Choosing the Right Flooring Underlayment: A Guide to Soundproofing Your Floors
When homeowners shop for new flooring, most of the attention goes to what they can see. What color should we choose? Engineered hardwood or luxury vinyl plank? Wide planks or narrow? Light oak or something darker?
But one of the most important parts of your new floor may be something you’ll never see once the installation is finished: the underlayment.
The right flooring underlayment can help reduce noise, improve comfort, provide moisture protection when appropriate, and support the flooring system above it. In condominiums and other multi-unit buildings, choosing the correct underlayment can be even more important because your HOA may have specific sound-control requirements.
And in a condo, remember one simple rule: Your floor is somebody else’s ceiling.
Here’s what homeowners should know before choosing an underlayment for engineered hardwood, luxury vinyl plank (LVP/LVT), or laminate flooring.
What Is Flooring Underlayment?
Underlayment is a layer installed between the subfloor and the finished flooring. Depending on the flooring product and installation method, underlayment may be made from materials such as:
- Rubber
- Cork
- Foam
- Felt
- Recycled fibers
- Combination acoustic materials
Some modern flooring products come with an attached pad, while others require a separate underlayment. But thicker doesn’t automatically mean better, and the most expensive product isn’t necessarily the right product. The underlayment must work as part of the complete flooring system.
Why Is Underlayment Important?
Depending on the product and installation, underlayment may serve several purposes.
Sound Reduction
This is particularly important in condominiums, apartments, and multi-story homes. Underlayment can help reduce impact sounds such as footsteps, dropped objects, chairs moving, and everyday activity.
Moisture Protection
Certain installations—particularly those over concrete—may require appropriate moisture protection. However, moisture requirements vary by flooring manufacturer, subfloor, installation method, and product.
Comfort
Some underlayments can make floating floors feel slightly more comfortable underfoot.
Flooring Support
The correct underlayment helps support the flooring system and allows it to perform as intended. This is why you should never simply buy the thickest foam you can find and assume you’ve improved your floor.
Understanding IIC and STC Ratings
If you’re installing flooring in a condominium, you’ve probably encountered two mysterious abbreviations:
IIC and STC.
They sound complicated, but the basic concepts are fairly simple.
What Is IIC?
IIC stands for Impact Insulation Class.
It relates to how effectively a floor/ceiling assembly reduces impact noise transmitted through the structure.
Think:
Footsteps.
High heels.
A child’s toy hitting the floor.
A chair being moved.
Something dropping in the kitchen.
Impact noise is particularly important when someone lives below you.
Generally, a higher IIC rating indicates better impact-sound isolation under the conditions in which the assembly was tested.
But there’s an important catch we’ll discuss shortly.
What Is STC?
STC stands for Sound Transmission Class.
STC is associated with airborne sound transmission.
Think:
Conversations.
Television.
Music.
Other airborne household sounds.
Again, higher numbers generally indicate better sound isolation for the tested assembly.
The Most Important Thing About IIC and STC Numbers
This is where homeowners can easily become confused. You may see an underlayment advertised with impressive numbers such as: IIC 70 or STC 67 and naturally assume that means the underlayment itself has that performance rating. Not necessarily. Sound ratings are generally based on a tested floor-and-ceiling assembly, not simply a loose piece of underlayment.
The test result can depend on many things, including:
- Flooring material
- Underlayment
- Concrete slab thickness
- Ceiling construction
- Insulation
- Suspended ceilings
- Installation method
Two underlayments with impressive-looking numbers may have been tested under very different conditions. So don’t compare products based only on the largest number printed on the package.
Ask:
“What assembly was used to achieve this rating?”
That’s a much better question.
Delta IIC: Another Number Worth Understanding
You may also encounter Delta IIC, sometimes written as ΔIIC. This measurement looks at the improvement in impact-sound performance created by a flooring system compared with the same reference assembly without it. For homeowners comparing acoustic flooring products, this can provide useful additional information. Again, however, laboratory results should be reviewed in context rather than treated as a guarantee of exactly what you’ll hear inside your particular building.
Condo Owners: Check Your HOA Requirements First
This is extremely important. If you live in a condominium, do not purchase your flooring and underlayment before checking your HOA requirements. Many San Francisco and Bay Area condominium associations have flooring rules intended to limit noise transmission between units.
Your HOA may require:
- A particular type of underlayment
- Minimum acoustic performance
- Laboratory test documentation
- Specific flooring/underlayment combinations
- Product specification sheets
- An architectural application
- Property-management approval before installation
Some buildings may have different requirements depending on whether you’re installing engineered hardwood, LVP, laminate, or another hard-surface floor. The fact that your neighbor installed hardwood doesn’t necessarily mean your proposed flooring system will automatically be approved. Get the requirements first. Choose the flooring system second.
Does Thicker Underlayment Mean Better Soundproofing?
Not necessarily. This is one of the most common misconceptions about flooring underlayment. It’s tempting to think: 2 mm must be good, so 6 mm must be three times better. Flooring doesn’t work that way. A flooring manufacturer may specify a particular type and maximum thickness of underlayment. Using something that’s too thick or too soft can potentially allow excessive movement in some floating flooring systems. That can place unnecessary stress on locking mechanisms and affect how the floor feels underfoot. The goal isn’t to find the thickest underlayment. It’s to find the correct underlayment for the flooring system and the building requirements.
Best Underlayment for Luxury Vinyl Plank (LVP/LVT)
Many modern luxury vinyl products already include an attached pad. That creates an important question:
Can another soundproofing underlayment be installed underneath it?
Sometimes yes. Sometimes no. It depends on the flooring manufacturer’s specifications. Adding an unauthorized second layer beneath flooring with an attached pad can create too much movement and may affect product performance or warranty coverage. If additional acoustic performance is needed for an HOA, select a flooring-and-underlayment combination specifically approved for that application. Don’t simply stack padding underneath the floor.
Best Underlayment for Engineered Hardwood
The appropriate underlayment for engineered hardwood depends heavily on the installation method. Engineered hardwood may be:
- Floating
- Glued down
- Installed using another manufacturer-approved method
A floating engineered floor may use an acoustic underlayment designed specifically for floating wood flooring. A glue-down installation may require an entirely different acoustic system or adhesive designed for that application. This is one reason homeowners should select the flooring and installation system together rather than purchasing each component independently.
Best Underlayment for Laminate Flooring
Laminate is commonly installed as a floating floor, making underlayment an important part of the system. Some laminate products include attached padding, while others require separate underlayment. When choosing laminate underlayment, consider:
- Manufacturer requirements
- Acoustic performance
- Subfloor type
- Moisture requirements
- HOA requirements
- Approved thickness
Again, more padding isn’t automatically better.
What About Concrete Subfloors?
Concrete is extremely common in high-rise condominiums throughout San Francisco and the Bay Area. Installing flooring over concrete introduces additional considerations. Depending on the flooring system, the contractor may need to evaluate:
- Moisture conditions
- Flatness
- Cracks
- High and low areas
- Existing adhesive
- Surface contaminants
- Previous flooring materials
An expensive acoustic underlayment won’t correct an improperly prepared subfloor. Soundproofing and subfloor preparation are two separate parts of a successful flooring installation. Both matter.
“Soundproof” Doesn’t Mean Completely Silent
This is another important expectation to understand. You’ll frequently see products marketed as soundproof flooring underlayment. In reality, underlayment generally reduces sound transmission. It doesn’t make a condominium completely silent. If someone lives below you, they’ll probably still know you exist. The objective is to reduce impact noise and meet the building’s acoustic requirements—not create a recording studio. A professional contractor should be realistic about this rather than promising that a particular underlayment will eliminate all noise.
Don’t Forget the Flooring Manufacturer’s Warranty
Your HOA may approve an underlayment, but the flooring manufacturer also needs to allow it. Ideally, the system should satisfy both: Building requirements + flooring manufacturer requirements. If a flooring manufacturer specifies a certain type or maximum thickness of underlayment, follow those instructions. Otherwise, you could potentially create installation problems or warranty issues later.
Common Underlayment Mistakes to Avoid
One of the biggest mistakes homeowners make is choosing underlayment as an afterthought. Avoid these common problems:
- Buying flooring before reviewing HOA requirements
- Assuming the highest advertised IIC number means the best product
- Assuming thicker underlayment is always better
- Adding extra padding beneath flooring with an attached pad without manufacturer approval
- Ignoring moisture requirements over concrete
- Expecting underlayment to correct an uneven subfloor
- Assuming “soundproof” means completely silent
- Choosing an underlayment that isn’t compatible with the flooring manufacturer’s specifications
A little research before installation can prevent a lot of frustration afterward.
How to Choose the Right Soundproof Flooring Underlayment
Instead of asking:
“What’s the best underlayment?”
Ask:
“What’s the best underlayment for my flooring, my subfloor, my installation method, and my building?”
Those are very different questions. Before choosing, determine:
- What flooring are you installing?
- Does it already have attached padding?
- What type of subfloor do you have?
- What installation method will be used?
- Does your HOA have minimum acoustic requirements?
- Does the HOA require laboratory test reports?
- What underlayment does the flooring manufacturer permit?
- Is moisture protection required?
- Is the proposed flooring assembly approved for your application?
Once those questions are answered, choosing the correct product becomes much easier.
Soundproof Flooring Installation in San Francisco and the Bay Area
For condominium owners, underlayment isn’t simply an accessory you grab from the flooring aisle. It can be an important part of getting your project approved by your HOA and installed correctly. At Concept Materials, we work with homeowners, HOAs, property managers, designers, and contractors throughout San Francisco and the Bay Area. Our team has experience with flooring projects in condominiums and multi-unit properties where acoustic requirements, building documentation, concrete subfloors, and HOA approvals can all be part of the installation process.
We install engineered hardwood, luxury vinyl plank (LVP/LVT), laminate, carpet, baseboards, transitions, and related flooring materials, and can help homeowners evaluate the flooring system appropriate for their particular project.
Before You Buy Your Flooring, Talk to Your Contractor
This may be the most valuable advice in this entire article. If you live in a condo, get your HOA flooring requirements before buying the flooring. Then bring those requirements to an experienced flooring contractor. It’s much easier to choose an appropriate flooring and underlayment combination from the beginning than to discover after purchasing thousands of dollars of material that your HOA won’t approve it. If you’re planning a flooring project in San Francisco or elsewhere in the Bay Area, we’d be happy to discuss the flooring, underlayment, subfloor, and installation requirements with you.
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The flooring is what you’ll see every day. The underlayment is what you—and your downstairs neighbors—may appreciate every day.