Mechanical insulation
Rated assemblies pass or they do not.
Grease duct wrap and firestopped penetrations are inspected against a specific tested system, by number. Install something that merely looks similar and the inspector fails it, usually after the ceiling is closed.
If you are reading about the IMC, you are reading about another state
Start here, because it is the single most common mistake on this subject. Most grease duct guidance online cites the International Mechanical Code. California does not adopt the IMC. California adopts the California Mechanical Code, which is based on the Uniform Mechanical Code, and the section numbers are different.
That is not a technicality. A submittal built on IMC section numbers gets questioned, a detail drawn to an IMC requirement may not satisfy the CMC, and an inspector in an East Bay jurisdiction is reading the CMC. The sections below are CMC sections.
Clearance to combustibles
CMC 507.4 sets the base clearances where no enclosure is required: 18 inches to combustible material, 3 inches to limited-combustible, and zero to noncombustible. CMC 510.2 applies those same provisions to grease ducts.
From there, the code gives you three ways to reduce the 18 inches, and the detail matters:
- CMC 507.4.2. Sheet metal of 0.013 inch, spaced out 1 inch on noncombustible spacers, brings it to 9 inches.
- CMC 507.4.2.1. Sheet metal of 0.027 inch over 1 inch of mineral wool or ceramic fiber, spaced 1 inch, brings it to 3 inches.
- CMC 507.4.2.3. Zero clearance to limited-combustible material where protected by specified assemblies such as metal lath and plaster, or ceramic tile.
There is a fourth and it is the one that matters for wrap. CMC 507.4.2.2 recognizes a listed field-applied grease duct enclosure tested to ASTM E2336, which then follows the clearances in its own listing rather than the table. That is the code path a duct wrap system takes.
One point people get backwards constantly: CMC 507.4.3.3 says the protection is applied to the combustible construction, not to the duct. If the detail protects the wrong surface, it does not comply no matter how well it is built.
When an enclosure is required
CMC 510.7 requires a continuous enclosure in interior installations, running from the lowest fire-rated ceiling or floor above the hood, through every concealed space, to or through the roof. It has to be sealed around the duct at the first fire-rated barrier it penetrates after the hood, and it has to be vented to the exterior through weather-protected openings.
The rating comes from building height. CMC 510.7.1 and 510.7.2: not less than one hour in buildings under four stories, not less than two hours in buildings of four stories or more.
Inside the enclosure, CMC 510.7.3 requires at least 18 inches from the duct or fan to a combustible enclosure, and at least 6 inches to a noncombustible or limited-combustible one. Critically, it states that the reductions in 507.4 through 507.4.3.3 do not apply to enclosures, though a listed field-applied or factory-built enclosure may reduce that clearance per its own listing.
A conflict worth raising before you build it. CMC 507.4.8 refers enclosure-interior clearances back to 507.4 through 507.4.3.3, while 510.7.3 states that those same reductions do not apply to enclosures. Those two readings do not sit comfortably together, and we are not going to pretend otherwise on a web page.
What we do is raise it with the authority having jurisdiction before fabrication, in writing, and build to the answer. An AHJ asked a clear question in advance will give you one. The same question asked at inspection, with the ceiling closed, costs money.
The rest of 510 that catches projects out
CMC 510.1.8 requires Type I exhaust systems to be independent of other exhaust systems. Combining a grease duct with general exhaust to save a shaft is not available.
CMC 510.7.8 limits each duct system to hoods in one fire zone on one floor, and allows multiple ducts to share an enclosure only where the AHJ accepts it.
CMC 510.7.5.1 requires the enclosure to be protected from physical damage, and any damage repaired so it still meets its listing and rating. This is the one that affects you after handover: a wrap system scuffed open by a ladder is no longer a rated assembly.
CMC 510.7.5.2 is the provision most owners have never heard of. After a kitchen exhaust fire, qualified personnel must inspect the duct, the enclosure and the covering. A fire that was put out quickly and did no visible damage still triggers an inspection obligation.
Throughout, the CMC points at NFPA 96 for ventilation control and fire protection of commercial cooking operations, and at NFPA 80 for fire doors.
Firestopping, which is a different discipline
Firestopping restores the rating of a wall or floor where something passes through it. It is not fireproofing, which protects structural steel, and the two get confused constantly.
The thing to understand is that a firestop is a tested system, not a product. A listed system specifies the wall type, the penetrant, the annular space, the material and the depth, together, and it is identified by number. A sealant installed in a configuration nobody tested is not a firestop, however much of it goes in.
Through-penetrations are tested to ASTM E814 and its UL counterpart, UL 1479. Joints, including head-of-wall conditions, are tested to ASTM E1966 or UL 2079, and perimeter fire barriers at the slab edge to ASTM E2307. Those are not interchangeable. A product tested only for joints is not valid in a through-penetration, which is a real and common field error.
Ratings come in four kinds. F is how long the system stops fire passing through. T is how long it also limits heat transfer to the unexposed side. W is water resistance. L is air and smoke leakage. Whether you need a T rating is a design decision, not a product decision, and it needs settling before anybody buys material.
For inspection, ASTM E2174 is the on-site practice for installed penetration firestops and ASTM E2393 covers joints and perimeter barriers. ASTM E3157 is the guide written for trades who install firestopping only part of the time, and it is explicit that responsibility for firestopping belongs in the specifications and the contract rather than being assumed.
Where the two scopes meet
An insulated chilled water line through a rated wall is the case that exposes a split scope. The firestop system has to be one tested with that insulation in place, because insulation changes how the assembly behaves. The insulation contractor leaves the pipe insulated to the wall, the firestop contractor finds the tested system does not match what is there, and the resolution happens by RFI while the wall stays open.
We hold the insulation classification and we install the firestopping, which means that handoff is internal. There is no second company to wait for and no argument about who changed the condition.
Scope
- Grease duct wrap
- Listed enclosure systems
- Shaft alternatives
- Clearance details
- Pipe penetrations
- Duct penetrations
- Cable and conduit
- Head-of-wall joints
- Insulated penetrations
- Documentation
- Repair of damaged systems
A note on wrap dimensions. Layer counts, overlaps, offsets, banding and pinning schedules come from the specific product listing and evaluation report, not from the code. Anyone quoting you a universal number for those is quoting a product they have decided on without telling you.
California Mechanical Code
The sections this work is inspected against.
| Section | What it covers |
|---|---|
| CMC 507.4 | Base clearances to combustibles: 18 in combustible, 3 in limited-combustible, 0 in noncombustible |
| CMC 507.4.1 | Listed products may use the clearances in their listing |
| CMC 507.4.2 | Reduction to 9 in using 0.013 in sheet metal on 1 in noncombustible spacers |
| CMC 507.4.2.1 | Reduction to 3 in using 0.027 in sheet over 1 in mineral wool or ceramic fiber |
| CMC 507.4.2.2 | Listed field-applied enclosure tested to ASTM E2336 follows its listing |
| CMC 507.4.3.3 | Protection is applied to the combustible construction, not to the duct |
| CMC 510.1.8 | Type I exhaust systems shall be independent of other exhaust systems |
| CMC 510.2 | Applies the 507.4 clearance provisions to grease ducts |
| CMC 510.7 | Continuous enclosure, sealed at the first rated barrier, vented to the exterior |
| CMC 510.7.1 / 510.7.2 | Not less than 1 hour under four stories, 2 hours at four stories or more |
| CMC 510.7.3 | 18 in to combustible enclosure, 6 in to noncombustible; 507.4 reductions do not apply |
| CMC 510.7.4 | Structural integrity and stability of the enclosure under fire and movement |
| CMC 510.7.5.1 | Protection from physical damage and repair to maintain the listing |
| CMC 510.7.5.2 | Inspection by qualified personnel after a kitchen exhaust fire |
| CMC 510.7.8 | One fire zone, one floor; shared enclosures only with AHJ acceptance |
| CMC 511.1.5 | Fan clearances |
| ASTM E2336 / UL 2221 | The two test standards for fire resistive grease duct enclosure systems |
Section numbers move between code editions. We confirm every citation against the edition your jurisdiction has adopted before it goes on a submittal, and we would encourage your engineer to do the same with anything quoted from a web page, including this one.
Questions we get asked
Rated work, answered.
Can I use duct wrap instead of building a shaft?
In most cases yes, and it is usually cheaper and takes less floor area. CMC 507.4.2.2 recognizes a listed field-applied enclosure tested to ASTM E2336, which is what a wrap system is, and it then follows the clearances in its own listing.
The catch is that it has to be installed exactly as the listing describes, with the manufacturer's instructions available on site. A wrap installed to a detail somebody remembered is not a listed assembly.
Does the grease duct enclosure really have to be vented?
Yes. CMC 510.7 requires the enclosure to be vented to the exterior through weather-protected openings. It is a frequent omission because it is easy to read the continuity requirement and miss the venting sentence in the same section.
It is also expensive to discover late, because the opening has to go through the exterior envelope.
Does my enclosure need one hour or two?
It follows building height. CMC 510.7.1 and 510.7.2 require not less than one hour in buildings under four stories, and not less than two hours at four stories and above.
Note that it is the building, not the floor the kitchen is on. A ground floor restaurant in a six story building is in the two hour category.
We had a small hood fire and put it out. Does anything need to happen?
Yes. CMC 510.7.5.2 requires qualified personnel to inspect the duct, the enclosure and the covering after a kitchen exhaust fire. There is no exception for a fire that looked minor.
The reason is that a wrap or enclosure can be compromised without obvious external damage, and the next fire is the one that finds out. It is a short inspection and worth doing properly.
Can I firestop around an insulated pipe the same way as a bare one?
No. The tested system has to be one that includes the insulation, because the insulation changes how the assembly behaves in a fire. Some systems require the insulation to be interrupted at the penetration and others require it continuous, and the system number tells you which.
This is the most common firestop error we find on mechanical work, and it is usually a scope split rather than a knowledge gap. The insulator and the firestop contractor each assumed the other had accounted for it.
Who is responsible for firestopping on a project?
Whoever the contract documents say, which is exactly the problem, because they frequently do not say. ASTM E3157 addresses this directly and is clear that responsibility belongs in the specifications and the contract rather than being inferred.
Practically, every trade that penetrates a rated assembly has a stake, and penetrations get made by mechanical, electrical, plumbing and low voltage. The cleanest arrangement is a single firestop scope held by one contractor, with the other trades required to leave penetrations accessible until it is done.
Will firestopping be inspected separately?
Often, and increasingly. ASTM E2174 is the on-site inspection practice for installed penetration firestops and ASTM E2393 covers joints and perimeter barriers. Where special inspection is required, the inspector is checking installed work against the specific listed systems in the submittal.
That means the documentation has to exist and has to match what was built. We photograph and log penetrations as they are completed, because reconstructing which system went where after the fact is unpleasant for everybody.
Related
Other scopes we self-perform.
Next step
Bring us in before the ceiling closes.
Rated work is far cheaper to get right the first time than to open up and redo. Send the plans and we will tell you which listed systems the details are calling for.