R2 Draftstops - 2018 IBC

[KHL]

For some reason I've found myself confused about the requirements for draftstopping in R2, 2018 IBC. Hoping someone can help me clarify the intent.

The building in question is VB, R2 (8 units), 2 story, 36ft in height, 24" open web floor trusses, pitched roof trusses.
Fully sprinkled, NFPA 13, including the attic, but not including the floor/ceiling cavity.

Floor/ceiling horiz assembly = 1/2 hr (711.2.4, exception) (dividing dwelling units)
Fire Partitions dividing dwelling units = 1/2 hr (per 420.2, 1/2 hr per 708.3, exception 2)
Fire Partitions at Corridors = 1/2 hr (fire partitions per 420.2, 1/2 hr per 708.3, exception 1)

Question is if draftstops are required in the attic and/or floor/ceiling space if the attic is sprinkled, but floor/ceiling is not?

For the attic (which is sprinkled), 708.4.2=
Exception 1 = no draftstopping required with sprinklers
Exception 4 = draftstops required because it is R2/etc
718.4 requires draftstops as required in 708.4.2, but an exception for being sprinkled.
Conclusion: sprinkled attic does not need draftstops.

For the floor/ceiling (not sprinkled), 708.4.2=
Exception 2 = draftstopping only required above one of the corridor walls
718.3 says draftstopping required per 708.4.2 - so above corridor walls
Conclusion: draftstopping required above corridor walls
(note if this was not R2, the floor/ceiling would be required to be subdivided into 1,000sf max areas)


708.4.2 Fireblocks and draftstops in combustible construction.
In combustible construction where fire partitions do not extend to the underside of the floor or roof sheathing, deck or slab above, the space above and along the line of the fire partition shall be provided with one of the following:

1. Fireblocking up to the underside of the floor or roof sheathing, deck or slab above using materials complying iwth Section 718.2.1.
2. Draftstopping up to the underside of the floor or roof sheathing, deck or slab above using materials complying with section 718.3.1 for floors or Section 718.4.1 for attics.

Exceptions:

1. Buildings equipped with an automatic sprinkler system installed throughout in accordance with Section 903.3.1.1, or in accordance with Section 903.3.1.2 provided that protection is provided in the space between the top of the fire partition and underside of the floor or roof sheathing, deck or slab above as required for systems complying with Section 903.3.1.1.
2. Where corridor walls provide a sleeping unit or dwelling unit separation, draftstopping shall only be required above one of the corridor walls.
3. In Group R2 occupancies with fewer than four dwelling units, fireblocking and draftstopping shall shall not be required.
4. In Group R2 occupancies up to and including four stories in height in buildings not exceeding 60 ft in height above grade plane, the attic space shall be subdivided by draftstops into areas not exceeding 3,000 square feet or above every two dwelling units, whichever is smaller.
5. In Group R3 occupancies with fewer than three dwelling units, fireblocking and draftstopping shall not be required in floor assemblies.

718.3 Draftstopping in floors. Draftstopping shall be installed to subdivide floor/ceiling assemblies where required by Section 708.4.2. In other than Group R occupancies, draftstopping shall be installed to subdivide combustible floor/ceiling assemblies so that the horizontal floor areas do not exceed 1,000 square feet.
Exception: Buildings equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1.

718.4 Draftstopping in attics. Draftstopping shall be installed to subdivide attic spaces where required by Section 708.4.2. In other than Group R, draftstopping shall be installed to subdivide combustible attic spaces and combustible concealed roof spaces such that any horizontal area does not exceed 3,000 square feet. Ventilation of concealed roof spaces shall be maintained in accordance with 1202.2.1.
Exception: Buildings equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1.

Sorry for the long post...hopefully I included enough info for feedback.
Thanks in advance!

 

[steveray]

I would like to see how they are getting out of sprinklering the floor system in a "full 13" building.....

 

[KHL]

There is a desire to not have sprinklers in the inaccessible/concealed floor/ceiling cavity.
So the thought was: NFPA 13, 8.15.1.2.7 Concealed spaces filled with noncombustible insulation shall not require sprinkler protection.

 

[steveray]

Watch out for that as adding insulation may change the rating of the floor/ceiling assembly....From UL....

17. Blanket Insulation

Unless specifically described in the individual designs, the addition of insulation in the concealed space between the ceiling membrane and the floor or roof structure may reduce the hourly rating of an assembly by causing premature disruption of the ceiling membrane and/or higher temperatures on structural components under fire-exposure conditions.

Insulation in G500, L500, M500 and P500 Series Designs — For 1-hour-rated G500, L500, M500 and P500 Series assemblies, fiberglass insulation, either loose-fill, or faced or unfaced batts or blankets may be added to the plenum or joist space above the gypsum board, provided an additional layer of gypsum board is added to the assembly. The gypsum board should be of the same type as shown in the individual designs. The base layer of gypsum board should be attached with the fastener type and spacing as described in the design. It is not necessary to tape the joints of the base layer. The finish layer of gypsum board should also be attached with the fastener type and spacing as described in the individual design. The length of the fasteners should be increased by a minimum of the gypsum board thickness of the additional layer. The joints in the finish layer should be finished as described in the design.

Other methods of adding insulation in the plenum or joist cavity are not permitted unless indicated in the individual designs.

 

[KHL]

Currently the plan was to use UL L521. Allows for an unlimited thickness of insulation in the cavity if resilient channels are spaced at 12"o.c. at the ceiling gyp. There are a few options within the assembly that accommodate unlimited thickness to the insulation without having to add the second layer of gyp. May depend on the choice of batts over blown in fiber.

 

[steveray]

Mod to 2021 IBC?

718.3 Draftstopping in floors. Draftstopping shall be
installed to subdivide floor/ceiling assemblies where
required by Section 708.4.2. In other than Group R occupancies,
draftstopping shall be installed to subdivide
combustible floor/ceiling assemblies so that horizontal floor
areas do not exceed 1,000 square feet (93 m2).
Exception: Buildings equipped throughout with an automatic
sprinkler system in accordance with Section
903.3.1.1.

Don't need it at all...in 13 building

EDIT....i see what you are saying about the dwelling unit separations....in 708

 

[KHL]

So reverse this a bit, and say "FULLY" 13 sprinkled, so including the floor/ceiling cavity - basically just take the unprotected cavity out of the equation.
You still have 708.4.2, exception 4 and 718.3 exception contradicting each other.

708.4.2 Fireblocks and draftstops in combustible construction.
Draftstopping up to the underside of the floor or roof sheathing, deck or slab above using materials complying with section 718.3.1 for floors or Section 718.4.1 for attics.
Exception 4: In Group R2 occupancies up to and including four stories in height in buildings not exceeding 60 ft in height above grade plane, the attic space shall be subdivided by draftstops into areas not exceeding 3,000 square feet or above every two dwelling units, whichever is smaller.

VERSUS

718.3 Draftstopping in floors.
Draftstopping shall be installed to subdivide floor/ceiling assemblies where required by Section 708.4.2. In other than Group R occupancies, draftstopping shall be installed to subdivide combustible floor/ceiling assemblies so that the horizontal floor areas do not exceed 1,000 square feet.
Exception: Buildings equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1.


Would it be reasonable to think that the 718.3 Exception is exempting the draftstopping in R2?

 

[Sifu]

Mod to 2021 IBC?

718.3 Draftstopping in floors. Draftstopping shall be
installed to subdivide floor/ceiling assemblies where
required by Section 708.4.2. In other than Group R occupancies,
draftstopping shall be installed to subdivide
combustible floor/ceiling assemblies so that horizontal floor
areas do not exceed 1,000 square feet (93 m2).
Exception: Buildings equipped throughout with an automatic
sprinkler system in accordance with Section
903.3.1.1.

Don't need it at all...in 13 building

EDIT....i see what you are saying about the dwelling unit separations....in 708

I have a proposal right now concerning this issue. A hotel really had their hopes pinned on using a 13R system, but it was not permitted. Their new strategy is to use a 13 system but utilize the cavity fill option to avoid sprinklers in the floor systems. All fine if the assembly permits it. The interesting thing is that their assembly is L511, a two hour, to layer assembly, without insulation. The general information for BXUV assemblies allows insulation in L series one hour assemblies if an additional layer of gyp is added. From a quick check of UL they could choose an L501, 1-hr, one layer assembly and add a layer of gyp, which would allow the insulation. Then the only difference I can see would be that the L511 allows resilient channels between layers of gyp, and the L501 does not specify them. Seems the same except for the RC, which they want for sound dampening. Not sure where this will go or how it will work, but I think it can be done one way or another.

 

[redeyedfly]

So reverse this a bit, and say "FULLY" 13 sprinkled, so including the floor/ceiling cavity - basically just take the unprotected cavity out of the equation.
You still have 708.4.2, exception 4 and 718.3 exception contradicting each other.

708.4.2 Fireblocks and draftstops in combustible construction.
Draftstopping up to the underside of the floor or roof sheathing, deck or slab above using materials complying with section 718.3.1 for floors or Section 718.4.1 for attics.
Exception 4: In Group R2 occupancies up to and including four stories in height in buildings not exceeding 60 ft in height above grade plane, the attic space shall be subdivided by draftstops into areas not exceeding 3,000 square feet or above every two dwelling units, whichever is smaller.

VERSUS

718.3 Draftstopping in floors.
Draftstopping shall be installed to subdivide floor/ceiling assemblies where required by Section 708.4.2. In other than Group R occupancies, draftstopping shall be installed to subdivide combustible floor/ceiling assemblies so that the horizontal floor areas do not exceed 1,000 square feet.
Exception: Buildings equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1.


Would it be reasonable to think that the 718.3 Exception is exempting the draftstopping in R2?

They aren't contradicting each other, they are two different requirements for different reasons. 708.4 is continuity of fire partitions. 718.3 is compartmentalizing concealed spaces. In both cases you can omit the draftstopping with sprinklers. See 708.4.2 Ex 1.

No one ever puts sprinklers in the floor cavity though. L521 with fully blown in insulation is the most typical wood floor truss assembly for R2.

 

[steveray]

Nope....no one...ever....

 

[redeyedfly]

Nope....no one...ever....

Exception that proves the rule

Definitely cheaper to fill with insulation and better sound performance too.

 

[KHL]

They aren't contradicting each other, they are two different requirements for different reasons. 708.4 is continuity of fire partitions. 718.3 is compartmentalizing concealed spaces. In both cases you can omit the draftstopping with sprinklers. See 708.4.2 Ex 1.

No one ever puts sprinklers in the floor cavity though. L521 with fully blown in insulation is the most typical wood floor truss assembly for R2.

Having 10,000+ sf of concealed space with sprinklers makes me pause, both for future maintenance issues, but also the coordination of plumbing, hvac, sprinkler, etc all vying for space in the floor truss cavity. We'd still have to have at least 3.5" of batt stapled to the underside of the floor deck for sound too.

When you've used blown-in insulation in the cavity did they just complete rough-ins, then net the bottom of the floor trusses?

I see what you are saying abut 708.4 vs 718.3. Pretty much 718.3 is saying "do what 708.4 says", unless you have sprinklers.
If we don't spinkle, then per 708.4 we'll have do per exception 3: In Group R2 occupancies up to and including four stories in height in buildings not exceeding 60 ft in height above grade plane, the attic space shall be subdivided by draftstops into areas not exceeding 3,000 square feet or above every two dwelling units, whichever is smaller.

 

[redeyedfly]

When you've used blown-in insulation in the cavity did they just complete rough-ins, then net the bottom of the floor trusses?

Yes, exactly. Goes in right before gypboard.

If you have an actual attic (not flat roof) then you will need sprinklers anyway with a full 13 (dry if you're in freezing climate) so you can skip the draftstopping. I only see the draftstopping in Type V that's small enough for a 13R.

 

[Builder Bob]

Remember that NFPA 13 requires the concealed space to be completely filled - Blown in insulation will not completely fill the void, batt insulation if stacked might if installed correctly - Spray foam full depth of truss or I joist perhabs- If open space or connectivity to other spaces is present, it does not meet the intent of NFPA 13 for concealed space filled with non combustible insulation.

 

[redeyedfly]

Remember that NFPA 13 requires the concealed space to be completely filled - Blown in insulation will not completely fill the void, batt insulation if stacked might if installed correctly

9.2.1.7.1: A maximum 2 in. (50mm) air gap at the top of the space shall be permitted.

Blown-in insul meets the code even when it settles.