The Importance of Fire Stops In A Double Wall System

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One of the least sexy topics in soundproofing is fire stopping. Okay there may be no sexy topic in soundproofing, but this one is not fun, but important. In this article you will learn what firestopping is, why we need it and how to incorporate it into your soundproof wall system. 

1) What is fire stopping

Fire stopping is a construction technique where you use a fire retardant barrier to block a cavity so fire cannot spread through that cavity. In soundproofing this is important in our double wall and double ceiling cavities. 

In normal wall framing our walls have bays with insulation. If a fire were to break out inside of one of the bays it would not be able to spread easily to the next bay or to your ceiling. See the diagram below. 

Notice how fire cannot escape the through the bottom, sides or top of a typical stud wall. However, with a double stud wall there is essentially a chimney in between your two walls. See below: 

This gap at the top means a fire could easily spread to your ceiling and throughout the inside of your wall system without you even knowing. For fire safety it is important to seal that opening at the top of the wall, but how do you do that without bridging your two walls and negating all the benefits of a double wall system? 

2) How To Build A Firestop

I learned this fire stop technique from Rod Gervais in his book, Home Recording Studio: Build It Like the Pros. He says to use compressed rockwool or any rigid insulation batt to decouple the fire stop from the inside wall. See the diagram below: 

Here is a side view section of a design I am working on for a client. Notice the rigid batt insulation sitting on top of the top plate. This insulation could be 3lb/ft3 of Rockwool, Thermafiber or any similar brand. In this diagram it is 2" thick and is compressed 50% to 1". This compression is what keep the isolation intact. We don't want to compress the batt 100 percent because then the drywall would touch both walls. 

On top of the insulation batt is 1/2" drywall that is cut to span the gap between the two walls. It should be screwed in to the inner wall top plate on top of the insulation with as few screws as possible or you could use contact adhesive to minimize the connection points. Then you should seal between the rockwool and 1/2" drywall with fire caulk to create an airtight seal.  

Below is another diagram with a perspective view of the fire stop around two walls. 

Notice how the firestop seals the gap around the entire room making it much more difficult for fire to spread up into the ceiling. 

Conclusion

Although this adds extra cost and labor to a build it is, nonetheless, an important part of the double wall design. It is always good to keep fire safety in mind when we are building a typical structures for sound isolation. 

Work Cited

Gervais, Rod. Home Recording Studio: Build It Like The Pros. 2nd Edition, Course Technology Cengage Learning, 2011.