Insulation R-Values & Title 24: Batt, Foam, Mineral Wool or Blown-In

What Title 24 actually asks of you

Insulation is the one building material sold entirely on a number, and the number gets misread constantly. R-value is resistance to heat flow, it is additive through an assembly, and it is only worth what the installation delivers — a batt compressed around a wire loses value the label never mentions. In California the number also has to satisfy Title 24, which is not a national rule of thumb but a climate-zone-specific energy code with paperwork attached. This is the hub guide: what R-value actually measures, which material belongs where, and how the choice connects to the compliance documents on your job.

Title 24, Part 6 is California's Building Energy Efficiency Standards, and it does not hand you one R-value for the whole state. Requirements are set by climate zone, and Los Angeles County alone spans several — coastal Santa Monica, the inland San Fernando Valley, and the mountain zones above the basin have different targets. The correct R-value for your project is the one on your energy compliance documents, not the one a supplier quotes from memory.

There are also two ways to comply. The prescriptive path gives you a fixed table of minimums to hit assembly by assembly. The performance path models the whole building, which lets a designer trade a weaker wall against better windows or a tighter duct system. Which path your project took determines whether swapping R-15 for R-13 on site is a paperwork problem or a non-event, and that is a question for whoever signed the compliance forms — not a field decision.

Two practical consequences. First, buy from the compliance documents, not from habit. Second, keep the labels: inspectors verify installed R-value against what was modeled, and a batt with the facing already stapled out of sight is hard to prove after the fact.

R-value per inch, and why depth is the real constraint

Most walls do not fail on material choice; they fail on cavity depth. A 2x4 wall gives you about 3-1/2 inches. No batt turns that into R-21 — you either go to a 2x6 cavity, add continuous exterior board, or accept what the depth allows. Comparing materials by R per inch makes the trade-off visible before you order.

Those figures come straight off the products in our Insulation collection — the 1 in. EPS board rates R-3.85, the 1 in. XPS board rates R-5, and the 2 in. polyiso board rates R-13.1. Read the label, not the category.

Fiberglass batts: the cavity default

Fiberglass is what fills most stud bays in Southern California, and the reason is fit. Batts are sized to standard framing — the R-13 kraft-faced batt at 15 in. wide by 93 in. long drops into a 2x4 wall on 16 in. centers with no cutting, and the R-19 batt at 23 in. wide covers 24 in. centers. Rolls like the R-30 unfaced roll suit long attic runs where cutting to joist length wastes material.

Faced or unfaced is the question people get wrong. The kraft facing is a vapor retarder and a stapling flange; it belongs toward the conditioned space and it is combustible, so it should not be left exposed in an attic or garage. Unfaced batts are the right call over an existing layer, in interior sound walls, and anywhere a separate vapor control layer is already in the assembly.

Installation quality is where fiberglass earns or loses its rating. Split the batt around wiring rather than crushing it, cut to fit rather than stuffing, and fill the thin gap behind a pipe instead of leaving it. Our guide to choosing fiberglass insulation goes deeper on grades and handling.

Stone wool: when fire and sound are also on the list

Stone wool buys you more than R per inch. It is non-combustible and water-repellent, and it is dense enough to make a real difference in a party wall or a bedroom wall against a street. The R-15 ComfortBatt at 3-1/2 in. thick is sized slightly oversized for a 2x4 cavity on 16 in. centers so it friction-fits without staples or facing — which is exactly why it holds a consistent R-value with no slumping. For 2x6 walls and ceilings, the R-23 batt at 5-1/2 in. and the R-30 batt at 7-1/4 in. cover the deeper cavities.

In Los Angeles the fire performance is not an abstraction. If your parcel sits in a Very High Fire Hazard Severity Zone or a WUI area, the assembly requirements in our fire-hardening guide for LA property govern the exterior, and non-combustible cavity insulation is a sensible companion choice. For soundproofing detail, see our guide to mineral wool insulation.

Rigid foam board: continuous insulation that stops thermal bridging

Every stud in a wall is a small heat leak. Wood frames at roughly R-1 per inch and steel is far worse, so a wall of R-15 batts framed at 16 in. centers performs meaningfully below R-15 across its whole area. Continuous exterior board is the fix, because it runs unbroken over the framing.

The three board types divide by job. EPS is the general-purpose, most vapor-open of the three. XPS — like the R-10 2 in. FOAMULAR NGX board — resists moisture well and is the usual pick for below-grade and high-moisture locations. Polyiso gives the highest R per inch and suits roof decks and above-grade sheathing, but it does not belong in ground contact.

Two rules that matter more than the R number. Foam boards need a code-required thermal barrier when they face an occupied space, so drywall or an approved covering is part of the assembly, not an option. And continuous insulation changes where the dew point sits in your wall — the board, the housewrap and the vapor retarder have to be planned as one system. Our guides to foam board insulation for Title 24 and housewrap applications cover that stack. Steel-framed walls need this treatment more than wood ones do, as our steel studs vs wood studs comparison explains.

Blown-in and the retrofit problem

Blown-in loose fill — cellulose or fiberglass — is the practical answer for an attic that already exists and for closed wall cavities you would otherwise have to open. It is installed with a machine rather than bought by the batt, so it is normally a contracted scope rather than a materials order, and settled depth rather than bag count is what determines the delivered R-value.

For LA attic retrofits the honest sequencing is: air-seal first, insulate second. Top plates, can lights, plumbing penetrations and the attic hatch move more heat through leakage than a thin layer of insulation does through conduction. Adding depth over an unsealed ceiling plane buys less than the label suggests. Where an attic is being reroofed anyway, a radiant barrier deck attacks the same problem from the other direction — it is a summer heat-gain measure, and in a Valley attic that is the load that actually hurts.

Denim and the occupied-remodel case

Recycled cotton batts sit in a narrow but real niche. The R-13 UltraTouch denim batt is made from recycled denim, fits standard stud and joist spacing, absorbs sound well, and — the part that decides it on a lot of jobs — is soft and non-irritating, so it installs without the itch and the protective gear a fiberglass day requires. On an occupied remodel where the client is home and the work is happening down the hall, that is worth something. It is not a wet-location material.

Buying and installing for the inspection

Order by assembly, not by pallet. Walls, ceilings, floors over crawlspaces and knee walls often carry different requirements on the same set of plans, and a single R-value ordered for the whole house is how a job ends up short in the attic and over-specified in the walls.

Then protect the number you bought. Insulation is verified by grade of installation as well as by R-value: cavities filled to the edges, no compression behind wiring or plumbing, facing installed in the right direction, and no gaps at the top and bottom plates. A gap of a few percent of the wall area costs more performance than most people would guess, because heat takes the easy route.

We stock batts, rolls, stone wool and rigid board across the Insulation collection and deliver across Los Angeles County. Send us the insulation schedule from your plans or your compliance forms and we will build the order to match it, assembly by assembly.

Frequently asked questions

What R-value does Title 24 require in Los Angeles?

There is no single answer, because requirements vary by climate zone, assembly and compliance path, and Los Angeles County covers more than one zone. Use the R-values on your project's energy compliance documents. If you do not have them, the designer or energy consultant who prepared them does.

Can I add new batts on top of old attic insulation?

Generally yes, and it is common. Use unfaced batts for the new layer so you are not installing a second vapor retarder mid-assembly, and lay them perpendicular to the joists to cover the framing. Air-seal the ceiling penetrations before you cover them, because you will not get back up there easily.

Does more insulation always help?

It helps with diminishing returns. Going from nothing to R-13 is transformative; going from R-38 to R-49 in an attic is a much smaller gain. Past a point, air sealing, duct sealing and window performance buy more comfort per dollar than additional depth.

Faced or unfaced — how do I choose?

Faced batts include a vapor retarder and a stapling flange and go with the facing toward the conditioned space. Use unfaced when you are adding a second layer, insulating an interior sound wall, or when the assembly already has a separate vapor control layer. Never leave kraft facing exposed where the code requires a thermal barrier.

Is rigid foam board worth it over thicker batts?

They solve different problems. Batts fill cavities; board covers framing. If thermal bridging through studs is your loss — which it usually is on a steel-framed or 16 in. on-center wall — continuous board does something no batt can, regardless of thickness.

Do I need a permit to add insulation?

Insulation added as part of permitted work is covered by that permit and inspected with it. Standalone attic insulation upgrades are handled differently by jurisdiction, and rebate or incentive programs often have their own documentation requirements. Check with your local building department before the crew shows up.