Floor Underlayment

Summary

Floor underlayment is the layer between the subfloor and the finish that smooths, stiffens, protects, or quiets the floor, as plywood, cement board, self-leveling compound, or acoustic rubber, cork, and foam. Pick the type by what the finish needs: flatness, stiffness for tile, or impact sound reduction, and confirm it with the floor covering maker.

Cols

Image

Underlayment: layer between subfloor and finish
Self-leveling underlayment (SLU): pourable cementitious or gypsum compound that levels a floor
Cement board: stiff, water-resistant board under tile
Acoustic underlayment: rubber, cork, or foam layer that reduces impact sound
IIC (impact insulation class): impact sound rating, tested to ASTM E492
Delta IIC: improvement over a reference floor, measured under ASTM E2179
Uncoupling membrane: layer that separates tile from substrate movement

Types

Plywood: usually 1/4 in. or 3/8 in. to level an uneven subfloor
Cement board: stiff, water-resistant base for tile, often thinset and screwed down over wood
Self-leveling underlayment: corrects dips in concrete or gypsum; one source limits it to defects under about 1 to 1.5 in.
Rubber, cork, and foam: acoustic mats; rubber is the most common high-IIC choice and foam is the lightest and least costly

Standards

ASTM E492: impact sound transmission in the laboratory
ASTM E2179: delta IIC of floor covering and underlayment
Flatness: one rubber-underlayment vendor asks 1/8 in. in 10 ft; other makers give 3/16 in. in 10 ft or 1/8 in. in 6 ft, so follow the floor covering maker
Code impact-sound minimums for dwelling-unit separations are in the parent code reference

Detailing

Match underlayment to the finish and read the covering maker's flatness and moisture limits
Keep the substrate dry; test slabs before covering
Use a lab report with the IIC and the full floor-ceiling assembly, not the mat alone
Seal acoustic mat seams and isolate the floor from walls at the perimeter
Over wood, fasten cement board and bed it in thinset as the tile maker requires
Keep floor height build-up in view at doors and thresholds

Uses

Specific uses

Leveling subfloors before tile or resilient flooring
Stiff base under tile
Impact sound control in multifamily and mixed-use floors
Floating floor installations

Pros and cons

Pros

Corrects flatness before expensive finishes
Acoustic types raise impact ratings
Many thicknesses and products

Cons

Adds thickness and cost
Ratings must be verified with lab test reports
Some types fail under moisture

Manufacturing and cost

Construction

Panels are fastened to the subfloor, compounds are poured and screeded, and mats are rolled out, taped, and covered by the finish

Main manufacturers and suppliers

Sources

Construction Specifier: choosing the right acoustical underlayment, https://www.constructionspecifier.com/choosing-the-right-acoustical-underlayment/
US Rubber: what is acoustic underlayment, https://www.usrubber.com/what-is-acoustic-underlayment/
Acoustical Surfaces: Duracoustic installation, https://www.acousticalsurfaces.com/acoust_flooring/images/Duracoustic_Installation.pdf
Hunker: types of underlayment, https://www.hunker.com/13723633/types-of-underlayment-a-homeowners-guide/
Wood Floor Business: the lowdown on wood flooring underlayments, https://www.woodfloorbusiness.com/installation/the-lowdown-on-wood-flooring-underlayments.html