Impact Insulation Class (IIC)

Code: IIC rates a floor/ceiling assembly’s ability to block impact sound.

Enforcement: Appendix Chapter 35 of the ’88 and ’91 UBC, Appendix Chapter 12, Division II of the ’94 and ’97 UBC will be contained in the forthcoming IBC. Although not all municipalities have adopted this appendix chapter, it is still recognized as an industry standard.

General Information: The Uniform Building Code (UBC) contains requirements for sound isolation for dwelling units in Group-R occupancies (including hotels, motels, apartments, condominiums, monasteries and convents).

UBC requirements for floor/ceiling assemblies: IIC ratings of 50 (if tested in a laboratory) or 45 (if tested in the field*).

* The field test evaluates the dwelling’s actual construction and includes all sound paths.

Definitions:

• Impact Insulation Class (sometimes referred to as Impact Isolation Class) measures a floor/ceiling assembly’s resistance to the transmission of structure-borne or impact noise.

IIC Strength: Helps to rate structure-borne noise such as footfall, a chair dragging on the floor, or other realistic sounds in a single number.

IIC Weakness: Due to the nature of the testing procedure, almost any assembly with carpet will meet the IIC requirement. Meeting the IIC requirement does not ensure the control of footfall noise. Conversely, if an assembly does not meet the IIC requirement, it does not necessarily mean that there will be a footfall noise issue.

The tapping machine frequently used for this test is not designed to simulate any one type of impact, such as a male or female footsteps, nor to simulate the weight of a human walker. Thus the subjectively annoying creak or boom generated by human footfalls on a limber floor assembly may not be adequately evaluated by this method (American Society for Testing and Materials – ASTM, E 1007, 5.2).

Recommended Isolation Level
An IIC rating of 50 will satisfy the building code requirements. As with STC, it is typically argued that luxury accommodations require a more stringent design goal. Bare in mind, some floor assemblies rated as high as IIC 70 could still transfer noticeable footfall noise.

This code is often used in conjunction with Sound Transmission Class (STC).