sprinklernotes
Concealed Spaces and Using Insulation as the Ceiling or Roof
You asked the following question: “We have 24” deep joists at the ceiling/roof assembly. Instead of attaching the 9 inches of insulation to the bottom of the roof deck, it is being installed at the upper side of the bottom chord. This leaves a 15-inch gap as a concealed combustible space above the insulation (that is being sprinklered). The underside of the insulation is the ceiling for the occupied room beneath it. We are installing sprinklers beneath the insulation as if it were the ceiling based on 13:A.8.5.4.1 statement that it creates an effective thermal barrier and can be considered the ceiling when determining the distance between the deflector and the ceiling. Is this an acceptable installation?” In response to your question, we have reviewed NFPA 13, 2016 edition as the applicable standard. Our informal interpretation is the insulation is not an acceptable ceiling. Let's start with the text discussing insulation as a thermal boundary. The annex material that discusses treating it as the ceiling for measuring the distance to the deflector (as a side note, I submitted the proposal that made this change to the 1999 edition). This addresses the thermal characteristic whereby the heat will bank down from the bottom of the insulation thus why the measurement should be made from the insulation. This is really an issue about the activation of the sprinkler during the initial portion of the fire event. The annex material readily leads one to the interpretation that the insulation alone can constitute a ceiling. A more appropriate way to state the question is whether insulation is an acceptable boundary for separating two compartments. This issue is not explicitly addressed by NFPA 13 but it is understood that the material must be sufficiently robust so that it will remain in place throughout the fire event. There is sufficient guidance in the body of the standard telling us that the insulation in combination with the ceiling or roof assembly is treated as the ceiling. Section 8.5.4.1.3 states: For ceilings that have insulation installed directly against the underside of the ceiling or roof structure, the deflector distance shall be measured from the bottom of the insulation and shall be in accordance with 8.5.4.1.3.1 or 8.5.4.1.3.2. In stating “directly against the underside,” it limits the application of separation of compartments to more robust materials. This later text was added to the 2010 edition and in reality, the older annex material should have been removed at that time. There is another piece of criteria that contributes to this issue. Section 8.15.1.2.13 states: Concealed spaces below insulation that is laid directly on top of or within wood joists or composite wood joists used as ceiling joists in an otherwise sprinklered concealed space, with the ceiling attached directly to the bottom of the joists, shall not require sprinkler protection. As indicated by the first of the sentence, this comes out of the section on concealed combustible spaces. When you have effectively two concealed spaces, both above and below the insulation, you do not have to install sprinklers below the insulation. This is based on the premise that if the fire is below the insulation, the sprinklers above will still activate (insulation melts around 1,300 F) although there will be a delay in activation. Once they activate, the waterlogged insulation below the sprinklers will readily fall out of the way. This supports the position that insulation alone is not an acceptable means to separate two compartments. This does beg the question of can we extend this scheme to occupied rooms below the insulation (and no sprinklers beneath the insulation). I would discourage such an attempt. The current application is for a small space beneath the insulation that presents a different fire hazard than an occupied space and more importantly, one would have to justify why the unspecified amount of the delay in activation for a concealed space is acceptable for an occupied space. Needless to say, it would also have to be approved by the AHJ. In conclusion, it is not the intent of NFPA 13 to allow insulation alone to be treated as a ceiling and any such application would require the approval of the AHJ.
