I'm recently involved in a project using SIP for exterior walls. Could anyone tell me what is STC rating for a 8-1/4" SIP with 5/8" gypsum board on one side? I know SIP is bad for low frequency sound. I just can't find a reliable source telling me what exactly the STC rating is. An approximate number should be fine. Thanks a lot!

Posted By ellisa015 on 08 Oct 2014 11:25 AM
I know SIP is bad for low frequency sound. I just can't find a reliable source telling me what exactly the STC rating is. An approximate number should be fine. Thanks a lot!

A typical SIP has a STC rating of 21-23.

By comparison, a standard 2x4 wood frame wall with 1/2" drywall has an STC of 30-34 and an ICF wall (6" core) has an STC rating of 50+

There are ways to increase that number with SIPs and it involves using decoupled hat channels. Simply screwing drywall to the SIP will not increase the STC rating. Sounds are basically vibrations and unless you decouple the vibration by using decoupled rubber isolators with a hat channel and screw the drywall to the decoupled hat channel, you will not increase the STC rating. I researched this quite a bit and spoke to sound engineers and the use of decoupled rubber/neoprene isolators is the best method.

Posted By Lbear on 08 Oct 2014 12:46 PM
...unless you decouple the vibration by using decoupled rubber isolators with a hat channel and screw the drywall to the decoupled hat channel, you will not increase the STC rating...

You could increase STC more with rubber isolators, but you can increase STC without them. Adding mass will increase STC, so screwing gypsum directly to the panel will increase it but the question is by how much. The addition of hat channels is better. The addition of resilient hat channels even more.

So does thickness of SIP matter to STC rating? I mean, if I choose a thicker SIP, will that increase its STC rating? What do you mean by typical SIP? Is it 4-1/2" thick? Thanks,

I don't think you'll gain much in the STC rating by increasing the thickness of the SIP - you certainly won't get double the value by going for example from 4.5 inches up to 8.25 inches. And that's because it's not the foam that is going to mitigate sound. Sorry I can't be of more scientific help to you. There is some interesting reading here in this forum about the topic in old threads.

The important thing to remember is there are methods to increase the rating, and some of them are what you would do already in the building process. For example furring out exterior walls with hat channel to hang sheetrock is often done to create a space for wiring and even water supply. Also using a high-mass exterior finish; brick is excellent because you have dense high mass material and the dead air space. But stucco or fiber cement will also help somewhat. This all depends on your needs of course.

OK. Thanks a lot! It is very helpful!

Posted By ellisa015 on 08 Oct 2014 04:37 PM
So does thickness of SIP matter to STC rating? I mean, if I choose a thicker SIP, will that increase its STC rating? What do you mean by typical SIP? Is it 4-1/2" thick? Thanks,

A standard SIP usually refers to a 4-6 inch thick SIP. SIPs by nature do not get good STC ratings. It's the low frequencies that transmit very easily so if you are near a highway, airport, etc. I would definitely invest in getting some resilient sound isolators and hat channels. The key is DECOUPLING the SIP from the rest of the wall assembly (drywall). Rubber isolators work best because they absorb vibrations and sound is basically a vibration. Metal hat channels without rubber isolators doesn't work as well since the metal hat channels will not decouple the vibrations.

where do you find the written STC data for SIPS

Posted By GeorgiaTom on 07 Nov 2014 02:15 AM
where do you find the written STC data for SIPS

SIP manufacturers don't like to publish the #'s because they are not good. The data is out there but one really has to dig deep to find it. Like anything, the manufacturers don't advertise the negatives, just the positives. Suffice it to say I researched this quite a bit and SIPs are around STC 21-23. An ICF wall is around STC 50.

• 20 - 25   Very Poor - even low speech can be heard

• 25 - 30   Poor - normal speech can usually be heard

• 30 - 35   Fair - speech would be quite loud to be heard

• 35 - 40   Good - one can hear loud speech through a barrier, but not understood

• 40 - 50   Very Good - loud speech can only faintly be heard

• 50 - 60   Excellent - one has to strain to hear loud sounds through a barrier

l bear, thanks, if you have found the data can you share where to find it since you have already done the research - Thanks

Posted By GeorgiaTom on 07 Nov 2014 07:56 PM
l bear, thanks, if you have found the data can you share where to find it since you have already done the research - Thanks

My research will cost you $$$$

All kidding aside, here is the info you requested:

Premier SIPs - Technical Bulletins - Click #25 for STC test results

Premier SIPs Panel (no finish either face) STC- 22
½” gyp, Premier SIPs Panel, no finish on other face STC- 28
5/8” gyp, Premier SIPs Panel, no finish on other face STC- 29
5/8” gyp, Premier SIPs Panel, 5/8” gyp STC- 33
2-layers 5/8” gyp, Premier SIPs Panel, 2-layers 5/8” gyp STC- 41

Joe Pasma, PE, Premier SIPS’ Technical Manager:
"The takeaway from Technical Bulletin #25 is that SIPs themselves, don’t have a real high STC rating and the effort, i.e. expense, involved in creating an assembly that is sound proof, has an STC near 50, is relatively high.  So, SIP buildings do a good job of reducing ambient noise but don’t do so well with the low frequency sounds.  Designers should keep this in mind when acoustical performance is a big design consideration."

Be aware that attenuating low frequency sounds (< 125 hz) is a completely different goal than improving the overall STC rating (more appropriate for speech). For the former, adding a small gap with a resilient channel will make things worse.