Key Takeaways
| Factor | What Matters |
|---|---|
| Material | Dense rubber or rebond foam—not standard foam |
| Thickness | 8mm minimum; 10mm ideal for concrete floors |
| IIC Rating | 65+ for multi-unit buildings (Impact Insulation Class) |
| Installation | Edge sealing required; gaps let noise through |
| Cost | $2–$5/sq ft for quality soundproof underlay |
| Carpet Type | Low-pile carpet performs better with thick underlay |
Why Underlay Matters in Shared Buildings
Footsteps carry through floors. Voices echo through walls. Most condo noise complaints trace back to one problem—thin or missing underlay beneath the carpet.
Underlay is the cushion between your carpet and the subfloor. It absorbs impact. Standard foam underlay costs less but does almost nothing for sound. Dense rubber underlay or high-grade rebond foam cuts airborne and impact noise by 20–30 decibels. That difference is the gap between hearing your neighbor’s conversation clearly and hearing a faint murmur.
Concrete subfloors amplify vibration. Wood subfloors transmit it. Both need a sound barrier. Most condo boards now require a minimum IIC (Impact Insulation Class) rating of 50 or 65 for flooring materials. The underlay—not the carpet—determines whether you meet that threshold.
If you’re replacing carpet in a condo or townhome, you’re already paying for carpet installation. Spending an extra dollar per square foot on proper underlay prevents noise complaints, future board violations, and the sound of your upstairs neighbor’s toddler at 6 a.m.
Dense Rubber Underlay: The Gold Standard
Rubber underlay weighs more than foam. That weight dampens vibration. Most rubber underlay products are recycled tire crumb mixed with a binding agent, then pressed into dense mats.
Thickness ranges from 6mm to 12mm. An 8mm rubber underlay provides noticeable soundproofing. A 10mm product performs better but costs more. Anything thinner than 6mm is a waste of money.
Rubber underlay does not compress permanently. Foam underlay flattens over time, especially in high-traffic areas. Compressed foam loses its sound-dampening ability. Rubber maintains density for 10–15 years.
Installation requires attention to seams. Gaps between underlay sections let sound through. Seams should be taped with acoustic tape—not duct tape. The installer must cut the underlay to fit snugly against walls and transitions. If your installer skips this step, you’ll hear it.
Rubber underlay feels firmer underfoot than foam. Some people prefer the softer feel of thick foam. The trade-off is sound. If you live above someone or share a wall, rubber is the correct choice.
Rebond Foam: The Budget-Friendly Alternative
Rebond foam is shredded scrap foam pressed into sheets. It costs less than rubber but still offers decent soundproofing if you choose the right density.
Density is measured in pounds per cubic foot. Look for 8lb or 10lb rebond foam. Anything lower than 6lb is standard builder-grade foam—it won’t block sound. Higher-density rebond performs nearly as well as rubber underlay at half the cost.
Rebond foam compresses over time. Expect 10–20% thickness loss in the first two years, especially under furniture or in hallways. That compression reduces sound dampening. If you plan to stay in your condo long-term, rubber is the better investment.
Installation is identical to rubber. Seams must be tight. Edges must be sealed. The foam should extend to the baseboards without gaps.
One advantage of rebond foam is cushion. It feels softer than rubber. If soundproofing is important but not critical—say, you’re on the top floor—rebond foam at 10mm thickness handles most noise issues without the premium cost of rubber.
IIC Ratings and Condo Board Requirements
IIC stands for Impact Insulation Class. It measures how well a floor assembly resists impact noise—footsteps, dropped objects, furniture scraping. The higher the IIC number, the better the soundproofing.
Most condo boards require an IIC rating of 50 minimum. Newer buildings often require 65. Your underlay and carpet combination must meet that standard or you may face fines or forced removal.
Carpet alone has an IIC rating of 40–45. Standard foam underlay adds 5–10 points. Dense rubber or high-grade rebond adds 15–25 points. That’s the difference between passing and failing the requirement.
Ask your installer to confirm the IIC rating before installation. Reputable carpet installation companies know which products meet local building codes. If they can’t answer, find a different installer.
Some condo boards require third-party acoustic testing after installation. That test measures real-world sound transmission. If your underlay choice fails, you’ll pay for removal and reinstallation. Spending an extra $200 on proper underlay now avoids a $2,000 problem later.
Thickness vs. Carpet Height
Thicker underlay does not always mean better soundproofing. The carpet type matters.
Low-pile carpet—office-style carpet or tight-loop carpet—pairs well with thick underlay. The firm carpet surface sits on top of the cushion without sagging. The combination maximizes sound absorption.
High-pile carpet—shag or plush carpet—already has built-in cushion. Adding 12mm underlay beneath high-pile carpet creates too much give. The carpet feels unstable. Furniture legs sink. The excessive softness reduces the underlay’s sound-dampening ability because the carpet itself absorbs impact before the underlay can.
For high-pile carpet, use 6–8mm underlay. For low-pile or berber carpet, use 8–10mm. If you’re installing stair carpet, use thinner underlay on treads—6mm maximum—to prevent the carpet from shifting.
Subfloor type also affects thickness choice. Concrete floors benefit from thicker underlay because concrete conducts vibration. Wood subfloors already dampen some sound, so 8mm underlay is sufficient in most cases.
Edge Sealing and Installation Details
Underlay does nothing if it’s installed poorly. Gaps, overlaps, and unsealed edges all leak sound.
The installer should measure and cut the underlay to fit the room exactly. It should extend to the baseboards without curling or bunching. Seams between underlay sheets must be butted tight—not overlapped—and taped with acoustic seam tape.
At doorways and transitions, the underlay should stop at the edge of the carpet. If it extends under the transition strip, it creates a bump. If it stops short, there’s a gap. The installer must cut it flush.
In condos with hydronic heating or concrete floors, a vapor barrier may be required beneath the underlay. The barrier prevents moisture from wicking up through the concrete and degrading the underlay. Not all buildings require this. Check with your condo board or property manager.
Some installers use adhesive spray to secure underlay to the subfloor. This prevents shifting but makes future removal harder. In rental units or condos where you may need to remove the carpet later, ask the installer to skip the adhesive and rely on the weight of the carpet to hold the underlay in place.
Cost Breakdown by Material
Here’s what you’ll pay per square foot for different underlay types:
| Material | Thickness | Density | Cost per sq ft | Soundproofing |
|---|---|---|---|---|
| Standard foam | 6mm | 3–5lb | $0.50–$1.00 | Minimal |
| Rebond foam | 8mm | 6lb | $1.00–$1.50 | Moderate |
| Rebond foam | 10mm | 8–10lb | $1.50–$2.50 | Good |
| Rubber underlay | 8mm | High | $2.50–$4.00 | Very good |
| Rubber underlay | 10mm | High | $3.50–$5.00 | Excellent |
Installation cost is the same regardless of underlay type—$1–$2 per square foot. The material cost is the variable.
For a 600-square-foot condo, upgrading from standard foam to 10mm rubber underlay adds $1,200–$2,400 to the project. That sounds steep. But the soundproofing benefit is immediate and permanent. If you’re already paying for carpet replacement, the incremental cost is worth it.
Most installers bundle underlay into their quotes. Ask for a line-item breakdown. If the quote lists “underlay included” without specifying type, thickness, or IIC rating, you’re probably getting standard foam. Request an upgrade in writing.
What to Ask Your Installer
Walk through these questions before signing the contract:
- What underlay material do you recommend for a condo?
- What is the IIC rating of the underlay and carpet combination?
- Will you tape seams with acoustic tape?
- Do you install a vapor barrier if needed?
- Can I see a sample of the underlay before installation?
- Is the underlay cost itemized in the quote?
If the installer hesitates or offers vague answers, that’s a red flag. Soundproofing is not optional in shared buildings. The wrong underlay causes problems you can’t fix without tearing out the carpet.
Use the carpet cost calculator to estimate total project costs including underlay upgrades. Then request a written quote from your installer. Compare the two. If the installer’s quote is significantly lower, ask what they’re cutting—it’s usually the underlay.
For questions about soundproofing underlay or to schedule an assessment, visit the contact page.
Frequently Asked Questions
Can I add soundproof underlay to existing carpet? No. Underlay goes beneath the carpet. You must remove the carpet, install the underlay, then reinstall or replace the carpet.
Does thicker underlay reduce floor height clearance at doors? Yes. Adding 10mm underlay raises the carpet height by 10mm. Check door clearances before choosing underlay thickness.
Will rubber underlay smell? Recycled rubber underlay may emit a faint odor for the first few days. Ventilate the room during installation and the smell dissipates within a week.
Can I use soundproof underlay on stairs? Yes, but use thinner underlay (6mm maximum) on stair treads to prevent shifting. Risers can handle standard thickness.
Does underlay prevent all noise? No. Underlay reduces impact noise—footsteps, dropped objects. It does not block airborne noise like voices or music. For airborne noise, address wall insulation.
What if my condo board requires a specific IIC rating? Ask your installer to provide the IIC rating in writing before installation. Most rubber underlay products meet IIC 65 when paired with low-pile carpet.
Can I install underlay myself? Yes, but improper seam sealing and edge gaps will reduce soundproofing. Professional installation ensures the underlay performs as rated.
How long does soundproof underlay last? Rubber underlay lasts 15+ years. Rebond foam lasts 10–12 years before compression reduces its effectiveness.
Does underlay help with thermal insulation? Yes. Thick underlay adds a layer of insulation between the floor and the room, reducing heat loss through concrete subfloors.
Should I tell my downstairs neighbor I’m installing soundproof underlay? You’re not required to, but it’s a courteous heads-up. Installation noise lasts 1–2 days, and they’ll appreciate the reduced footstep noise afterward.
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