Mycelium bricks are made by growing fungal root networks through agricultural waste inside a mold, then drying the result to kill the fungus and lock in the shape. The process takes roughly 10 to 14 days from start to finish, requires no special equipment beyond what you’d find in a home kitchen, and produces lightweight blocks useful for insulation, packaging, or experimental construction. Here’s how to do it step by step.
Choosing Your Fungal Species
Oyster mushrooms (genus Pleurotus) are the go-to choice for beginners. They colonize substrate aggressively, tolerate imperfect conditions, and are widely available as grain spawn from online suppliers. Oyster mycelium grows best at around 28°C (82°F) but will tolerate swings of 10 to 20 degrees in either direction, which makes it forgiving if your workspace isn’t climate-controlled.
Reishi (Ganoderma lucidum) is the other common option. It produces a thicker, denser mycelial mat, which can improve the surface finish and water resistance of your final brick. Some researchers have reported compressive strengths of up to 2.49 MPa using reishi and oyster mushroom strains grown on sawdust-based substrates, though most low-density home-grown bricks land well below that. Either species works. Oyster is easier; reishi gives a denser product.
Preparing the Substrate
The substrate is whatever organic material your mycelium will eat and bind together. Good choices include chopped straw, sawdust, hemp hurds, or cottonseed hulls. Each material produces a slightly different brick. Hemp substrates absorb the least water (about 24% after 24 hours of soaking, compared to 27% for straw and 30% for flax), making them a strong choice if moisture resistance matters to you. Straw is the cheapest and easiest to source. Sawdust works but colonizes more slowly because of its high lignin content: oyster mycelium may only cover 60 to 70% of untreated sawdust after three weeks, while straw can be fully colonized in two weeks.
Chop or shred your substrate into pieces roughly 1 to 5 centimeters long. Smaller pieces pack more tightly and give the mycelium more surface area to grip, which improves the final brick’s integrity.
Pasteurizing to Prevent Contamination
Raw agricultural waste is full of competing molds and bacteria that will outgrow your mycelium if you don’t knock them back first. Pasteurization is the simplest method: heat your substrate to 60 to 80°C (140 to 175°F) and hold it there for at least one hour. You can do this by submerging the material in a large pot of hot water, or by steaming it inside a pillowcase set over boiling water. You’re not trying to sterilize the substrate completely, just reduce the microbial competition enough to give your mycelium a head start.
Let the substrate cool to room temperature before moving on. If it’s too hot when you add the spawn, you’ll kill the fungus before it gets started.
Mixing Spawn and Filling the Mold
Grain spawn is the easiest inoculant for home projects. You’ll want roughly 10 to 15% spawn by weight relative to your wet substrate. Break the spawn apart with clean hands or a sanitized spoon, then mix it evenly through the cooled substrate. The more uniformly you distribute the spawn, the faster and more completely the mycelium will colonize.
Pack the mixture firmly into your mold. Standard bricks work well in rectangular plastic containers, bread loaf pans, or custom forms made from laser-cut acrylic. Whatever you use, poke a few small holes (about 5mm) in the sides and lid for gas exchange. Mycelium needs oxygen and produces carbon dioxide, so fully sealed containers lead to stalled growth. Loosely fitting lids or containers wrapped in plastic wrap with a handful of holes punched through work fine.
Pack the substrate tightly enough that it holds shape but not so compressed that air can’t reach the interior. A gentle, even press with your palm is about right.
Incubation: Growing the Brick
Place your filled mold in a dark space at 25 to 28°C (77 to 82°F). A closet, cabinet, or plastic storage bin works well. Darkness isn’t strictly required, but light can trigger the mycelium to start forming mushroom fruiting bodies, which wastes energy that should go toward colonizing the substrate.
Humidity should stay high, around 80 to 90%, to prevent the surface from drying out. If you’re incubating inside a plastic bin, the substrate’s own moisture usually maintains this level. If you notice the surface looking dry and cracked, mist it lightly with a spray bottle of clean water. Over the first three to five days, you’ll see white threads spreading from each grain of spawn into the surrounding material. By day 7 to 14, depending on your species and conditions, the entire block should be a solid white mass with no visible loose substrate.
Watch for green, black, or orange patches during this stage. Those are competing molds. Small spots can sometimes be overcome by vigorous mycelium, but large contaminated areas mean starting over. Working in a clean environment and pasteurizing your substrate thoroughly are the best defenses.
Drying and Inactivation
Once the substrate is fully colonized (white throughout, firm to the touch), remove the brick from its mold and dry it to kill the living fungus and lock in the structure. The ideal drying temperature is 60°C (140°F) for 24 hours. Research on mycelium composites shows that this temperature produces the best combination of strength, hardness, and impact resistance, while completely preventing regrowth after drying.
Temperatures below 40°C risk incomplete drying, which can lead to mold regrowth later. Temperatures above 80°C damage the fungal network itself, degrade the plant fibers in the substrate, and cause warping and shrinkage from rapid water loss. A standard kitchen oven set to its lowest temperature (often around 70°C with the door cracked) or a food dehydrator works. If you don’t have access to either, air-drying in direct sunlight over several days can work in dry climates, though it’s less reliable for fully inactivating the fungus.
The brick is done when it feels hard, light, and completely dry throughout. Tap it and it should sound somewhat hollow, similar to knocking on cork.
What the Finished Brick Can (and Can’t) Do
Mycelium bricks are not a replacement for concrete or fired clay. Standard low-density mycelium composites have compressive strengths in the range of 0.02 to 0.17 MPa, while ordinary concrete sits around 20 to 40 MPa. That’s a difference of roughly 100 to 1,000 times. These bricks won’t bear structural loads.
Where they excel is insulation, acoustic dampening, and lightweight interior applications. Mycelium composites have thermal conductivity values comparable to expanded polystyrene (rigid foam board), making them genuinely useful as insulating panels or cavity fill. They’re also significantly less flammable than common plastics. Combustion testing performed according to ASTM standards shows that mycelium’s tendency to ignite and sustain flame is notably lower than standard plastics like acrylic or polylactic acid, making it a safer choice for interior applications.
Improving Water Resistance
Untreated mycelium bricks absorb water readily, which limits their use in damp environments. The best defense starts during manufacturing: growing a dense outer skin of mycelium on the brick’s surface significantly reduces absorption. You can encourage this by leaving the colonized brick in its mold for an extra two to three days after full colonization, allowing the fungus to build up a thick hydrophobic layer on the exposed surfaces.
Hemp-based substrates absorb water more slowly than straw or flax alternatives, so your material choice matters here too. For outdoor or moisture-exposed applications, a coating of natural linseed oil, beeswax, or shellac adds a further barrier. Even with these precautions, mycelium bricks are best suited for dry, indoor, or protected environments.
Tips for Better Results
- Mix substrates. Combining straw with a smaller portion of sawdust or rice bran can boost mycelial growth rates and improve density. Adding 10 to 15% rice bran as a nutritional supplement has been shown to significantly increase colonization speed and yield.
- Use fresh spawn. Grain spawn older than a few weeks colonizes more slowly and is more vulnerable to contamination. Buy from reputable suppliers and use it promptly.
- Match species to temperature. If your workspace runs cool (below 20°C), oyster mycelium will still grow, just slowly. Below 16°C, growth essentially stalls. A small space heater in an enclosed area can solve this cheaply.
- Press harder for stronger bricks. Denser packing produces denser, stronger finished bricks. Some makers use a simple lever press or clamp to compact the substrate in the mold before incubation. Densified composites have been shown to reach compressive strengths several times higher than loosely packed versions.
- Grow in the dark. Light triggers fruiting. Mushroom caps growing from your brick aren’t harmful, but they divert the organism’s energy away from binding the substrate together.