A building's R-value refers to the capacity of the insulating material to resist the flow of heat. The higher that the number of the R-value is, the stronger the insulating performance of the material. In new construction projects, there is a minimum mandatory level of insulation that needs to be established in order to keep up with Title 24 insulation requires. This R-30 mandatory level applies when specific compliance methods are used for the project itself. For all other purposes, the R-30 minimum will be superseded by other requirements, which may call for either R-30 or R-38 standards, depending on the climate zone of the installation.
In order to keep up with the mandatory Title 24 requirements, metal-framed ceiling and roofing constructions need to have a U-factor of 0.031 or less. For most roofing systems, if the insulation has not been penetrated by the framing, then the rigid insulation can actually pass compliance testing with an R-value that is less than R-30. This is only acceptable if the U-factor is not greater than 0.031.
If the insulation is going to be established in existing buildings through the standard performance method of compliance, then the mandatory minimum will always be R-30. Otherwise, if a prescriptive method of compliance is used for the building's insulation, then it must be R-30 or R-38, depending on the local climate zone. Furthermore, if an already constructed rafter roof is altered with the performance method of compliance, then the insulation may have an R-value of R-19.
In roofing systems, the insulation must be placed in immediate and direct connect with the rest of the infiltration barrier. In the vast majority of cases, the attic will be ventilated according to the drywall ceiling infiltration barrier, with the insulation lying directly above the ceiling.
Care should always be taken to make sure that the ceiling's insulation extends as far as it can outside of the walls in order to cover the truss's bottom chord. Regardless of the installation, it is important for the insulation to not block the eave vents in the attic. If the flow of air is blocked in these locations, there may be moisture buildup, with water vapor condensing underneath the roof. This defeats the purpose of the Title 24 insulation compliance, while causing structural damage to the roof itself. When professionals follow the Title 24 requirements, they are guaranteeing that the building has a sufficient amount of insulation to be comfortable and energy efficient for residents.