Underlayment Rules Explained What's Allowed · Thickness Limits · Compressive Strength · Sound Rating Implications
Underlayment rules define whether a separate foam or cork pad may be used beneath a floating rigid core LVT installation — and if so, what thickness and compressive strength limits apply. The rules vary significantly between products: some prohibit any additional underlayment, some require it (for products with no attached pad), and some permit it within specific limits. Using an incompatible underlayment destabilizes the locking joint system and typically voids the structural warranty. Reference-only: no product recommendations.
Products with factory-attached underlayment typically prohibit any additional pad. Products without an attached pad may require or permit separate underlayment with a maximum thickness (usually 3mm) and minimum compressive strength (usually 45 psi). The installed underlayment configuration also affects acoustic ratings — the published IIC/STC numbers reflect the tested assembly, which may differ from your actual installation.
Three Underlayment Scenarios
Scenario 1: Product has attached underlayment — no additional pad
When a rigid core LVT product ships with a factory-attached IXPE, EVA, or cork pad, most manufacturers prohibit adding any separate underlayment beneath it. Stacking two compressible layers creates excessive total compliance beneath the floor — the locking joints flex with every footstep rather than resting on a stable base. This cyclic flexing fatigues the click-lock profile over time, resulting in joint loosening, clicking sounds, and eventual separation. Published sound ratings for the product were established with only the factory pad; adding additional underlayment changes the assembly and voids both the acoustic rating and the structural warranty.
Scenario 2: Product has no attached pad — separate underlayment permitted with limits
When a product ships without any attached pad, the installation guide will typically specify whether a separate underlayment is required or optional, and if so, what properties it must have. Commonly specified limits are: maximum thickness of 3mm (some manufacturers allow 2mm maximum); minimum compressive strength of 45 psi; and sometimes specific approved products. These limits exist to ensure the underlayment is firm enough to support the locking joints without excessive compression. Soft foam underlayments marketed for use under hardwood — which may be very thick and soft — are typically not suitable for rigid core LVT unless the manufacturer explicitly approves them.
Scenario 3: Product has no attached pad — underlayment required
Some rigid core products without attached pads require a separate underlayment as part of the installation — particularly when being installed in jurisdictions with multi-family acoustic code requirements. In this scenario, the manufacturer will specify approved underlayment options that have been tested in combination with the flooring product to achieve the stated IIC/STC ratings. Using a different underlayment not included in the approved list may affect acoustic performance and may not satisfy code requirements for multi-family installations.
Why Thickness and Compressive Strength Limits Matter
The stability of a floating floor's locking system depends on the floor resting on a sufficiently firm base. When a plank is loaded by foot traffic at a point near a locking joint, the joint experiences a small amount of rotation or flex. If the underlayment beneath the plank compresses significantly under load — as soft foam does — this flex is amplified. Over thousands of loading cycles, even small amounts of joint flex cause the locking profile's tongue and groove to wear against each other, producing a "clicking" sound and eventually allowing visible gaps to form. Underlayment thickness limits (3mm) and compressive strength minimums (45 psi) are derived from testing to identify the threshold below which joint fatigue becomes a predictable failure mode within the warranty period.
Spec Sheet Checklist
- First, determine whether the product has a factory-attached underlayment — if yes, additional underlayment is typically prohibited.
- If no attached pad, check the installation guide for whether separate underlayment is required, permitted, or prohibited.
- If underlayment is permitted, verify the maximum allowable thickness (commonly 3mm or less) and minimum compressive strength (commonly 45 psi or higher).
- Note whether specific approved underlayment products or brands are listed — some manufacturers require approved products to maintain warranty coverage.
- Check whether the published IIC/STC sound ratings reflect the product with or without a separate underlayment — the rating only applies to the tested assembly.
FAQ
Can I use any foam underlayment with rigid core LVT? ⌄
No. For products with factory-attached underlayment, additional underlayment is typically prohibited. For products without attached pads, manufacturers specify maximum thickness and compressive strength limits. Using foam that is too thick or too soft creates an unstable base that causes locking joints to flex under foot traffic, accelerating joint fatigue. Common limits are 3mm maximum thickness and 45 psi minimum compressive strength.
Do underlayment rules apply to glue-down installations? ⌄
Glue-down LVT does not use an underlayment layer — the adhesive is applied directly to the prepared subfloor and the LVT is pressed into it. Using any foam or cushion layer under glue-down LVT would interfere with the adhesive bond and is universally prohibited in glue-down installations.
Does the underlayment affect the product's published sound ratings? ⌄
Yes, significantly. A product's published IIC and STC ratings reflect a specific tested assembly — including whatever underlayment was used. Installing with a different underlayment may change the actual acoustic performance. This is important when acoustic performance must meet a specific code standard — the installed assembly must match the tested assembly for the published rating to apply.
What is the maximum underlayment thickness typically allowed? ⌄
For products that permit separate underlayment, most manufacturers specify a maximum of 3mm (approximately 1/8 inch). Some manufacturers are more restrictive — 2mm maximum. Compressive strength specifications (typically 45 psi minimum) are often listed alongside the thickness limit — both constraints must be satisfied by the selected underlayment product.
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Reference-Only Information
This page provides general informational reference about underlayment rules for rigid core LVT. It does not provide installation guidance, professional advice, or product recommendations.