Author Topic: Soundproofing duplex right next to freeway  (Read 6373 times)


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Soundproofing duplex right next to freeway
« on: August 22, 2008, 09:50:18 PM »
I am currently building a stick duplex right next to a freeway.  The home sits about 20 feet above and 40 feet back from a 6 lane freeway).

Currently, our exterior walls consist of 5/8" QuietRock (model 525) drywall on 2 x 4 wood studs, 1/2" plywood w/waterproof membrane (Tyvek or similar) and standard fiberglass insulation in between.  Attached to the exterior plywood are 1 x 4 wood furring strips and I plan to attach 1/4" thick 4 x 8 foot HardiPanels to the wood furring strips.  The HardiPanels will not be placed butt to each other but will rather have a space of approx 5/8". This is to create a "grid" pattern to the exterior wall.  My goal is to build a modern, minimalist home.

So essentially, (from the outside in) I have 1/4" HardiPanel concrete board, 1/2" plywood sheet,  2 x 4 stud and  5/8" QuietRock on one wall.  Would anyone be able to tell me what the STC rating of such an assembly would be?

I've been told by a local dealer of QuietRock that I should look into the "triple leaf" effect and that spacing the HardiPanel away from the plywood could cause the STC rating of the wall assembly to drop as a result of this phenomena.  My reasoning would be that placing the 1/4" HardiPanel 3/4" away from the plywood would increase the STC rating since we are adding a third but very small airgap.  Keep in mind, however, that the HardiPanel assembly is not completely sealed and air/sound tight because I am spacing each panel away from each other by roughly 5/8".   

The other options I was looking at was going with 2 sheets of standard 5/8" drywall and GreenGlue between.  I have heard that this assembly would reduce low frequency penetration into the home much better than one sheet of QuietRock 525.  Is this true?

And another facade option would be to go with stucco so instead of a 1/4" sheet of HardiPanel and 5/8" "air space", I would have densglass, a cementitious coat and the stucco coat on top.  I'm not sure if this would provide a better STC than spacing 1/4" HardiPanel away from the plywood but I'm assuming it would.  If anyone can chime in on this I'd really appreciate it. 

For the windows, I am looking closely at Migard's Quietline product.  I may just use these windows for the bedroom windows (because of cost) and other windows in the home where people are not so sensitive to noise would likely be a standard window with thicker panes of glass (to achieve at least a 35 STC).  Is this a good idea?  And is it true that vinyl windows are a better sound insulator than aluminum?  Everyone I talked to here says that vinyl is superior to aluminum for soundproofing but I've found some information online to the contrary.  My concern with the vinyl is that it is only available in white and an off-white "tan" color, both of which won't match the modern exterior of my building.  What would my options be if I went with vinyl but didn't want those frame colors?

Regarding flooring, the building is 3 stories high (garage at bottom, kitchen/living second and bedrooms third) I am looking at standard TJI for the floor between the bedroom and kitchen/living area but standard 12" joists for the kitchen/living floor (garage below) and the roof (nobody above). 

Sorry for all the questions.  I've been doing some research here locally for quite sometime but nobody I've talked to seems to be a real professional about soundproofing, hence my visit to this site.  Like most of the people that visit this forum, my goal it to achieve soundproofing at the best "STC per buck".  I am willing to invest money into my project just as long as it's within reason.   Thank you.

Mark Daveis

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Re: Soundproofing duplex right next to freeway
« Reply #1 on: August 23, 2008, 08:23:55 PM »
Aluminium is better than vinyl even though the vinyl damps better but has much less mass. We have aluminium frames on all our windows.
Anything less than 2 inches of air space will cause the air gap to act more like a solid making things and LF performance worse.
Leaving a gap between the cement board panels of that size will render it almost useless for soundproffing.
If I was building right next to that much traffic then I would use concrete block or poured concrete as its cheap and I really dont think a few thin layers of drywal/plywood will cut it with that much loud sound coming at you. :)


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Re: Soundproofing duplex right next to freeway
« Reply #2 on: August 23, 2008, 09:32:14 PM »
My suggestion is to construct staggered stud walls - 2X6 plates, with 2X4 studs.  You can decouple the walls and get more insulation in the cavity. 

Skip Quietrock.  2 layers of 5/8" drywall is plenty, adding Green Glue between the sheets is even better, but hardly necessary if you build the wall right in the first place. 

Go for the maximum practical amount of mass for the exterior siding - stucco would work best.

The frame of a window isn't that important - how well the window seals when closed will determine the reduction in noise.  It would be much better to install two sets of windows per opening with as large as possible airspace. 

Even with these measures, your house will still be (relatively) noisy....   


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Re: Soundproofing duplex right next to freeway
« Reply #3 on: September 26, 2008, 04:52:59 PM »
In order to get "the best STC for the buck" you need to understand a few basic elements regarding soundproofing.  The first element is to decouple any vibrtating surfaces from the structural they're attached to.  This refers to wall/ceiling/floor panels.  The most cost effective is isolation/padding tape for info see
The second element is to absorb and block sound within a contained space - insulation does this.  The third element is to barrier the remaining air-borne sound (MLV is excellent for this).  And the fourth is to lower the resonant frequency of the assembly in order to stop low frequency (impact) sound transmission which is below STC measured range in hz (Green Glue damped panels do this). 
The mix of these four elements depends on the types of noise present, the quietness goals of the occupant, the skill sets of the builder and tradesman, and of course the budget constraints.  All four elements must be present if you expect to get good results - leave one out and the whole suffers.