The layer nobody sees helps decide how long the floor lasts.
What underlayment does
Underlayment is the layer between the subfloor and the new floor. It supports the floor, smooths small subfloor imperfections, and reduces impact sound between levels. If the flooring has an attached underlayment, do not add a second one. In some cases, a double layer can damage the floor.
Thickness is not the most important point. Type, sound ratings, and density matter as much or more. Most underlayments are 2 to 3 mm thick. Our underlayment guide gives an IIC rating of 50 and above as the minimum for most building codes and HOA requirements.
Match the underlayment to the floor
| Floor | Underlayment our guide recommends | Watch for |
|---|---|---|
| Laminate | Foam; foam with a vapor barrier in moisture-prone areas | Cork gives more sound absorption between units |
| Vinyl | Cork, if the manufacturer allows underlayment | Rubber and rubber cork can stain vinyl |
| Carpet tile | Rubber pad | Use a foam pad for broadloom carpet |
| Tile set in thinset | Rubber cork, for crack suppression | Tile can also go on plywood or cement board |
| Hardwood or engineered wood | Cork or foam | Cork has less give, so planks flex less |
When to use a moisture barrier
A moisture barrier and a vapor barrier are the same product. Look at three factors: the climate, the grade level, and the subfloor. Below-grade spaces are more prone to moisture. Concrete is porous, so our guide recommends a vapor barrier over a concrete subfloor before the floor goes in.
- Test the slab: tape a plastic sheet over the concrete for 24 hours. Condensation under the sheet means you need a moisture barrier.
- Pick the thickness: 6 mil is the minimum. Use 6 to 11 mil in dry areas and 12 mil or more in wet or humid areas.
- Floating floors: under laminate or vinyl planks, use a plastic sheet of 6 mil or more, or an underlayment with a built-in barrier.
Tell us the floor type and the subfloor. We will match the underlayment and the barrier.
Install the barrier correctly
Lay the barrier over all areas where moisture is a concern. Overlap the sheets by six inches and bond them with vapor barrier tape. Repair each puncture or tear before you install the floor. Roll out underlayment one row at a time, perpendicular to the flooring direction. Do not overlap underlayment unless the manufacturer tells you to.