Gypcrete is a gypsum-based floor underlayment used primarily for sound reduction, fire protection, radiant heating, and floor leveling in wood-frame and concrete buildings. It’s not a structural material you’d use instead of concrete. Instead, it’s a lightweight layer poured over subfloors to improve how a building performs acoustically, thermally, and in terms of fire safety.
What Gypcrete Actually Is
Gypcrete (a brand name from Maxxon Corporation that’s become a generic term) is a mixture of calcined gypsum, sand, water, and a small amount of Portland cement. A typical mix runs about 24% gypsum, 57% sand, and 19% water by weight. The Portland cement, added at 5 to 8%, was introduced after early formulations had problems with expansion and cracking. The gypsum-to-sand ratio generally falls between 1:1.3 and 1:3.
The result is a pourable material that can be pumped through hoses and spread across a floor in thin layers, typically three-quarters of an inch to an inch and a half thick. It self-levels to some degree, making it useful for smoothing out uneven subfloors before finished flooring goes down.
Sound Reduction in Multi-Story Buildings
This is one of the most common reasons builders specify gypcrete, especially in apartments, condos, and hotels. A layer of gypcrete over a wood-frame floor adds mass, which helps block both airborne noise (voices, music) and impact noise (footsteps, dropped objects). When combined with acoustic mats, insulation, and resilient channels, a gypcrete assembly can achieve an STC (Sound Transmission Class) rating of 60 and an IIC (Impact Insulation Class) rating of 61. For context, an STC of 50 is considered the minimum for most residential codes, so 60 represents a noticeable improvement in how much sound passes between floors.
Gypcrete alone won’t deliver those numbers. It works as one layer in a multi-component system. A typical high-performing assembly stacks vinyl flooring, acoustic underlayment, gypcrete, an acoustic mat, plywood subfloor, fiberglass insulation between joists, resilient channels, and gypsum board on the ceiling below. Each layer addresses a different part of the sound problem.
Fire Ratings for Wood-Frame Construction
Wood-frame buildings need fire-rated floor and ceiling assemblies, and gypcrete is a standard way to meet those requirements. Gypsum is naturally fire-resistant because it contains chemically bound water. When exposed to heat, that water slowly releases as steam, which delays the transfer of heat through the material. A layer of gypcrete over a wood subfloor can help an assembly achieve a one-hour or two-hour fire rating depending on the rest of the system’s components.
Radiant Floor Heating
Gypcrete is widely used to encase hydronic radiant heating tubing. In these systems, warm water circulates through plastic tubing laid across a subfloor, and gypcrete is poured over the tubing to lock it in place. The U.S. Department of Energy identifies gypsum as one of the materials used to embed radiant tubing in thin layers on top of subfloors, alongside concrete and other options.
Gypcrete works well here for a few reasons. It flows easily around the tubing without air pockets, it conducts heat reasonably well so warmth radiates up through the finished floor, and it adds thermal mass that helps the system hold and distribute heat more evenly. Because it’s lighter than concrete, it can be used in upper-story installations where a full concrete pour would be too heavy for the framing.
Floor Leveling
Older buildings and even new construction can have subfloors that aren’t perfectly flat. Gypcrete can be poured in varying thicknesses to create a smooth, level surface for hardwood, tile, carpet, or vinyl. This is especially useful in renovation projects where the original subfloor has settled unevenly over time.
How It Compares to Regular Concrete
The biggest practical difference is weight. At a depth of one and a half inches, gypcrete weighs roughly 9 pounds per square foot. Standard concrete at the same depth weighs about 18 pounds per square foot, roughly double. That weight savings matters in wood-frame construction, where floor joists have load limits. Pouring two inches of regular concrete on an upper floor could exceed what the framing was designed to carry.
Strength is a different story. Standard gypcrete has lower compressive strength than structural concrete, though the high-performance version from Maxxon reaches 3,000 to 4,000 psi, which exceeds commercial flooring requirements. That’s strong enough to support foot traffic, furniture, and typical floor loads, but gypcrete is not designed to replace structural concrete in foundations, driveways, or load-bearing slabs.
Drying Time and Installation
Gypcrete is poured as a liquid slurry, usually pumped from a truck-mounted mixer through hoses into the building. It’s a fast process compared to traditional concrete work. A crew can pour an entire floor of an apartment building in a single day.
Drying time depends on thickness. A three-quarter-inch pour typically takes 5 to 7 days to dry under normal conditions. A one-inch pour takes 7 to 10 days. These timelines assume adequate ventilation and reasonable temperature and humidity levels. Finished flooring cannot be installed until the gypcrete has dried thoroughly, since trapped moisture can damage adhesives and cause problems with flooring materials like hardwood or vinyl.
Installation follows ASTM F2419, a standard that covers how thick-poured gypsum concrete underlayments should be installed over wood-panel subfloors or concrete floors in commercial buildings, and how the surface should be prepared before resilient flooring goes on top.
Where Gypcrete Should Not Be Used
Gypcrete is strictly an interior product. Gypsum is water-soluble, so prolonged moisture exposure will soften and degrade it. It should not be used in exterior applications, below-grade installations, or any area prone to flooding or persistent dampness. Bathrooms and kitchens are acceptable as long as the gypcrete is properly sealed and covered with appropriate flooring, but standing water is a problem.
It’s also not a structural material. You can’t use it for foundations, countertops, or any application where it would need to support loads independently. It always functions as an underlayment, sitting on top of a structural subfloor and beneath the finished floor covering.