Growing In Rockwool Cubes – Is Rockwool Safe For Plants

23 Jun.,2025

 

Growing In Rockwool Cubes – Is Rockwool Safe For Plants

If you're looking for a soilless substrate for seed starting, stem rooting or hydroponics, consider using rockwool growing medium. This wool-like material is made by melting basaltic rock and spinning it into fine fibers. Rockwool for plants is then formed into easy-to-use cubes and blocks. But is rockwool safe to use for the production of food?

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Advantages and Disadvantages of Growing in Rockwool

Safety: Formed from natural materials, rockwool contains no harmful chemicals. It's safe to use as a rooting medium and substrate material for plants. On the other hand, human exposure to rockwool represents a health issue. Due to its physical properties, rockwool growing medium can cause irritation to skin, eyes and lungs.

Sterile: Since rockwool for plants is a manufactured product, it's contains no weed seeds, disease pathogens or pests. This also means it contains no nutrients, organic compounds or microbes. Plants growing in rockwool require a balanced and complete hydroponic solution to meet their nutritional needs.

Water Retention: Due its physical structure, rockwool drains excess water quickly. Yet, it retains small amounts of water near the bottom of the cube. This unique property allows plants to attain adequate hydration while allowing more air to circulate and oxygenate the roots. This difference in moisture levels from the top to the bottom of the cube makes rockwool ideal for hydroponics, but it can also make it difficult to determine when to irrigate the plants. This can result in over-watering.

Reusable: As a rock derivative, rockwool doesn't break down or erode over time, thus, it can be reused many times. Boiling or steaming between uses is recommended to kill pathogens. Being non-biodegradable also means it will last forever in a landfill, making rockwool for plants a not-so environmentally friendly product.

How to Plant in Rockwool

Follow these easy instructions when using rockwool growing medium cubes or blocks:

  • Preparation: Rockwool has a naturally high pH of 7 to 8. Prepare a solution of slightly acidic water (pH 5.5 to 6.5) by adding several drops of lemon juice using pH test strips to attain the correct acidity. Soak the rockwool cubes in this solution for about an hour.
  • Sowing Seed: Place two or three seeds in the hole at the top of the rockwool growing medium. Water using a hydroponic nutrient solution. When the plants are 2 to 3 inches (5 to 7.6 cm.) tall, they can be transplanted into soil or placed in a hydroponic garden.
  • Stem Cuttings: The night before taking the stem cutting, water the mother plant thoroughly. In the morning, remove a 4 inch (10 cm.) cutting from the mother plant. Dip the cut end of the stem in honey or rooting hormone. Place the cutting in the rockwool. Water using hydroponic nutrient solution.

Mineral Wool Insulation Board Problems & Alternative Solutions

Mineral wool insulation board, commonly known as Rockwool, is a building material used in construction to insulate walls and attic spaces. The fibers that make up mineral wool insulation come from the minerals in volcanic rock and slag waste from steel production, that are spun and pressed together into board form. Mineral wool in board form is more rigid and dense than batts, another common form of mineral wool.

Mineral wool insulation boards come in various sizes and thicknesses, from 1” up to 5” thick. Standard sizes are limited to 24” modular increments, including:

  • 24” x 48” (2’ x 4’)

  • 24” x 72” (2’ x 6’)

  • 24” x 96” (2’ x 8’)

  • 48” x 96” (4’ x 8’)

Mineral wool insulation boards are often used in exterior wall applications where continuous insulation is desired. They are installed flush over a substrate like plywood and attached with nails and 3” washers.

Advantages of Mineral Wool Insulation Board

Mineral wool insulation board has several physical characteristics that make it desirable for use in building construction, including:

  • Natural fire-resistance

  • High sound absorbance

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  • R-value of 4.2 per inch

Drawbacks of Mineral Wool Insulation Board

Mineral wool insulation board isn’t right for every application, however. Here are some of the problems with using mineral wool insulation boards in construction projects:

It’s Heavy

At 8 pounds per cubic foot, mineral wool insulation is dense and heavy, which makes it difficult to work with. A single 2” thick, 4’ x 8’ board weighs over 40 lbs.

The size and weight of mineral wool insulation boards present a challenge for installation without doubling labor. Often, one laborer is required to hold the sheet in place and another to secure it. The alternative is to purchase smaller sheets for easier handling, but this slows labor down by decreasing the coverage per sheet.

It’s Expensive

If you’re seeking the additional fireproofing and sound absorbance properties of mineral wool insulation, keep in mind that it will cost you more initially than standard fiberglass insulation. Expect to pay 25-50% more than fiberglass for the material, plus a higher installation cost.

Not for Custom Jobs

Mineral wool insulation boards are only produced in specific sizes and must be cut to fit around obstacles. If you have a custom job that requires irregular insulation shapes, mineral wool will make the job very difficult because there are fewer sizes available and the material does not conform to the cavity space, and must be cut on the job.

No Vapor Barrier

Some building insulation materials include a built-in vapor barrier facer. Mineral wool is not one of them since it is vapor permeable and is only available unfaced. With mineral wool insulation boards, a separate vapor barrier is required in order to keep moisture out of your walls and structure.

Requires a Nailable Substrate

Mineral wool insulation boards are designed to be installed over a flush substrate, such as weather-sealed plywood. That means that you’ll need additional furring strips over the top to attach your exterior finishes, like siding.

A Superior Alternative to Mineral Wool Insulation Boards

Polyiso insulation is another building material that is used in construction projects. It solves many of the challenges and downfalls of mineral wool insulation boards, including:

  • Higher R-Value: R6.0 per inch

  • Custom Facer: Polyiso adds a reflective or weather barrier option to the insulation board facer

  • Lightweight: Polyiso is very lightweight and easily handled by a single installer

  • Competitive Pricing: Polyiso is less expensive than mineral wool

  • Versatile Installation: Polyiso can be installed over multiple substrates or subgirts

Wall Insulation Board Options

Rmax specializes in manufacturing quality building insulation products. For your next construction project, consider the following superior polyiso products:

If you want to learn more, please visit our website Rubber And Plastic Insulated Pipe.

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