One of the most common things a DIYer neglects to do when finishing a basement is to install proper firestop per code. Firestop is required by building code to impede a potential fire from spreading- which could come in handy if you’re doing your own electrical work. :)
You will not pass your inspections from the local building department if you don’t install firestop as required. So don’t forget to firestop! If you don’t know how to install it, I’ll explain the code and how to meet these firestop requirements.
Basement Firestop Code Requirements
In the International Residential Code (IRC) section R602.8 it explains that firestop is required in basements:
- to form a fire barrier between stories.
- to separate wood-framed vertical chases from horizontal chases including soffits, drop ceilings, and cove ceilings, etc.
- in walls, every 10 feet minimum.
- at openings around vents, pipes, ducts, cables, and wires at ceiling/floor level.
- 2-inch nominal lumber (2×4, 2×6, 2×8, etc.)
- double layer of 1-inch nominal lumber (1×4, 1×6, 1×8, etc.)
- 3/4″ (22/32″) OSB, or double layer of 1/2″ (7/16″) OSB
- 3/4″ particle board
- 1/2″ drywall
- 1/4″ cement board
- mineral or rock wool, installed securely in place
- unfaced fiberglass batt insulation, tightly packed
Requirement #1: Form a fire barrier between stories
This first rule is intended to prevent a potential basement fire from spreading to the main level of the house. The top of exterior basement walls is one place where this rule applies. Installing firestop at the top of exterior basement walls prevents a potential fire from spreading from this wall through the gap between the top plate and foundation wall, to the basement ceiling (or main level floor) above.
The typical way to firestop this area is by installing pieces of 3/4″ (or 22/32″) OSB at the top of all exterior basement walls. It needs to be pushed tight against the mud sill and extend out above the top plate. Here’s a section drawing of the top of a typical exterior basement wall (as if you sliced through it and looked at it from the side), showing the location of the firestop:
Start by laying out the exterior walls on the basement floor with chalk lines, then transfer these layout lines to the bottom of the floor joists above using a level and chalk line. Then measure from the mud sill to this chalk line to determine how wide to cut the piece of firestop. Once cut, the strip of OSB firestop is nailed to the bottom of the floor joists.
Here’s what it looks like installed, before building the exterior wall:
If there’s a pipe in the way, just notch the firestop material to fit around it like this:
Here’s how it looks after the wall is framed:
Finally, seal any penetrations through your firestop, such as voids caused by pipes or wires passing through the firestop. This can be done with firestop spray foam, such as this (that can be purchased at a home center for around $5 per can).
The firestop spray foam works great for filling small voids and penetrations, but if the void is too big, you might have to stuff rock wool or compressed unfaced fiberglass batt insulation (which are also approved firestop materials). Here’s how it looks after this is done:
Interior basement walls don’t need this type of OSB firestop because the 1/2″ sheetrock on both sides of the wall and the 2×4 top plate contains a potential fire, preventing it from spreading to the main level floor above. (There is no air gap for the fire to pass through like there is at exterior walls).
Requirement #2: Separate vertical chases from horizontal chases
The second rule requires an approved firestop material to separate vertical chases from horizontal chases. Stud cavities in your basement walls are considered vertical chases. Fur-downs, bulk-heads, or drops (whatever you call the framing over ductwork and pipes) are considered horizontal chases. So this second rule requires a firestop material to separate your basement wall stud cavities from adjacent drops. (For this article I will refer to fur-down/bulk-heads as drops).
There are two typical methods to do this. The first is to use a piece of 3/4″ (or 22/32″) OSB between the wall and the drop. The other way is to use blocking in the wall along the bottom of the drop.
If you are using 3/4″ (or 22/32″) OSB for the firestop, you cut and nail pieces to the wall from the bottom of the floor joists above to a chalk line marking the bottom of the drop, as shown here at the end of the drop in this basement:
Sometimes the ductwork or pipes you are covering with the drop are right up against the wall you are trying to firestop, preventing you from sliding in a piece of OSB. In this case, the other method (using blocking in the wall along the bottom of the drop) is the way to go. Nail blocks in the wall along the chalk line marking the bottom of the drop as shown here:
Even though they look different, both the OSB and blocking methods separate the wall from the drop. With the blocking method, the space in the stud cavity above the blocks just becomes part of the drop’s horizontal chase. The key here is to separate the wall from the drop with a firestop material.
On this basement job there was an existing ABS plumbing pipe that would be difficult to notch a firestop block around, so I used the OSB method instead for just that stud cavity:
Here’s what the finish drop framing looked like using both the OSB and firestop blocking methods to separate the vertical chases (the stud cavities) from the horizontal chase (the drop):
Finally, seal any penetrations through the OSB or firestop blocking with firestop spray foam like this:
Requirement #3: Firestop walls every 10 feet
In typical basements, the place this rule applies is the gap between the exterior framed wall and the basement foundation wall. At least every 10 feet horizontally, a firestop material is required to prevent a potential fire from spreading horizontally behind these exterior framed walls. Common materials used for this requirement are 2×4 lumber, 3/4″ (23/32″) OSB, or compressed fiberglass insulation. In the picture below, a 2×4 was placed behind a 2×4 in the wall to create a fire barrier.
I nailed pieces of OSB to the stud and the 2×4 firestop to hold it in place, then foamed any voids or penetrations in the firestop.
A firestop like this is then placed at least every 10 feet horizontally in exterior framed walls. If there isn’t room to put a 2×4 directly behind a stud, nail it to the side of an exterior wall stud, like the picture below. Just make sure the firestop is pushed tight against the foundation wall or compressing the batt insulation behind.
It doesn’t matter whether you use a 2×4, OSB, or compressed insulation, or a combination of these approved firestop materials, just make sure you firestop exterior framed walls at least every 10 feet horizontally along your exterior basement walls.
Requirement #4: Firestop openings around vents, pipes, ducts, cables and wires at the ceiling and floor level
The last step to installing firestop in your basement is to foam any penetrations with fire stop spray foam insulation to seal off any voids around wires and pipes through your top plates as shown in these pictures:
Following these steps will help you meet firestop code requirements, pass your inspections, and give you peace of mind that you’ve taken approved measures to prevent a potential fire from spreading.