IIC 55 or Not: When Acoustic Underlay Works in New Zealand
Acoustic underlay cuts a real chunk of footfall and mid to high-frequency impact noise through most common flooring types, but it rarely solves heavy, low-frequency thumps on lightweight timber floors by itself. Soft coverings like carpet with a quality underlay often perform fine on their own. Hard floors over shared ceilings usually need a verified floating assembly, and sometimes a full site survey, to get anywhere near a reliable result.
TL;DR:
Rubber sheet or reclaimed-rubber underlays provide the best density and damping for soundproofing timber, laminate, and tile floors, especially when properly installed.
Low-frequency impact noise, such as heavy thuds, requires mass or ceiling treatments rather than just resilient underlays alone to achieve effective sound reduction.
Correct site preparation, including leveling, moisture control, and sealing seams, is essential to preserve the acoustic performance of the underlay system.
Compliance with Building Code G6 relies on whole-system tested assemblies, not just individual underlay products, and field verification is crucial for accurate noise performance.
For severe noise issues from below, ceiling-side treatments like extra plasterboard or cavity insulation are more effective than underlay upgrades alone.
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How does acoustic underlay actually reduce noise?
Acoustic underlay works through resilience and damping. Resilience is the layer’s ability to compress under an impact and spring back, absorbing energy instead of transmitting it straight into the structure below. Damping is what happens to the vibration once it’s in the material. A denser, more elastic underlay converts more of that vibration into heat rather than passing it on as sound. Together, these two properties are why a rubber or foam layer under laminate can noticeably soften the tap of heels or a dropped phone.
The catch is frequency. Resilient layers are genuinely good at knocking down mid and high-frequency impact noise, the sharp clicks and taps you hear most clearly. Low-frequency impact, roughly 20 to 200Hz, is a different problem entirely. That’s the dull thud of a dropped weight or a child jumping, and it travels through a structure as much as through the air. Controlling it needs mass, not just cushioning, which is why a timber design acoustics guide notes that resilient underlays often do little for low-frequency impact on lightweight timber floors.
This is where decoupling comes in, and it’s a different mechanism from simply softening the surface:
Adding a soft layer reduces the energy transmitted at the point of impact, helping with surface-level tapping and clicking.
Decoupling (as in a floating floor) physically separates the finished floor from the structure below, interrupting the path vibration takes through the building.
Mass addition slows down low-frequency energy that a thin resilient layer can’t touch on its own.
A floating engineered timber floor over battens with resilient clips behaves very differently from a rubber underlay glued straight to a concrete slab. The concrete slab already has mass working for it. The timber floor needs the decoupling and the damping to do the job the concrete gets for free.
Types of acoustic underlay and where each one earns its place
Not every underlay suits every job, and picking one on price alone is how a lot of renovations end up disappointed. The right choice depends on the flooring going on top, the room below, and how much floor height you can afford to lose.
Rubber-sheet underlay is dense, durable, and handles moisture well, making it a strong pick under tile, vinyl, or engineered timber in wet areas like bathrooms and laundries. Thicknesses typically run 3 to 10mm, and the denser sheets carry more mass, which helps mid-frequency damping.
Bonded foam underlay is lighter and cheaper, common under laminate and carpet. It compresses more than rubber, which is great for comfort underfoot but can be too soft for point loads under tile.
Cork underlay offers natural resilience and reasonable thermal insulation, and it’s a favourite under floating timber floors where a firmer, more stable feel matters. It’s less forgiving with sustained moisture than rubber.
Fibre-based felt underlay (often recycled fibre) is a mid-range option under carpet and laminate, valued for its balance of cost and acoustic softening rather than outright performance.
Reclaimed-rubber panels, made from recycled tyre rubber, deliver some of the best density-for-cost ratios on the market and are popular in multi-unit projects where budget and performance both matter.
Specialised tile underlay membranes combine crack-isolation with acoustic damping, built to handle the compressive loads tile demands without crushing over time.
Cost bands vary widely. Basic foam sits at the affordable end, fibre felt sits just above it, and rubber sheet and reclaimed-rubber panels sit at the premium end because of their density and durability. Foam and felt are generally DIY-friendly for a confident renovator. Rubber sheet under tile, and anything forming part of a tested floating assembly, is better left to an installer who knows how to preserve the tested performance, because cutting corners on adhesive or seams undoes the acoustic benefit fast.
Choosing underlay for timber, laminate, vinyl and tile floors
Work through this checklist before you buy anything, because the flooring finish alone doesn’t tell you what underlay you need:
Identify the floor finish. Engineered timber and laminate favour resilience and rebound. Tile and stone need high compressive strength so the underlay doesn’t crush and crack the finish.
Check the subfloor. Concrete slabs need moisture-appropriate products; timber subfloors need underlay compatible with structural movement.
Consider footfall type. A family home with kids and pets needs stronger low to mid-frequency damping than a quiet home office conversion.
Know what’s below. A bedroom over a living room needs far more attention than a floor over a garage or crawl space.
Set a realistic budget. Premium rubber and reclaimed-rubber panels cost more upfront but often save money over a failed retrofit later.
Check your height allowance. Some underlays add 8 to 10mm; door clearances and stair nosings can force a compromise.
Under engineered timber, prioritise resilience and rebound over raw compressive strength. Under tile, flip that priority. A tile underlay needs to resist crushing under point loads (furniture legs, dropped items) without losing its acoustic properties over years of use.
Trade-offs show up fast once you start comparing products. Thicker underlay usually means better acoustic figures but more height loss, which matters at doorways and stair transitions. Some products require specific adhesives to maintain their tested performance, and swapping to a cheaper adhesive can void the warranty and quietly wreck the acoustic result.
Pro Tip: Ask any supplier for the specific IIC or FIIC figure tied to a named, tested flooring assembly, not a generic underlay-only number. A rating with no flooring type, subfloor, and installation method attached is close to meaningless.
Watch for these red flags: acoustic claims with no named test standard attached, underlay sold with no installation guide at all, and adhesives recommended generically rather than matched to the specific product. Manufacturer sheets, like the QuietFloor™ Acoustic Underlay technical data sheet, show measured improvements for a specific assembly. That figure won’t transfer automatically to a different flooring type or a botched installation.
Getting installation right so the acoustic benefit survives
An underlay’s lab-tested performance is only ever a starting point. Site preparation and installation quality decide whether you get anywhere near it in the finished room.
Level the subfloor properly. Most flooring manufacturers work to a 3mm variation over a 3m span, and underlay does not fix an uneven subfloor, it just telegraphs the dip through the finished surface. A guide on levelling before flooring covers when levelling compound is worth it and when it isn’t.
Test for moisture before laying anything over concrete. A vapour barrier is cheap insurance against a warped floor and mould further down the track, and it matters even more when retrofitting over older slabs, similar to the moisture considerations that come up in wall retrofits.
Match the adhesive to the underlay. A generic adhesive can compress the underlay’s cell structure over time, quietly reducing its resilience.
Detail the seams and edges carefully. Gaps and overlaps at joins create weak points where impact energy leaks through untreated.
Leave a perimeter gap on floating floors and seal it with an acoustic sealant, not rigid skirting fixed hard against the floor. A floating floor screwed or glued to a wall at any point stops being a floating floor.
Bring in a licensed installer or acoustic consultant for any multi-unit or inter-tenancy job, where getting it wrong is expensive to undo and potentially a compliance issue.
Get these details wrong and even a genuinely good underlay product can underperform its rating by a wide margin once it’s actually in the floor.
What the Building Code expects and how performance gets verified
New Zealand’s Building Code Clause G6 sets a minimum Impact Insulation Class (IIC) of 55 for inter-tenancy floors, the shared floors between separate households in apartments, townhouses, and similar multi-unit builds. That number isn’t arbitrary. A bare concrete slab typically measures around IIC 25, which shows how far a raw structure sits below the standard before any acoustic treatment goes in.
Getting from IIC 25 to IIC 55 isn’t a matter of picking a thick underlay off a shelf. It requires a tested floor and ceiling assembly, specified as a whole system rather than product by product. Building points renovators toward whole-system measures, tested assemblies, ceiling treatments, and airtightness, rather than relying on underlay claims in isolation.
Here’s the gap that catches people out: lab results and field results are not the same thing.
Lab testing happens under controlled conditions with a fixed, correctly installed assembly.
Field testing uses Verification Method G6/VM1, which measures the actual finished result on site.
On-site variability is real. Field reports have shown a tested underlay like Mapefonic deliver strong FIIC gains on some projects, and noticeably weaker results on others, purely down to installation quality and site conditions, as documented in field impact insulation testing.
If compliance matters for your project, specify the tested assembly by name, not just the underlay brand, and commission a field test once it’s built. That’s the only way to know what you’ve actually got, rather than what the data sheet promised.
When underlay alone won’t be enough
Low-frequency footfall on a lightweight timber floor is the scenario where underlay consistently disappoints people. A resilient layer softens the sharp edge of an impact, but the deep thud that travels through the joists needs mass or damping to slow it down, and a 5mm foam sheet has neither in any meaningful quantity.
When that’s the problem, the fix usually lives elsewhere in the assembly:
Ceiling-side treatments, resilient hangers, an additional layer of plasterboard, or cavity insulation, can meaningfully cut what comes through from above without touching the flooring at all.
Mass-based options, including sand or sawdust vibration-damping infills in the floor topping, add weight and damping directly where the low-frequency energy travels. Research from the Acoustical Society of New Zealand shows these infills can genuinely improve low-frequency impact insulation in timber-framed floors.
Heavier subfloor toppings shift the whole floor’s mass profile, which is often the most reliable lever on older timber structures.
Budget for this honestly. A $40 to $60 per square metre underlay upgrade won’t touch a genuine low-frequency problem. Ceiling work or a topping system costs considerably more but is the only route that actually addresses the physics involved.
How a managed renovation delivers a verified result
Getting from “we bought good underlay” to “the floor actually performs” is where most DIY acoustic projects stall. Sortedhomesolutions runs it as one coordinated job: All Trades, one team, SORTED, handling everything from the initial survey through to the finished floor so nothing falls through the gaps between trades.
A properly managed acoustic floor job covers a site survey to understand what’s above and below, specification of a tested assembly rather than a guessed-at product stack, coordination of the flooring, plastering, and any ceiling trades involved, and oversight through installation so the seams, adhesives, and edge details don’t quietly undo the acoustic benefit. A guide to levelling before flooring and a flooring cost guide are useful starting points for planning what a job like this involves before you commit to anything.
What actually moves the needle on a real renovation
The three mistakes I see most often: skipping the levelling step, mixing adhesives with whatever’s in the shed, and buying underlay based on a headline acoustic number with no assembly attached to it. Before spending big, try the cheap fixes first, rugs, door seals, checking existing skirting isn’t rigid against a floating floor. A site survey earns its cost the moment shared walls or ceilings are involved, where getting it wrong is expensive and hard to undo.
— Rudi
Get a fixed-fee quote before you commit to a floor
There are other routes to a quieter floor, buying underlay yourself and hiring a flooring contractor separately, or DIY-ing the whole thing on a weekend. A different route is available: one fixed-fee quote covering the survey, the specified assembly, and every trade needed to install it correctly, with a single point of contact managing the lot instead of you chasing three different tradespeople.
The first step is a site survey and a fixed-fee quote, based on your existing floor construction, the finish you want, and what’s happening in the room below. Check current pricing on the Sortedhomesolutions pricing page, or if you’re planning a broader renovation alongside the flooring, look at the Full Home Reno & Flood Restoration service. Before you get in touch, have your floor plan, the flooring type you’re considering, and any known subfloor issues (cracks, damp, unevenness) ready. It speeds up the survey and gets you an accurate quote faster.
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FAQ
Does acoustic underlay work?
Yes, for mid and high-frequency impact noise like footsteps and taps on hard flooring, most quality underlays make a noticeable difference. For low-frequency thuds on lightweight timber floors, underlay alone rarely solves the problem without added mass or ceiling-side treatment.
What type of underlayment is best for soundproofing floors?
Dense rubber-sheet or reclaimed-rubber panel underlay generally delivers the strongest acoustic result under timber, laminate, and tile, thanks to their mass and damping properties. The best choice still depends on your flooring type, subfloor, and what’s happening in the room below.
What is the best underlay for soundproofing?
For most homes, a dense rubber or reclaimed-rubber underlay in a correctly installed, tested assembly gives the most reliable result. Compliance projects should specify the whole tested floor-ceiling system, since Building Code G6 sets a minimum IIC of 55 for inter-tenancy floors.
How can I soundproof my floor to reduce noise from the floor below?
Noise from below is a ceiling problem more than a flooring one, resilient hangers, extra plasterboard layers, or cavity insulation on the ceiling underneath usually help more than any underlay upgrade on your own floor. If the noise is severe or affects a shared wall or ceiling, a site survey is worth the cost before spending on materials.
Do I need a professional to install acoustic underlay?
Foam and felt underlays under carpet or laminate are generally manageable for a confident DIYer. Anything under tile, or forming part of a tested compliance assembly, is worth having installed by a professional, since incorrect adhesive or seam work can undo the acoustic benefit entirely.
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