Insulation and Title 24

Roof/Ceiling Insulation Title 24 Mandatory Measures

  • In new construction, an R-30 mandatory minimum level of insulation applies when using the performance method of compliance. Otherwise, the R-30 minimum is superseded by the prescriptive requirements, which call for either R-30 or R-38 depending on the climate zone.
  • Metal-framed and roof/ceiling constructions other than wood framed must have a U-factor of 0.031 or less in order to comply with the Title 24 mandatory measures.
  • If the insulation is not penetrated by framing, such as rigid insulation lain over a structural deck, then the rigid insulation can actually have a rated R-value of less than R-30 as long as the total roof/ceiling assembly has a u-factor not greater than .031.
  • If insulation is going to be installed in the attics of existing buildings and the performance method of compliance is used the mandatory minimum is R-30.  If the prescriptive method of compliance is used the insulation must be R-30 or R-38 depending on the climate zone. If an existing rafter roof is altered the insulation may be R-19 if the performance method of compliance is used.


Insulation in roof/ceiling constructions must be placed in direct contact with the infiltration barrier. In most cases the attic is ventilated and the infiltration barrier is the drywall ceiling; in this case, the insulation must lie directly on top of the ceiling.


Ceiling insulation should extend far enough to the outside walls to cover the bottom chord of the truss. However, insulation should not block eave vents in attics because if the flow of air is blocked, moisture may build up in the attic and water vapor may condense on the underside of the roof. This can cause structural damage and reduce the insulation’s effectiveness. Insulation may be tapered near the eave, but it must be applied at a rate to cover the entire ceiling at the specified level.


Loose fill insulation must be blown in evenly, and insulation levels can be verified by checking that the depth of insulation conforms to the manufacturer’s coverage chart for achieving the required R-value. The insulation must also meet the manufacturer’s specified minimum weight per sq.ft. for the corresponding R-value.

Wall Insulation Title 24 Mandatory Measures

  • The mandatory measures require that wood-framed walls above grade have at least R-13 insulation installed in the cavities between the framing members. The R-13 mandatory minimum level of wall insulation applies when using the performance method of compliance. Otherwise, the R-13 minimum is superseded by the prescriptive requirements. 
  • Wall constructions with insulation that is not penetrated by framing members, or with metal framing, comply with this mandatory measure if they have a U-factor lower than 0.102, which is the U-factor of a wood-framed wall with R-13.
  • The mandatory measures apply to framed foundation walls of heated basements or heated crawl spaces that are located above grade, but not to the portion that is located below grade.
  • Existing wood-framed walls of an addition that are already insulated with R-11 insulation need not comply with the mandatory R-13 wall insulation, but this exception applies only with the performance method.
  • Rim joists between the stories of a multi-story building are deemed to comply with the mandatory measures if they have R-13 insulation installed on the inside of the rim joist and carefully fitted between the joists.

Batt insulation should fill the wall cavity evenly. If Kraft or foil-faced insulation is used, it should be installed per manufacturer recommendations to minimize air leakage and avoid sagging of the insulation. Wall insulation should extend into the perimeter floor joist (rim joist) cavities along the same plane as the wall. If a vapor barrier is required, it must be installed on the conditioned space side of the framing. Because it is difficult to inspect wall insulation behind tub/shower enclosures after the enclosures are installed, insulation of these wall sections should be inspected during the framing inspection. 

Floor Insulation Title 24 Mandatory Measures

  • Wood-framed raised floors must have at least R-19 insulation installed between framing members, or the construction must have a U-factor of 0.037 or less. The R-19 mandatory minimum level of floor insulation applies when using the performance or prescriptive methods of compliance. The equivalent U-factor is based on R-19 insulation in a wood-framed floor and no crawlspace or buffer zone beneath the floor. 
  • Other types of raised floors, except for concrete raised floors, must also meet these maximum U-factors.
  • Raised slab floors with radiant heat must meet special insulation requirements that are described in the Title 24 residential compliance manual.
  • There is an exception to the mandatory measures for controlled ventilation crawlspaces. If all eligibility and installation criteria for a controlled ventilated crawlspace are met, raised floors above the controlled ventilation crawlspace need not meet the minimum insulation requirement.

Floor insulation should be installed in direct contact with the subfloor so that there is no air space between the insulation and the floor. Support is needed to prevent the insulation from falling, sagging, or deteriorating. Options for support include netting stapled to the underside of floor joists, insulation hangers running perpendicular to the joists, or other suitable means. Insulation hangers should be spaced at 18 inch or less prior to rolling out the insulation. Insulation hangers are heavy wires up to 48 inches long with pointed ends, which provide positive wood penetration. Netting or mesh should be nailed or stapled to the underside of the joists. Floor insulation should not cover foundation vents.


Slab Insulation

The Title 24 mandatory measures do not require slab insulation, but when the prescriptive requirements call for it, the mandatory measures require that the insulation material must be suitable for the application and meet the requirements. An example of an insulating material that meets the requirements is smooth-skin extruded polystyrene.


Slab insulation must be protected from physical and UV degradation by either installing a water-resistant protection board, extending sheet metal flashing below grade, choosing an insulation product that has a hard durable surface on one side, or by other suitable means. Slab edge insulation is mandatory with heated slabs, as required by the Title 24 energy standards. See the Title 24 residential compliance manual for details.


(Excerpted from the Title 24 Residential Compliance Manual)