Can I Use Rockwool Insulation For Hydroponics?

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By Chris Lipsey

Hydroponic gardening can become expensive, especially when your local supply of growing cubes begins to dwindle and prices go up. If this has happened to you, it’s completely understandable that you are wondering if Rockwool insulation can be used as a replacement for the traditional Rockwool cube.

Rockwool insulation and Rockwool hydroponic cubes are made from the exact same fibrous material. This means that you can, indeed, use insulation as a replacement growing media for hydroponic gardening. However, before you use it, you should take some precautions to ensure your plants stay healthy.

If you’re wanting to save some money on your hydroponic setup, switching to insulation could be the method you’re looking for. Let’s take a look at how you can replace your old Rockwool cubes with mineral wool and how to prepare it safely.

Hydroponic Rockwool and Rockwool Insulation – What’s the Difference?

hydroponic rockwool vs rockwool insulation

If you take a close look at both hydroponic Rockwool cubes and standard Rockwool insulation, you’ll find that they are made from almost exactly the same rockwool fibers. Yet, that doesn’t make these two products equal in every way. You will likely find that:

  • Hydroponic Rockwool cubes are far more expensive than insulation
  • Prices vary depending on where you live in the world
  • Hydroponic Rockwool cubes are often in short supply in much of the world
  • Insulation is made using toxic chemicals, which you should be careful to wash out

With some parts of the world paying twice as much for growing cubes than other parts of the world, matched with the sheer scarcity of hydroponic rock wool cubes outside of the USA, it’s reasonable that many hydroponic growers have turned to insulation.

However, before you start using insulation as a replacement for Rockwool cubes, understand that it could kill your plants if you don’t treat it properly.

What to Watch Out for in Rockwool Insulation

Rockwool insulation wasn’t originally designed to be used in a hydroponic garden. Instead, it was designed to maintain the climate inside of a house.

As a safety precaution, manufacturers treat it with several harsh chemicals to make it fire retardant in the case there is a house fire. If you decide to switch to insulation for your hydroponic grower, know that some of these chemicals could leach into your plants.

These chemicals include formaldehyde and asbestos, which can be harmful to you and your plants, especially if you are growing fruits or vegetables to eat. Let’s explore why insulation is made using these chemicals and how you can prepare it for safe use as a growing medium.


Formaldehyde is a chemical binder added to insulation during the production process. It’s used to bind microfibers of glass into a single sheet of insulation. Throughout the process, most of the formaldehyde is eliminated but there are still small trace amounts left behind in the final product.

If you use insulation as a replacement hydroponic rock wool cube, trace amounts of formaldehyde will begin to leach into your plants if you don’t take the appropriate steps to prepare your growing medium.

High concentrations of formaldehyde will reduce the amount of water your plants can absorb and can also affect your eyes, nose, and throat if you plan to consume fruits or vegetables grown in the insulation.


Although asbestos has been a banned material since the mid-1980s, it does still find its way into some Rockwool insulations throughout the developing world. This is a potential health problem because, according to the World Health Organization, asbestos still contributes to cancer throughout the world.

Furthermore, asbestos can hinder your plants’ ability to absorb nutrients from their base nutrient solution. Over time, this will cause your plants to grow slower and produce a smaller yield.

How Rockwool Insulation Rockwool Affects the Root Zone

Compared to horticultural Rockwool cubes, insulation is generally more compact. As your plants begin to grow from seeds, they will have to push through more resistance in insulation than they would in a growing cube.

Greater resistance can cause your plants to grow more slowly than they would in hydroponic cubes. Some of your seeds may not even make it through the insulation and will die before they pass the seedling stage.

As long as you treat your insulation properly, though, you should be able to use it to grow healthy hydroponics without spending an arm and a leg. However, if you are still unsure about using insulation, you can also use coco coir, rice hulls, or coconut fiber as hydroponic media. All are available at a low cost in most of the world.

How to Prepare Rockwool Insulation for Hydroponic Use

If you are still committed to using stone wool insulation for hydroponic gardening, you’ll want to follow these steps to properly prepare it. This method will help you avoid the harmful side effects of insulation while still creating a useful growing medium:

Use dishwasher detergent

Before you plant anything in insulation, you’ll need to wash away the harmful formaldehyde using a bath of dishwasher detergent. Let the insulation sit in soapy water for at least 24 hours before you use it.

Rinse it in hydrogen peroxide

If you plan to reuse insulation – or any rockwool product – it’s best to wash them thoroughly in hydrogen peroxide. This will help prevent any disease from spreading from old crops to new seeds. After washing it in detergent, let the insulation soak in peroxide for another 24 hours.

Rinse in water

Finally, after soaking your insulation in both dishwasher detergent and hydrogen peroxide, give it a final rinse in clean water. Insulation is highly useful for growing plants because it retains so much moisture. If the moisture is contaminated with peroxide, though, you could kill your plants.

Soak it in freshwater so your plants have a clean, moist environment to grow in.

After cleaning, rinsing, and soaking your insulation, you can then form it into cubes and begin planting seeds into your hydroponic system.

It’s best to let your cubes sit in the water again until they are fully saturated and all of the air bubbles have risen to the surface. This way, any additional moisture reaches your seeds instead of soaking them into the Rockwool insulation.

Adjust the pH of Your Insulation

One final note we should make is that insulation typically has a higher pH level than other growing media. This can affect the nutrient balance in your growing solution and mess with how well your plants bloom. After installing your insulation, be sure to recheck the pH levels and adjust them if needed. Check out our recommended ph meters.

If you are still early in the setup stage, you can also soak your insulation in a pH-optimized nutrient solution to stabilize its pH before planting any seeds. This way, your plants can start growing in an ideal environment.


If you are struggling to find a Rockwool block supplier or cannot afford to use them as the main growing medium for your entire hydroponic system, you can use mineral wool as a replacement.

Both hydroponic cubes and insulation are made from the same material, making them ideal growing media. However, be sure to treat the insulation before using it.

Insulation is often soaked in formaldehyde and can contain asbestos. If you are not careful to remove these toxins, they will leach into your plants and can even affect your health. Be sure to wash your insulation in detergent AND hydrogen peroxide before use, then give it a good soak in both water and a nutrient solution.