Most guys skip the leveling compound. They think the underlayment will hide the dip. It won’t. I spent three days grinding concrete on a job last month just so the floor wouldn’t click like a castanet. That job was an upstairs remodel where the homeowner had already bought a pallet of cheap, thin foam. I told them to send it back. If you are putting laminate on a second floor, you are essentially building a giant drum. Every footstep, every dropped toy, and every shifted chair vibrates through the floorboards, into the subfloor, and down the wall studs to the room below. My hands are permanently stained with subfloor adhesive and my knees have seen better decades, but I know this much for certain. The thin blue foam from the big box store is a lie told by accountants to save five cents a square foot. It offers zero structural support and even less acoustic decoupling.
The subfloor secret that builders hide from you
Soundproofing an upstairs laminate floor is not about the thickness of the foam but about the density of the material and the decoupling of the surface. High density underlayments like recycled rubber or heavy duty felt are essential for absorbing impact energy. When we talk about sound in flooring, we are dealing with two distinct physics problems. First, there is airborne sound, like a television or a conversation. Second, and more difficult, is impact sound. This is the kinetic energy of a heel hitting the floor. Without a high quality underlayment, that energy travels unimpeded through the laminate’s rigid core. Because laminate is a floating floor, it is never nailed down. It sits on top of the underlayment. If that underlayment is too soft, the locking mechanisms will flex and eventually snap. If it is too hard, the floor will feel like walking on a sidewalk. The balance is found in the compression set and the density of the material measured in pounds per cubic foot.
“A floor is only as good as the subfloor beneath it; deflection is the enemy of every joint.” – Master Flooring Axiom
The science of impact insulation class ratings
To understand what underlayment to buy, you must look at the IIC and STC ratings. The Impact Insulation Class rating measures how well a floor assembly blocks impact noise from reaching the room below. A rating of 50 is the bare minimum for most building codes, but for a truly quiet upstairs, you want an IIC of 60 or higher. The Sound Transmission Class rating measures airborne noise. While the underlayment helps with STC, its primary job is the IIC. You also need to look for the Delta IIC rating. This number tells you exactly how many decibels of impact noise the underlayment alone is removing from the equation. If a product claims an IIC of 72, read the fine print. They likely tested it on a fourteen inch thick concrete slab with a dropped ceiling. In a real world wood-framed house, that same underlayment might only provide a Delta IIC of 20. You need to account for the physics of your specific structure before writing the check.
| Material Type | Typical IIC Rating | Density Level | Compression Strength |
|---|---|---|---|
| Standard PE Foam | 50-52 | Low | Poor |
| Cross-linked Polyethylene | 58-62 | Medium | Fair |
| High Density Rubber | 65-70 | High | Excellent |
| Heavy Duty Felt | 62-66 | Medium-High | Very Good |
Why too much cushion destroys your locking joints
A common mistake I see is the homeowner who thinks more cushion equals a better floor. They buy the thickest, squishiest underlayment they can find. This is a recipe for a mechanical failure within six months. Laminate flooring relies on a tongue and groove locking system that cannot handle excessive vertical deflection. When you walk across a floor with too much cushion, the planks dip under your weight. This puts massive stress on the plastic or HDF locking joints. Eventually, those joints will fatigue and break. Once they break, you have gaps and