Fireblocking and Draftstopping

Why this matters

Fire in a house spreads fastest where nobody can see it: up a stud bay, along a soffit over the kitchen cabinets, across a floor cavity between stories. Fireblocking and draftstopping are the code's answer, and they are mostly lumber, drywall and insulation that a framer installed without ceremony. A tech running a circuit from the basement to the attic drills through three of them in twenty minutes. Each hole you leave open turns a closed cavity back into a chimney, and in an ordinary single-family house this is the fire protection you are most likely to remove.

Three words that are not the same thing

The trade uses these interchangeably. The code does not, and the difference decides what material you are allowed to use.

• Fireblocking closes concealed paths, vertical and horizontal, so fire and hot gas cannot travel through hidden spaces from one floor to the next or from a wall into the attic. It is required in ordinary combustible construction whether or not anything is fire-rated.
• Draftstopping subdivides large horizontal concealed spaces, typically a floor-ceiling cavity, so fire cannot run across an entire floor inside it.
• Firestopping restores a fire-resistance-rated assembly at a penetration with a tested system. That belongs to rated walls and floors and is covered elsewhere: see related, How to Firestop a Penetration So the Rating Survives.

What fireblocking deliberately is not: it is not a rating. A fireblock has no hours attached and no test listing behind most of its materials. That is why a scrap of 2x lumber or a packed fiberglass batt qualifies, and why a fireblocking sealant does not make a hole in a rated separation acceptable. Using rated firestop in a fireblock location is fine and over-built. Using a fireblock product in a rated assembly is wrong.

Where the IRC requires fireblocking

IRC R302.11, as adopted by your jurisdiction (2018 and 2021 editions consulted), lists the locations. The ones a service tech crosses:

1. Stud walls, vertically at the ceiling and floor levels. In platform framing the top and bottom plates do this job. Every hole you bore through a plate is a hole in a fireblock.
2. Stud walls, horizontally at intervals not over 10 feet, which mostly matters in tall walls and in parallel or staggered stud walls.
3. Every interconnection between a concealed vertical space and a concealed horizontal one: soffits, dropped ceilings, cove ceilings. The soffit over upper cabinets is the classic, because it often opens straight into the stud bays behind it.
4. Between stair stringers at the top and bottom of the run.
5. Around vents, pipes, ducts, cables and wires at ceiling and floor level, with an approved material that resists the free passage of flame and products of combustion. The code says the material filling this annular space does not have to be noncombustible under ASTM E136.
6. Chimneys and fireplaces, sent to their own section (R1003.19), where the material is noncombustible. See related: Penetrations Near Flues and Chimneys.
7. Cornices of a two-family dwelling at the line of the unit separation.

Item 5 is the one this card is mostly about. It means the gap around whatever you ran through a plate or a ceiling has to be filled, not just the original plate left in place.

What counts as a fireblock

R302.11.1 names the materials:

• 2 inch nominal lumber, or two thicknesses of 1 inch nominal with broken lap joints;
• 23/32 inch wood structural panel or 3/4 inch particleboard, with joints backed by the same;
• 1/2 inch gypsum board;
• 1/4 inch cement-based millboard;
• batts or blankets of mineral wool or glass fiber, or other approved materials, installed so they are held securely in place;
• cellulose insulation installed as tested for that specific application.

Two conditions are attached to the insulation options. Unfaced fiberglass batt used as fireblocking must fill the entire cross section of the cavity to at least 16 inches high, packed tightly around pipes or conduit (R302.11.1.2). Loose-fill insulation is not a fireblock unless tested for that use. A wisp of fiberglass stuffed into a plate hole is neither.

For the annular gap around a cable or pipe, the usual materials are a sealant or one-part foam marketed for fireblocking (commonly colored orange or red so an inspector can see it), or mineral wool packed tight. "Approved" means approved by your building official, and many accept those products; some want to see the product's evaluation report. Ordinary gap-filling foam is a different product and is not a fireblock just because it fills the hole.

Fireblocking integrity is a requirement in its own right: R302.11.2 says it has to be maintained. That sentence is what makes restoring it after your work an obligation and not a courtesy.

Draftstopping, and when you meet it

IRC R302.12 applies when there is usable space both above and below a concealed floor-ceiling space. Then draftstops divide the concealed space so no section exceeds 1,000 square feet, in roughly equal areas. In a floor with a membrane on top and a ceiling below, it is required in two cases: the ceiling is suspended below the framing, or the floor framing is open-web or perforated, such as floor trusses. Solid joists already compartmentalize the cavity, which is why you rarely see draftstops in a joist floor.

Materials (R302.12.1): at least 1/2 inch gypsum board, 3/8 inch wood structural panel, or other approved material, adequately supported, installed parallel to the framing unless otherwise approved. The same integrity requirement applies. In practice it looks like a vertical sheet of drywall running down the middle of a truss floor, which an HVAC tech pulling a duct across the floor will want to cut.

Worked example: one circuit, one condensate line, one duct

Two-story house, platform framed, open-web floor trusses between the first and second floors, 2,400 square feet of floor. A crew is adding a ducted mini-split air handler in the attic with a new circuit from the basement panel, a condensate line down to a basement drain, and a 6 inch supply branch across the truss floor to a first-floor room.

The path, counted. Cable and condensate line run together from the basement up an interior partition, then to the attic. From bottom to top they cross: the first-floor bottom plate, the first-floor double top plate, the second-floor bottom plate, the second-floor double top plate into the attic. Four fireblock crossings under item 1, each one bored.

The holes. The crew bored 1-1/4 inch holes for a cable about 1/2 inch across sharing with a 3/4 inch condensate tube. The leftover gap at each plate is irregular and up to about 1/4 inch. At every plate they packed mineral wool into the gap and finished with fireblocking sealant, so the plate is closed except for the penetrants. Hole size against the plate width is a framing question, not a fire one; see related, Notching and Boring Limits in Wood Framing.

The soffit they nearly missed. The partition they chose runs behind a kitchen soffit, and the soffit is open to the stud bay. That interconnection should have been fireblocked at construction and was not: the bay rose straight into the soffit, which ran the length of the kitchen. Under item 3 the crew is not obliged to fix a builder's omission they did not touch, but they now knew the cable ran in a cavity connected horizontally to everything along that wall. They told the owner, wrote it up, and quoted closing it with 1/2 inch gypsum as a separate line. They did not quietly close it on the job ticket.

The draftstop. The truss floor carries two draftstops, dividing 2,400 square feet into three compartments of about 800 square feet each, under the 1,000 square foot limit. The 6 inch duct has to cross one. The crew cut a 7 inch hole in the draftstop gypsum, ran the duct, and closed the gap around the duct with fireblocking sealant on a strip of drywall cut to fit. They did not remove the panel, which is the shortcut that matters: take one draftstop out and two compartments merge into 1,600 square feet, which is over the limit.

At the attic. The last top-plate hole opens into the attic over blown insulation. That hole is both a fireblock and an air leak, so it was sealed once and serves both jobs. See related: How to Air Seal Penetrations Into the Attic.

What the record says. Four plate crossings, sealed with named materials; one draftstop opening, closed; one pre-existing open soffit, reported and quoted separately; photos at each.

What changes the answer

• Balloon framing. In older houses studs run continuously from sill to roof with no plate at the floors, so the stud bay is an open path from basement to attic. Your new cable did not create that path, but you may be the first person to see it. Report it; a fireblock added at the floor lines is normal remodel work.
• Rated assemblies. At a garage separation or a dwelling unit separation, the wall or ceiling is rated, and fireblock sealant does not restore it. See related: The Garage-to-House Fire Separation; Dwelling Unit Separations in Townhouses and Duplexes.
• Chimneys and factory-built flues. The fireblock there is noncombustible and keeps a clearance. Foam or ordinary sealant against a flue is a fire, not a fireblock.
• Plastic pipe and large ducts. A fireblock does not need an intumescent product, but a large gap around a duct needs a backer such as a drywall collar; sealant alone will not span it.

How to verify you got it right

Walk the route with the list above in hand and name each crossing out loud: plate, ceiling, soffit, draftstop. For each, confirm with eyes or a phone camera that the gap is filled with one of the named or approved materials, not left open behind insulation. Check that no draftstop panel was removed or left with an unclosed hole, and that the compartments on either side are still separated. Photograph each closed crossing before it disappears behind finish, and enter the crossings in the job record. See related: Restoring What You Cut Through: The Closing Checklist.

References

• IRC R302.11 (fireblocking), R302.11.1 and R302.11.2 (materials and integrity), R302.12 (draftstopping), as adopted by your jurisdiction; 2018 and 2021 editions consulted.
• IRC R1003.19 for chimney fireblocking, as adopted.
• Manufacturer evaluation reports for fireblocking sealants and foams, where your building official requires them.
• See related: Before You Cut: The Four Things a Hole Can Break; Notching and Boring Limits in Wood Framing; How to Air Seal Penetrations Into the Attic; Restoring What You Cut Through: The Closing Checklist.