How To Soundproof Exterior Walls

double wall drywall exterior wall green glue mass soundproof wall stc rating Sep 18, 2023

I have gotten so many questions about how to soundproof the outside exterior wall of a double wall system. This makes sense since all the double wall diagrams show two interior walls with sheetrock on both sides. This leads you to wonder, well how do I replace the sheet rock with an exterior wall material and still have it be soundproof? In this article I will answer exactly that question. 


1) Double Wall Construction 

First, let's take a brief look at how our walls are constructed to help us understand how we can modify this design. A typical interior double wall will have two layers of 5/8" drywall on a wood or metal framed stud wall. Then there will be at least a 1" air gap. After that there is another wood or metal frame wall. Insulation is installed in the wall cavity either filling both framing bays or having insulation in the bays of each wall. Either is fine. Lastly, the outer wall has 2 layers of 5/8" drywall. 

This is your typical double wall design and it would look something like the far right wall design in the diagram below:


 This gives you an STC of 63 and everything is great, but what about if that left or right side of that wall is an exterior wall. You can't just put drywall on the outside of a building, so what are you to do? 


2) How To Soundproof Exterior Walls

So to solve this problem we just need to understand some soundproofing theory. The reason we use 5/8" drywall is twofold. First, it is relatively cheap and is a common building material. Second, it has mass. It weighs 2.2lb per square foot whereas 1/2" drywall is much lighter. 

Knowing that our inside wall will weigh 2.2lb per square foot per sheet of 5/8" drywall we can do some super simple math and know that our inside walls weights 4.4lb per square foot. 

Instead of focusing on the drywall, the key is to focus on the mass of the wall it self. Now we can build our exterior wall knowing it needs to be 4.4lb per square foot or more. 


3) A Real World Example

In my studio, most of my walls are exterior outside walls, meaning they face the outside. This means my second wall has one layer of 9/16" sheathing and one layer of hardy plank siding. Let's calculate how much that weighs. 

9/16" Sheathing (nominal 5/8") = 66.37lb (4'x8' sheet) = 2.07lb per square foot


Hardy Plank Siding =  2.3lb per square foot

*(James Hardie® Siding FAQs - Homescapes of New England: 603.734.4282)

This means my total outside wall mass or weight = 4.37lb per square foot. 

This is just shy of my inside wall mass, but actually worked out perfectly. 

Now, I will be honest. I did not think about any of the math when I was building my studio. It was my first studio build and I really didn't know what I know now. So, to save you some trouble simply match your studio mass on both walls. Easy and you don't have to overthink it. 


4) But What About Green Glue???

Yup, I knew we had to go there next. So, if you use Green Glue on your inside wall you certainly could use it between your sheathing and outside exterior siding. This would guarantee you have equal walls. I did not do this in my studio and I don't really need it. 

You could also say that Green Glue is adding soundproofing to your inside wall and thus add more mass to your outside wall. How much mass, well that is up for debate. At one point Rod Gervais had said that GG is like having four layers of drywall. He has since pulled back that claim, but it is adding to transmission loss, especially at low frequencies. 

My advice, would be to not obsess over these minor details. Green Glue or no green glue, mass is still more important in my humble opinion. Focus on the mass and then add Green Glue if you want that extra boost. 


5) But wait...Wilson, my walls are too thin on the exterior. 

If you are building in an existing structure then your walls may not have much mass. A shed or garage may have thin low mass construction. In this case you can add mass as long as the structure can handle it. You can add drywall to the inside of the exterior wall or add more mass to the outside. How you do it is not as important as that you do it and the mass is correct. That magic 4.4lb per square foot is your mark. That is the number that matters on both walls. 


Works Cited: 

James Hardie® Siding FAQs - Homescapes of New England: 603.734.4282. 26 Mar. 2010, Accessed 12 Sept. 2023.

jonathan. “How Much Does OSB Weigh? (+ OSB Weight Calculator).” ToolCrowd, 17 Oct. 2021, Accessed 12 Sept. 2023.


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