What to Put Under Laminate Flooring: Complete Subfloor Guide

Installing laminate flooring is an exciting home improvement project, but the success of the final result hinges entirely on what lies beneath the surface. The underlayment is not merely a optional extra; it is the critical foundation that ensures your new floors look incredible and function perfectly for years to come. Choosing the right materials and preparation methods protects your investment and dictates the longevity of the installation.

The Essential Sub-Flooring Preparation

Before any decorative layer is considered, the existing sub-floor must be meticulously prepared. This foundational step determines the stability and smoothness of the entire flooring system. A sub-floor that is uneven, loose, or damp will inevitably cause problems such as creaking, gaps, or premature wear in the laminate planks above.

You must ensure the sub-floor is structurally sound, meaning there are no loose boards or squeaks. Any cracks or dips should be filled and sanded smooth to create a level surface. Moisture is the enemy of laminate, so you must verify that the area is completely dry. Ideally, the sub-floor should be flat within a 3/16 inch radius over a 10-foot span; if this standard is not met, you must address the high and low points before proceeding.

Understanding Laminate Underlayment

Laminate underlayment serves three primary functions: sound absorption, moisture protection, and cushioning. While some laminate planks come with a attached foam underlay, purchasing a separate underlayment product is often necessary to meet installation requirements and provide superior performance.

The most common material is a thin foam sheet, typically made from polyethylene or EVA (ethylene vinyl acetate). This foam provides the necessary slip resistance to prevent the planks from sliding during installation. It also acts as a vapor barrier, blocking moisture from seeping up from the sub-floor and potentially warping the boards. Furthermore, the foam compresses slightly underfoot, reducing the impact of footsteps and creating a softer, more comfortable feel underfoot.

Moisture Barrier Solutions

One of the most critical decisions regarding what to put under laminate flooring is addressing moisture, particularly when installing over concrete slabs. Even if the concrete feels dry to the touch, capillary action can draw moisture up from the ground, which will destroy the integrity of the laminate.

For moisture-prone areas, a heavy-duty plastic vapor barrier is essential. You should roll out 6-mil polyethylene sheeting across the entire surface, overlapping the seams by at least 6 inches and taping them shut with heavy-duty duct tape. This creates a continuous barrier that prevents vapor from reaching the back of the laminate planks. In commercial settings or areas with extreme moisture concerns, cork underlayment is an excellent alternative, as it offers natural resistance to dampness and provides superior sound insulation.

Acoustic and Thermal Considerations

If your laminate installation is going over a finished attic, a home theater, or a multi-story living space, acoustic underlayment becomes a priority. Standard foam underlayment dampens the impact noise of footsteps and dropped objects, reducing the "hollow" sound often associated with floating floors. Look for underlayment specifically rated for noise reduction, which can significantly improve the acoustic comfort of the room.

Similarly, thermal underlayment plays a role in energy efficiency. While laminate itself is not a great insulator, a thin layer of underlayment can help retain heat, particularly in rooms built over unheated spaces like basements or garages. This not only makes the floor feel warmer in the winter but can also contribute to lower heating bills, making the investment in a slightly thicker underlayment worthwhile.

Specific Product Recommendations and Installation Tips

When selecting a product, you will encounter terms like "universal underlayment" or "sound insulation underlayment." A universal underlayment is a good choice for standard residential installations over wood sub-floors, providing a balance of moisture protection and cushioning. For high-traffic areas or apartments where noise transfer is a concern, you should invest in a specialized acoustic underlayment that is thicker and denser.

Regardless of the type you choose, the installation technique is just as important as the material itself. The underlayment should be rolled out smoothly, without wrinkles or folds, as these irregularities will telegraph through the laminate surface. Tape the seams to prevent shifting during the installation process. Finally, ensure that the underlayment extends slightly up the walls to allow for expansion, but remember to cut it away before installing the baseboards to prevent wicking.

Summary of Under-Layer Requirements

To guarantee a flawless and durable laminate floor, the materials you select must match the environment of the room. Below is a quick reference guide to help you determine the right combination of products for your specific needs.