Complaints from homeowners about noisy neighbors are a constant issue. Some of these grievances involve airborne sound sources (talking, music, plumbing), but many are due to the reverberation of footfall (people walking) or structure-borne noise that differs from its airborne counterpart.
The most commonly used terms in multi-family residential construction are the sound transmission class (STC) and the impact insulation class (IIC). These describe different types of noise, but are often interchanged or misunderstood.
What is noise?There are two basic types of noise and measurements used to rate the acoustical performance of the floor-ceiling assembly. They are airborne noise and structure-borne noise. The measurements used to determine the acoustical performance of airborne and structure-borne noise is the Sound Transmission Class (STC) and the Impact Insulation Class (IIC). The STC measures the performance of the floor-ceiling assembly to stop or isolate airborne noise such as voices, radio, televisions, etc. The IIC measures the performance of the floor-ceiling assembly to reduce or isolate impact noise (footfall) people walking, dropping objects, furniture being moved, etc.
There are two types of noises: airborne noise (voices, TV, radio) - always associated with STC; and structure-borne noise (footfall, object dropped) – always associated with IIC.
Building codesBuilding codes have established standards for minimum noise performances of partitions between residences. Most states use the International Building Code (IBC) which calls for a minimum IIC & STC rating for multi-family dwellings for new construction at 50(or 45 if field tested). The minimum IIC rating of 50 would be considered low or an affordable housing rating, while a 55 would be considered more of an average or mid range rating and a 60+ would be considered a luxury or high-end rating.
One of the most common mistakes in selecting a floor underlayment is to look at a product IIC rating and think that it will perform to the same rating in your building. An IIC rating is determined on the complete floor-ceiling assembly, not just the floor underlayment. When comparing floor underlayments it is important to request the official IIC report from the company. The test report should come from an accredited lab and should detail what the floor-ceiling assembly was tested at. In recent years there have been some companies that advertise IIC rating of 72 without explaining how the product was tested, leaving many uneducated builders thinking that’s all they need.
A recent article from Dave Adams of DL Adams Associates Acoustical Consultant compared the recent rise of these companies to snake oil sales people who made outrageous claims, but many times the product failed to perform to the level claimed.
Now that we understand that the IIC rating is determined on the complete floor-ceiling assembly and not just the underlayment, we need to take a look at flooring. The harder or denser the floor covering the less the IIC rating will be. A marble floor will have a lower IIC rating than a ceramic tile or solid hardwood floor because it is a harder or denser floor. Carpet is by itself an underlayment; it has the ability to cushion and absorb impact energy and has the highest IIC rating when used in a floor-ceiling assembly. When selecting floor covering keep in mind that a floated engineered hardwood or laminate floor will have a higher IIC rating than a nailed or glue-down floor. There are many floor underlayment products on the market that have been specially engineered for the various installation methods of floor coverings, from floated to glue-down and nail-down systems.
In recent years, next to moisture issues, sound isolation issues are becoming the most litigated problem in multi-family housing. Recently we began work on a very large condo project that was facing litigation; the owner decided to face the problem head-on and called in an acoustical consultant who examined and field tested the space. The consultant made his recommendations and the owner called us for material. The owner had to rip up the existing floors, as well as the ceiling below and install high-performance underlayments and resilient clips and then re-install new flooring and new wall board. The owner ended up incurring overwhelming costs of the new material, removal and re-installation, hiring movers and storage and putting people up in temporary housing.
Whenever architects, builders and contractors are concerned with impact noise they should take the time to evaluate what underlayment will be installed and have an experienced acoustical consultant review their plans.