Sound transmission through floor/ceiling assemblies is essential when designing for multi-family housing, hotels, and mixed-use occupancy. It should also be a consideration for single family homes. This article will address multiple aspects that affect the sound performance of your Trus Joist® floor system and demonstrate how sound performance can be improved.
There are two common measures of sound performance: Sound Transmission Class (STC) and Impact Insulation Class (IIC) ratings. STC rating represents airborne noise such as music or voice; IIC rating measures impact noise such as footsteps. Higher values reflect better sound performance.
STC ratings are described in Table 1:
Table 1: Privacy Afforded According to STC Rating
IIC ratings aren’t as well described but do follow a similar trend. An IIC rating of 30 represents typical single-family floor ceiling assemblies with hardwood or vinyl flooring. Ratings below 30 are poor and ratings of 50 and higher reflect the high performance levels required and expected for multifamily construction.
Improving Sound Performance
The high levels of sound performance required for multifamily construction (STC and IIC above 50) typically require special detailing and large amounts of mass. This level of effort and expense is not justifiable for single-family homes; however, significant improvements in sound performance can be made to single-family floors through the use of resilient channels and insulation.
In the typical house, gypsum is attached directly to the underside of the joists with no insulation in the cavities. In some cases, basements may be unfinished with no gypsum ceiling. This construction results in relatively low STC and IIC ratings. Gypsum ceiling attached to wood or stiff metal furring will not provide significant improvement. However, the addition of resilient channels and insulation to a floor/ceiling assembly can considerably reduce sound transmission. Table 2 illustrates the approximate performance for a range of floor constructions.1
Table 2: Approximate STC and IIC Values for Base Assemblies
As shown in Table 2, the use of insulation alone has small benefit for STC (+5 points), but no benefit for IIC. However, the use of only resilient channels greatly improves both STC and IIC (+10 points). The logic – sound travels through solids much easier than through air. A resilient channel decouples or mechanically separates the drywall from the structural member and minimizes direct pathways for sound. Figure 1a shows a standard system with directly attached drywall thus allowing sound to directly travel through the assembly. Figure 1b shows how resilient channels can prevent the direct pathway of sound. The greatest improvement comes with use of insulation in combination with resilient channels, for which the total improvement is more than the sum of their individual improvements.
Finished Flooring Effects
The ratings in Table 2 represent baseline data for assemblies tested without finished flooring, which will affect performance. Finished flooring does not typically change the STC rating, but can significantly affect the IIC rating. The most effective way to improve IIC rating is by using absorbing surface materials – such as carpet and padding. These floor coverings absorb footfall traffic.2 The difference between assemblies 1-2 and between assemblies 3-4 in Table 3 below shows how carpet and padding greatly improves the IIC rating.
Table 3: Finished Floor Affects on STC and IIC Rating2
Impact noises are a larger concern when a habitable space is below a hard floor (i.e. below tiled/vinyl kitchens and bathrooms). Direct-nailed hardwood will perform similarly to bare OSB. Tile applied directly over wood subfloor will actually reduce IIC by 10-15 points due to an increase in high frequency noise transmission. Thick cushioned vinyl flooring over wood subfloor may improve IIC by a few points (less than 5). The use of ¼” thick recycled rubber mats* (such as ECORE 5mm QTScu®or Proflex™ RCU 250) under tile or floating hardwood will produce results similar to a thick cushioned vinyl. Luxury vinyl tile typically performs similar to or better than thick cushioned sheet vinyl.*
*contact manufacturers for more information
When selecting channels, it is critical that the single-leg resilient channels are used. Hat channels do not provide substantial benefits for sound attenuation. Hat channels are solid and do not provide the decoupling mechanism found in resilient channels.
Choosing and installing the right resilient channel makes a big difference in how a floor assembly performs. The original resilient channel USG RC-1, developed by USG in the 1960s, uses large slotted holes that are centered on pre-punched screw holes at 4” on center (Figure 3).
Since then, many other types of resilient channels have emerged in the market place. With various hole shapes and sizes, all channels do not perform equally. Resilient channels with holes resembling the long slots that are found on the original USGS RC-1 outperform resilient channels with smaller oval or round holes.3
The accidental use of screws that are too long can result in connecting the channel and framing structure together. This causes a ‘short-circuit’ connection and will diminish any decoupling action. As short-circuits increase, the STC rating can decrease as much as 20%.4
Another common installation error is attaching the resilient channel on a solid surface such as floor or wall sheathing thus sandwiching the channel between the drywall and sheathing. This can result in greatly reduced sound attenuation and in some cases even completely negate the effectiveness of the resilient channels.4
Flanking noise is not accounted for in the acoustical ratings. By going around the assembly, flanking noise can increase noise transmission. Flanking noise occur because of conditions like continuous joist space over partition walls, non-isolated duct paths and back-to-back electrical and plumbing outlets. Furthermore, one can minimize places where sound can leak through by reducing the amount of openings, joints, and cracks in the assembly.5
Finally, poured toppings or an additional ceiling layer can be used to increase mass and improve sound ratings. These strategies are commonly used in multi-family construction. For tested assemblies meeting the requirements for multi-family construction see Technical Resource Sheet TJ-4035 (http://www.woodbywy.com/document/tj-4035/).
Changing the mass of the floor system or adding resilient channels may affect the feel of the floor as well. For best floor performance, work with your Trus Joist® representative to evaluate the TJ-Pro® Rating for the floor. (Please refer to our technical bulletin addressing floor performance here http://www.woodbywy.com/document/tb-104/)
A designer should consider sound transmission as this performance factor will be tested daily throughout the life of the structure. If you have any questions about Trus Joist® products in floor/ceiling assemblies, please contact your Trus Joist® representative or submit an e-mail to firstname.lastname@example.org.
(1) Warnock, A.C.C and Birta, J.A, Detailed Report for Consortium on Fire Resistance and Sound Insulation of Floors: Sound Transmission and Impact Insulation Data in 1/3 Octave Bands, IRC Internal Report IR-811, National Research Council of Canada, 2000
(2) Form No. W460N: Noise-Rated Systems, APA The Engineered Wood Association, 2000
(3) Lilly, Jerry, Resilient Channel Update, http://www.jglacoustics.com/acoustics-rc_1.html, August 2015
(4) LoVerde and Dong, Quantitative comparisons of resilient channel designs and installation methods, http://www.pac-intl.com/pdf/IN09_737_Submitted.pdf, August 2015
(5) Warnock, A.C.C, Construction Technology Update No.35: Controlling the Transmission of Impact Sound through Floors, National Research Council of Canada, 2009