Do You Need Green Glue?
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Lately I have been thinking a lot about if Green Glue is really necessary in a soundproof studio build. The first hint, was that Rod Gervais only mentions Green Glue in a small section of his book. His entire process involves 5/8" drywall with specific construction techniques. So, why then do we need Green Glue? This is what I will answer in this article.
1) What Is Green Glue
Green Glue is a damping system. Damping involves reducing sound energy by converting it to heat by reducing the vibrations in the material. There are many damping materials on the market, the most notable competitor to Green Glue being Mass Loaded Vinyl or MLV.
In his book Home Recording Studio: Build It Like The Pros, Rod Gervais states the following: "There are a large number of damping systems on the market...But the only one that has caught my attention (providing not only greater isolation at low frequencies than standard drywall, while maintaining a cost performance that is reasonable) is a product called Green Glue..." (Gervais Chapter 4 Floor, Wall and Ceiling Construction Details pg.89).
2) How Green Glue Increases Isolation
To understand how Green Glue works we need to compare it to the mass needed to get the same isolation without using Green Glue. To understand this we need to look at Mass Law. Mass Law states that each doubling of mass adds 6dB of additional isolation, but in the real world that number is closer to 5dB. (Gervais Chapter 4 Floor, Wall and Ceiling Construction Details pg.89).
So, if you have two sheets of drywall on each side of your wall the next step is four sheets on each side to increase the isolation by 5dB.
Gervais shares a graph in his book from lab tests done by the Green Glue Company. These tests were done by a reputable third party lab with no connections to the Green Glue Company.
In the diagram below you can see that the top two lines in the graph show the Green Glue Assembly (two layers of drywall with green glue in the middle on a single stud wall) and the estimated performance of 4 layers of drywall per side.
What is important to note is first, the Green Glue simply outperforms the four layers of drywall across the entire frequency spectrum from 250Hz upward. Starting around 125Hz and below the green glue is superior to the four layers of drywall.
If we look at two layers of drywall without Green Glue it is clear that the Green Glue certainly outperforms the two layers of drywall across the entire frequency spectrum.
3) How Much Isolation Do You Need?
Now the initial instinct after looking at the graph is to say, great! Let me use Green Glue because it clearly gets the best results. Before you jump to that conclusion you need to ask yourself how much isolation you really need.
If you are primarily recording acoustic instruments and listening back to your speakers at a normal listening volume then you probably don't need to spend the extra money on Green Glue. However, if you are playing drums at all hours of the day or rehearsing with a heavy metal band at full blast then you would be wise to use Green Glue to get the maximum isolation possible.
4) Your Studio Is Only As Soundproof As Its Weakest Link
The next logical step is to think if your walls are getting really great isolation then you are good to go. However, if you use Green Glue and your walls have a really high STC value, but your doors and windows do not then you will not have a soundproof room with an STC as high as your walls.
Your room is only as soundproof as the least soundproof assembly in it. This means if your walls get an STC of 73, but your door gets an STC of 57 then you probably did not need to spend all that money adding Green Glue to your walls.
STC is not even the best way to compare the door and the walls. The best way is to look at the Transmission Loss over the entire frequency spectrum.
For example, let's look at the transmission loss of the best Overly Acoustical Door. This door gets an STC of 57. Overly, by the way, is one of the most reputable soundproof door companies on the market.
Below is a diagram from the Overly door product catalog.
If we compare the frequencies of the transmission loss chart to the double wall with Green Glue we can see some comparisons. Now remember, the graph from the Green Glue company shows a single stud wall. In studios it is best to use a double stud wall so your transmission loss will be even better.
This said, this high end door achieves really good results across the frequency spectrum and would perform better than the drywall with Green Glue in the low end. The only downside is this door probably costs upwards of $5,000. If you build your own door you need to try and match the weight of your walls with your door.
If you are using Green Glue, remember the mass of your walls will be close to the equivalent of four layers of drywall on each side. For a double door system that means both doors need to weigh close to 176 LBs or more to match the mass of your walls.
Conclusion
For most DIY soundproofing projects Green Glue may be overkill. However, if you are a drummer or a very loud band then I would consider Green Glue and building a very heavy door or buying a professionally built one. The key is to remember your soundproofing is only as good as your weakest link. Companies like Green Glue want to sell you their product, but be aware of the bias towards getting superior results without taking into account your doors and windows as well.
Works Cited:
Gervais, Rod. “Chapter 4 Floor, Wall and Ceiling Construction Details.” Home Recording Studio: Build It like the Pros, Delmar, Clifton Park, NY, 2011.
“Product Catalog.” Overly Door Company, https://door.overly.com/products?PT_Guid=d942747f-96d5-4dc3-8b3b-58a3b5b70a55&PM_Guid=2f9a63cb-9c60-4b12-8cc9-8c852003606e&PA_Guid=b5b287a4-917d-4771-bac9-07cc0941bd0c.
