2009 IBC; rigid insulation thermal barrier

2009 IBC.  Rigid roof insulation sheets, reflective faced.  Section 719 describes the requirements for insulation.  In 719.1 there are four exceptions.  Exception 2 requires foam plastic insulation to comply with Chapter 26.  Exception 4 requires "All layers of single and multilayer reflective plastic core insulation shall comply with Section 2613."


Rigid insulation is a plastic insulation.  So must it meet Chapter 26 requirements?  Or, if it has a reflective membrane, is it ONLY required to meet Section 2613?  


the intent of the question is to determine if reflective faced rigid roof insulation is required to have a thermal barrier on the exposed to room side.  


thanks for your input!

Original Post

1. Not all rigid insulation is plastic insulation. Mineral-wool and glass-fiber also are available in rigid forms.  Also, cellular glass is a rigid insulation.


2. The IBC defines reflective plastic core insulation (although the actual definition includes the word "foil," too).  This type of insulation is not the same as foam plastic insulation.  Reflective plastic core foil insulation involves plastic formed into bubbles (like bubble wrap) with a layer of reflective foil applied on one or both sides.  The insulation cannot be more than 1/2" thick.


Therefore, foam plastic insulation (i.e. polyisocyanurate, EPS, XPS, etc.) used above the roof deck for roofing insulation, still must comply with Chapter 26. Section 2603.4.1.5 does not require a thermal barrier for foam insulation that is part of a Class A, B, or C roof assembly, provided the roof assembly, with the foam insulation, is tested per FM 4450 or UL 1256.

thanks riga for the response.

1. agreed.

2. i can't find that definition.  i see "foam plastic insulation", but not the other.


sorry, my bad.  the rigid insulation would be installed below the roof slab above.  not on top.

assuming it is a plastic foam rigid insulation, then why not ONLY comply with 2613, instead of the whole chapter?  otherwise, what value does 2613 have beyond the other chapter requirements?



sorry hil, you lost me.


if the rigid insulation material is plastic (per earlier conversations above), then exception 2 would take it OUT of chapter 7 and into chapter 26.  AND all the requirements for plastic, whether blown or in pre-manufactured boards would apply per that chapter.

there is no code distinction i can find between roof insulation or building insulation per the plastic material.


so clearly i'm missing something.  maybe a little more help?


No, Exception 4 only applies to reflective insulation (also called radiant barriers).  See http://www.radiantbarrier.com/...-foil-insulation.htm for an example.


Rigid insulation that is fabricated of foam plastic must comply with Chapter 26.  And, as hil mentioned, if used below the deck, the insulation is not really insulation that is part of a roofing assembly, but standard thermal insulation used just like insulation in a wall and must have a thermal barrier.


BTW, it is "rlga" as in "RLGA."

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