Impact Insulation Class, or IIC, is a rating of impact sound insulation. Impact sound is the sound of impact on a floor surface, like the sound of footsteps on the floor above. It only applies to sound transmitted down through a floor and ceiling. It does not apply to walls.  The International Building Code (IBC) requires that a floor-ceiling separating condominiums, apartments, hotel rooms, or other dwelling units be designed for an IIC of 50 or higher.  If the floor-ceiling is tested in the field, a field IIC, or FIIC is allowed to be 45 or higher.

I’m often asked “What product will achieve an IIC 50?”  IIC ratings can be confusing.  Information published by floor product manufacturers can be even more confusing.

One way to increase a floor-ceiling assembly’s IIC rating is to put a “soft” underlayment under a hard floor.  For example, there are many resilient mats that are designed to be placed under ceramic tile, under wood laminate, or under vinyl flooring.

The most important thing to remember is that none of these products have an IIC rating by themselves.  The IIC rating is a rating of the entire assembly.  If you are considering the IIC rating of a tile floor on a resilient mat underlayment with a gypsum board ceiling below, the rating is dependent on all of these components.   What is the subfloor?  …plywood (one layer or two)? …gypsum concrete (how thick)? … concrete?  These things make a big difference.  How is the gypsum board ceiling mounted? …directly on joists? …on resilient channels? …on resilient clips?  This makes a big difference too.

A common mistake is to use a published IIC rating for a product only to find out later that the product was tested on an assembly that included a heavy floor like 6″ concrete, when the project in question is using a lightweight wood subfloor and joist assembly.

If you are evaluating materials for your floor-ceiling, make sure that you find out how the material was tested.  Sometimes this information is published, but sometimes you may have to request a full IIC test report.  Some manufacturers will provide tests on different types of assemblies and this is very helpful.