of high intensity indoor gardens (without AC or supplementary CO2)

The CFM calculation is pretty easy. Your room's total volume in cubic ft. is the basis of the figure as we need to move this every minute. For the CMH equation we need to multiply the volume (in cubic meters) by 60 to step it up to the amount of air changes needed per hour.

You may have noticed the x 1.25 in the equations above. What's all that about?

Well, this allows for the use of a carbon filter attached to the extractor fan. Typically we can expect a drop in fan efficiency of approximately 25% when attached to a carbon filter. This figure is not universally definitive; it depends on the make and age of the filter and the length and course of ducting between the fan and filter and some other factors too!

The long and short of it is that we need to allow for this efficiency drop of 25% by simply multiplying our CFM or CMH figure by 1.25.

Example Calculation:

If we run this equation through our example indoor garden it gives us;

Required Fan size (CFM) = (Volume of Active Growing Area) x 1.25

(8 x 8 x 8.2) x 1.25 =656 CFM

These final figures represent theminimumsize extractor you'll need for the growing area in this example. A well-insulated location such as a basement should be fine using an extractor of this specification. Note that less-insulated and more sun-exposed location such as an upstairs, south-facing bedroom or attic will require an extractor around 25% more powerful.

Note that you will need to match your required extractor size to the nearest size available. In this instance the nearest widely available inline fan size is an 8" (200mm) 745CFM extractor.

If we illuminate more of the garden area (15ft by 10ft with 6 x 1000W lights) we end up with very different numbers:

Required Fan size (CFM) =  (Room volume) x 1.25

(15 x 10 x 8.2) x 1.25 =1537.5 CFM

This garden would probably be best served by multiple extractor fans.
One possible solution is:

1 x 12" 1060 CFM extractor.          (E.g. Hurricane 12" Inline Fan)
1 x 8" 745 CFM extractor.          (E.g. Hurricane 8" Inline Fan)

The combined extraction power of the above two fans is: 1805 CFM.

Screw all this math-Why not just go BIG?

Some growers think "Why bother with the math? Just go for the biggest extractor fan/s you can." However "bigger is better" does not always apply to extractor fans. When you remove air from your indoor garden you're not just removing heat, you're also removing humidity. This means that an oversized extractor fan can dry indoor gardens out and cause low relative humidity. If levels sink too low (below 45%) this will cause excessive transpiration and poor plant growth.

But my grow room Gets TOO HOT in the Summer! Surely I need a supersized extraction fan?

Supersized extraction fans are also not always the answer to a warm indoor garden-especially when it's warm outside too! There comes a point where it doesn't matter how much air you're extracting, if your incoming air is warm your room will simply stay warm. If you can't keep the heat down and you're changing the air in your garden more than three times a minute, you need to consider installing air conditioning or using air-cooled or water-cooled grow lights.

Getting Fresh Air into Your Indoor Garden: Passive vs. Active

The simplest way of making sure fresh air gets into your garden is to create passive vents (basically holes) through which fresh air can be drawn in. The act of extracting warm air naturally draws fresh air in from any hole available.

However, that word "fresh" is important! When using passive vents you have to ensure there actually is adequate fresh air outside the growing area. It's no good if you're pulling in stale or warm air. This means you may need to have a window open so fresh air can be drawn in from outside and into the indoor garden. As a rule of thumb, the passive vents should be two to three times the size of the surface area of the extractor fan outlet. This means if the extractor has a 6" (150mm) spigot size, the garden will need 2-3 x 6" holes or rectangular vents with and equal surface area. When installing passive vents always have the extractor fan at the opposite end of the room. It's better to have oversized passive vents than undersized. If the vents are too small, the extractor fan will struggle to pull in sufficient quantities of fresh air.

Some growers prefer to use extraction fans to actively pull fresh air into their gardens.

Indoor gardens with active intake fans often run more efficiently than those with passive vents. By pushing in fresh air you not putting as much strain on the extractor fan and you also get to choose where to pull the fresh air from. During the cooler winter months it's best practice not to pump in very cold air, so a lot of growers pull slightly warmer air from inside their home. If it's a room you spend time in, like your bedroom or living room, it will also have the added benefit of the air being slightly higher in CO2. During the summer months it's best to pull fresh cooler air in directly from outside as air from inside you house is likely to be warmer. Whenever you pull air straight from outside it's best to use an intake filter or 'bug screen' to limit the possibility of sucking in pests.

The golden rule when installing an intake fan is to make sure you're blowing in less air than is being removed by the extractor. This creates a 'negative pressure' and ensures that all the air exits through the carbon filter. If you input more air than the extractor can remove the air will start to build up and cause a 'positive pressure' forcing untreated air out of the garden.

When selecting an intake fan it should have a maximum capacity that is 10-20% lower than the actual output of the extractor. This will maintain adequate negative pressure while not putting too much strain on the extractor and intake fans.

To work out the intake fan size we will need to take the extractor fan size and apply an estimated reduction for the carbon filter- 25% (only reduce by 25% if no intake filter is being used.) If our target for the intake fan is 15% less air than the exhaust we need to multiply the reduced output by 0.85. Below is a work through of how to size up the intake fan for both or the example rooms.

4 x 600W garden:

4 x 600W High Intensity Indoor Garden

Extractor size - 1,080 CFM

Estimated extractor power with carbon filter - 1,080 x 0.75 = 810

Reduction to insure negative pressure = 810 x 0.85 = Intake Fan Size 688 CFM

6 x 1000W garden:

6 x 1000W High Intensity Indoor Garden

Extractor size - 1,805 CFM

Estimated extractor power with carbon filter - 1805 x 0.75 = 1,354

Reduction to insure negative pressure = 1,354 x 0.85 = Intake Fan Size 1,151 CFM

When installing the intake fan, make sure the extractor is at the opposite end of the garden to the extraction fan so that your plants fully benefit. It's a good idea to split the intake air with a solid 'T' or 'Y' piece so that the cooler fresh air is distributed evenly. Using air socks or longer lengths of ducting with holes in is a good way of evenly distributing the incoming fresh air.

One last factor to consider is that inline fans are better at pushing than pulling air through ducting. This means than when positioning your intake fan, it's better to place it nearer the source of fresh air and push it towards the indoor garden. To make the air reach the garden efficiently, make sure the duct runs are as smooth and straight as possible.

Oscillating fans should be positioned to move the air between the grow lights and the tops of the plants.

Moving Air Inside Your Indoor Garden

It's not just about getting fresh air in and warm, spent air out. You need to move it all around your plants too! Moving the air within the garden is of utmost importance. A light breeze moving air over the plants' leaves refreshes the CO2 depleted air, gets rid of heat and humidity and encourages transpiration. The area of an indoor garden where most unwanted heat will accumulate is between the lights and the canopy, so it's absolutely crucial that this air is removed to avoid heat build up. To achieve good air movement between the lights and the canopy you can install fixed or oscillating air circulation fans. These can be wall mounted or floor standing and should be powerful enough to mix the air well, while not causing the plants to be blown too vigorously. You want to move the air, not your plants! If you point strong air circulators straight at your plants the air will move past the leaves so quickly that it will strip away the humidity surrounding the leaf and encourage rapid transpiration. This leads to the leaves losing water rapidly and can cause them to appear burnt at the edges crispy to touch; this is known as 'wind burn'. If you need to enhance the air movement around your plants, it's a good idea to point air circulators towards walls rather than directly at the plants to mix the air adequately while not causing the plants to be flapping around in turbulent wind.

Keep Your Garden Just For Plants, not for Ballasts and other Heat Generating Equipment!

To avoid unnecessary heat transfer, any equipment that generates heat should to be stored outside the garden. Most notably, the power packs (aka ballasts) that can get quite warm need to be situated outside the garden on a shelf or any non flammable surface. Having them outside the room also is best practice for electrical safety as they won't be operating in a warm and humid environment and will not have risks of stray foliar sprays landing on them or accidental splashes of nutrient solution.

Nutrient solution will also benefit from staying outside the garden. Your reservoir will quickly heat up under the direct light from your grow lights so its best practice to locate your reservoir outside the garden.

Any liquid nutrients and additives should not be stored in hot or cold environments. It's best to consult the packaging and see what the best environment is for your products but most appreciate a constant moderate temperature. This should again be outside your garden.


Following the above principles you can construct yourself a great, budget indoor garden, suited around you, while creating the ideal environment for your plants. All you need to do after this is choose a method to grow your plants whether it's growing passively in plant pots, or using an active hydroponics system such as an Ebb and Flow, Drip, or NFT - all will flourish in your well planned and ventilated indoor garden!