How to Firestop Your Basement

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:

  1. to form a fire barrier between stories.
  2. to separate wood-framed vertical chases from horizontal chases including soffits, drop ceilings, and cove ceilings, etc.
  3. in walls, every 10 feet minimum.
  4. at openings around vents, pipes, ducts, cables, and wires at ceiling/floor level.
The next section of the code, IRC R602.8.1, provides a list of approved firestop materials including:
  • 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).

firestop foam

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:

foam wires at top plate with firestop foam

foam wire penetrations through top plates

foam around pipes at top plates

seal voids and penetrations

Conclusion

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.

31 Replies to How to Firestop Your Basement

  1. Don Hall says:

    Kurt,

    Thank you for the firestoping article – I now understand the requirements and can get on with my basement. One related question: what does one do to firestop around the jumble of electrical wires going in and out of the electric panel?

  2. Kevin says:

    Kurt – Great article on fire stopping!!!! I’m in the process of finishing my basement and this by far has been the best piece I’ve found on how to do it.

    I do have one question however. You recommend placing a piece of OSB tightly against the mud sill and mounts your wall header to that. But I have one section of my basement wall where it’ll be impossible to do that because I have a toilet drain that follows almost the entire length of the wall. It’s very similar to the 3rd picture you have with the black pipe which you notched out but mine runs the length of the wall before it comes down to the floor. It pretty much hugs the floor joists too so there’s not way to squeeze anything above it. I was planning on building my wall about 3.5″ from the poured basement wall so I can hide the pipe in the wall cavity. Any recommendations? I was thinking of building a two-piece wall…the first would go from the floor to just below the pipe and would be capped with a fire stop across the length of the wall directly UNDER the pipe. And then I’d build a very small, probably 8-10″ wall structure right above it. Would that work? Also, for the majority of the basement I plan on using metal studs if that makes a difference. I’m thinking wood might be better for this wall however. Ugh – so confused on this one darn wall. Thanks for your help.

    • Kevin, The pipe issue your having is a fairly common challenge, and your idea of running a wall from the floor to just below the pipe with firestop at the top of this wall would be a great approach. Sometimes, just a 2×4 for backing can then be used above the pipe that is nailed to the bottom of the floor joists. Most inspectors would then consider the pipe area a horizontal chase- so you wouldn’t need to separate it from the joists above. You can definitely use metal studs placing firestop in the same locations as you would with wood framing. (I used metal studs on some of my basement framing).

  3. Chris Werle says:

    Can you expand your photos and explanation to include how to properly fire block when installing a drop ceiling? Thanks, Chris

    • Out here in Utah dropped ceilings are uncommon. We sheetrock our basement ceilings, so I’m a little unfamiliar with dropped ceiling applications. However I believe that no matter what kind of ceiling you have, the firestop requirements would still need to be met in a similar manner.

  4. Chris Werle says:

    I used an incorrect term for this………should have said draft stopping. As I understand it approved material must be installed parallel to the floor joists above the suspended ceiling to block fire movement between the ceiling and floor above. I assume that means the entire length of the floor joist but only at 10 foot intervals along the length of the room……….is that correct? Thanks, Chris

    • I’ve heard the terms draft stop and fire stop used synonymously. I’m not sure how the code would apply for dropped ceilings. I would recommend calling your local building department to ask what they require.

  5. Jeff says:

    Requirement for a horizontal firestop every 10 feet.
    My basement wall is not straight and has a couple of bump outs and bump ins that are only perhaps 3-6 feet in length. My question is how is the 10 feet measured…a straight line from the front of the house to back or around inside and outside corners along the perimeter no matter how many jogs there happens to be? Thanks

  6. Kent says:

    Great article!! I have one question about the horizontal firestop every 10′. I am using 2″ foam board insulation glued to the concrete wall. Does the firestop that is attached to the studs need to go through the foam board all the way to the concrete, or just butt firmly against the foam? If it needs to contact the concrete wall directly, then shouldn’t it be treated lumber? Thanks in advance!

    • Great questions Kent- I would find out from your building department if they consider the foam board you have an approved firestop material. If so, you can just butt the 2×4 you’re using for firestop firmly against the foam. If not you’ll need to run it through the foam to the concrete. As a general rule of thumb, it is a good idea to use treated wood anywhere it’s in contact with concrete. Where I live, inspectors allow framers to use untreated lumber for firestop against the foundation wall. I’m not sure if it’s because it’s non-structural and just serving the purpose of preventing fire from spreading or some other reason. (Maybe it’s just that they haven’t realized it’s probably a better idea to use treated there- I’m actually not sure on that).

  7. David Clay Gonsalves says:

    Hi Kurt, Great write up. Too many people tackle projects and have no idea about fire stop.

    I have done a ton of ceiling drops and they are typically done exactly as you have shown in some of your photos of bulk heads/horizontal chases.

    Again, great job on the write up!

    Dave – Claygon Construction

  8. Jim Ellis says:

    Kurt, I have a kind of unique situation. The main beam in the 1950′s house (made up of 4 2×8′s) has the floor joists nailed directly to it, with a 1×1 sill beneath the joists for support. Wiring crossing from front to rear wraps down under the main beam. We built 2×4 walls about 4 inches off the main beam, and nailed into the joists above, but now there is that 4″ opening leading into the ceiling above the new room. We weren’t planning on finishing the ceiling but now believe that wallboarding it would be the only way to close it off from the mechanical room on the other side of the wall. would that work ? There are a lot of cables and heating pipes to work around otherwise…Or maybe insulation batts fitted around and between would be easier ?
    Thanks,
    Jim

  9. Max says:

    Kurt,

    Great article and photos. Question for you…. if the exterior walls are filled with fiberglass insulation, and the wallboard (drywall) covers the entire wall cavity, is the 3/4″ OSB firestop between the top plate and the floor joist necessary? Thanks for your help!

  10. Jessica says:

    Thank you for posting this article and photos. Does this requirement and materials vary by state?

    • The IRC or International Residential Code developed by the ICC (International Code Councel) has been adopted throughout most of the United States, so the code should be the same regardless of where you live, but I have found that sometimes the local jurisdictions interpret code a little differently from place to place. I would recommend asking any questions you have to the local building department as you apply for a permit for your project.

    • Steve says:

      Actually, inspections vary not just by state, but by county, and sometimes by municipality. But usually there is more agreement in a state than between states, and most everything goes back to national building codes and international standards, but sometimes with variations and especially variations in enforcement.

  11. Jessica says:

    Thank you. What would you do if you lived in a home that had a finished basement but the previous owners didn’t install the fireblocking? (The basement isn’t completely finished and you can see that it wasn’t done). We also checked with the county, no permits were pulled.

    • This is a great question Jessica- If you are buying a house, I think it’s a good idea to find out first if a permit was pulled when the basement was finished. If a fire were to start in the basement and burned down the house, you run the chance of having trouble with the insurance company not wanting to pay for the damages because the home owner didn’t fulfill their responsibility to firestop per code and have it inspected and approved by the building department. Even though it’s required by law to pull a permit when you finish your basement, it seems like there are a lot of people that don’t do it.

    • You could also talk to your building department about it. They may be able to issue you a permit to retrofit the firestop. The downside is you’ll most likely have to pull off or cut out sheetrock, and in general, it’s just more difficult to install after the fact.

      I was asked by a customer to finish a basement that they had already started. I told them even though the framing was “done” that I would need to bring a framer back to install the firestop to pass inspections. And then I remember instead of being able to do 3/4″ OSB at the top of basement perimeter walls we had to use blocks between the studs that extended back to the foundation at the top of the outside walls. Then we had to foam to voids left inbetween all the blocks. It was pretty tedious. It’s a lot easier to do it the first time.

    • Steve says:

      This might be worth a sit down with the building dept in your jurisdiction. But the bottom line is that in order to get it permitted, they are going to want you to meet code, so you may be faced with either living with unpermitted improvements or redoing them so that they meet code. I think if I knew the firestop wasn’t done, I might just plan on removing and replacing the drywall. That is actually not a super huge thing, just messy, and probably the drywall isn’t that well done either given what you know, and you’ll probably find other things that should be improved when you pull the drywall off. This also will make your inspections much more straightforward and get you off on an excellent footing with your inspectors.

  12. Steve says:

    I just read through this article and all the comments. All my questions were answered. Thank you!!

  13. Steve says:

    Non fireblocking related, is there anything else I need to know about framing with inspections? Until recently, I didn’t even know of the fireblock requirements.

    Thanks,
    Steve

    • As long as you’re not changing the existing framing, the stuff you’ll be doing in a basement finish is typically non-load-bearing and non-structural. You’re really just providing backing and support for the drywall and any finishes to be installed later. Be sure to provide proper backing for sheetrock- check all the corners of the ceiling and walls to make sure the edge of the sheetrock will have backing and something to secure the drywall to along the edges. Use treated lumber where your framing is in contact with concrete (bottom plates). Some building departments will be more picky about your pin & washer spacing for fastening the bottom plate to the concrete floor and some will require coated nails (that you can find at home improvement stores) that resist corrosion from the chemicals in the pressure-treated wood. Other than that, I would recommend chatting with the building department when you go in to apply for a permit to see if there’s anything else to keep in mind.

  14. Mark Hays says:

    Thanks Kurt for this great article on fireblocking — which helped me avoid a major inspection problem on a recent project.

    One question: you highlighted DOW “Fireblock” Great Stuff foam, which led to a discussion with our inspector. I was surprised to discover that Fireblock Great Stuff ignites at 240 degrees, and DOW does not recommend it as a “firestop”!! Here is a quote directly from the DOW website:

    “This product is defined by the International Building Codes as the use of approved building materials installed in concealed spaces to resist the migration of fire and hot gases. GREAT STUFF™ FireBlock is tinted orange to be more recognizable to building code officials. This product is not approved for use in firestop systems.”

    Note that the third sentence contradicts the first two, and the label on the can. (Source: http://greatstuff.dow.com/products/fireblock/ See the “reviews” option.)

    I contacted DOW and asked them for clarification, with no luck. Maybe you can convince DOW to reply.

    Based on what I learned about Great Stuff, it would be a good idea to recommend another option. There are a number of good fireproof caulk products that are also easy to apply, e.g. 3M Firebarrier and DAP Fire Stop caulk, which are both available at Home Depot and Lowes. If you read their MSDS data sheets, you will see that both are nonflammable and 4-hour fire rated — unlike “Great Stuff”!

    Thanks for all of your helpful advice and info!

    Mark

    • Thanks for your comment Mark! I was recently made aware of firestop caulk, and have seen it at home improvement stores, and was told it was required by some building departments. In the areas I’ve worked, the inspectors approve/require the spray foam. There may be other areas out there that want to see the firestop caulking instead. It probably would be good to check with the local building department on that to see what they require. But from your research it sounds like the caulking is better than the spray foam- interesting.

  15. Steve says:

    There are different levels of requirement here in Colorado for residential and commercial. Residential construction is approved using the foam, but commercial requires the caulk. Obviously the caulk is going to end up covering less and costing more per area, but it is definitely “beefier”.

    Also note that there is a technical difference between “firestop” and “fireblock”. The idea of firestop is to not allow any air to flow through. The idea of fireblock is the “2 hour burn.” So you make your barriers with materials that will not burn through faster, but you seal any air passage so that the air can’t fuel fire between sections.

    • Mark Hays says:

      Thanks Steve for your reply and info. As you noted, there is a key difference in the IRC between a “fire block” and “fire stop”. This highlights the confusion behind the DOW Great Stuff Fireblock product. If you believe the name on the can, this should qualify as a “fireblock” — which it definitely does not. In fact, it will ignite and burn faster than the wood studs. Plus, according to the post from a DOW tech rep, “This product is not approved for use in firestop systems.” That’s right — it doesn’t work as a basic “firestop” either!! OK, so what should it be used for? I asked DOW this question and received no reply.

      Frankly, I am surprised that this flammable foam is approved by any inspector. They probably read the name on the can and the directions on the back — not the MSDS data sheet. No more orange foam for me; the 3M and DAP products are clearly superior fireblock solutions for residential and commercial applications.

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