Home Systems Guide · No. 3 · Theory guide, set v2 · draft 1

Insulation

How heat, air and moisture cross the envelope of a two-story wood-frame house under a truss roof, what limits each assembly, and what the energy code requires when an assembly is opened.

Refer to the following guides for concepts:

Framing (truss uplift and slip joints, no cutting of truss members, dead load of a floor topping)
Soundproofing (cavity fill as an acoustic requirement, decoupling, the partition-head gap as a sound path)
Windows & Exterior Doors (whole-window U, glazing code values, rough-opening seal)
HVAC, DHW & Ventilation (design loads, radiant emission coefficients)

Applied to this house: House Project Guide, chapter 3

DepthModerate

Governing code2025 Title 24 Part 6 and CRC, effective 2026-01-01

ClimateTitle 24 CZ 3–4; IECC 3C or 3B

RoofEngineered trusses over a vented attic

VersionSet v2 draft 1 · 2026-10-23

Read this first

R-value describes conduction only, so air leakage and radiation, which it leaves out, can match or exceed it at some surfaces. Part 1
In a 2×4 wall the studs cap the assembly: no cavity fill reaches the new-construction wall value, and with the framing fixed only continuous insulation moves the cap. Part 2
A standard truss heel thins the insulation at the eave, so a nominal R-49 ceiling performs near R-40 and the code's two ceiling routes are different requirements. Part 3
At the ceiling plane air transport is larger than conduction once the attic is deep, so sealing acts on the larger term and added depth on the smaller. Part 4
The garage puts two boundary surfaces in the middle of the house, and both read as interior construction, so their fill is easy to miss. Part 4
Stucco walls in IECC zone 3 can dry toward the interior, so a low-perm interior layer carried over from cold-climate practice removes their drying path. Part 5
Heating a slab changes its rules: the edge loss rises and Title 24 makes slab insulation mandatory. Part 6
Most of the code's insulation requirements are triggered by what the work touches, not by the house as a whole. Part 8

Decisions this guide informs
Decision	Depends on	Treated in
D1 Ceiling plane and attic: seal, add depth, or both; vented attic kept or moved to the roof deck	Existing depth; heel height and pitch; leakage and its location; anything mechanical in the attic; recessed fixture plan	Part 3, 4
D2 Exterior walls that are opened: cavity fill, and whether interior continuous insulation is added	Framing fraction; room depth available; drying direction; fire barrier over foam	Part 2, 5
D3 Garage boundary: floor over the garage and the shared walls	Whether the cavities are filled now; joist depth; the fire-rated membrane	Part 4
D4 Slab: left unheated, or heated with which insulation route	The radiant floor or ceiling decision; floor height available; slab and footing as built	Part 6, 8
D5 Compliance route: prescriptive alteration values or the performance path	Which provisions the work triggers; climate zone; whether credits such as QII are claimed	Part 8

Table 1 Which part treats which boundary surface
Part	Ceiling and attic	Exterior walls	Garage boundary	Slab
1 · Heat flow	Series and parallel paths; the envelope on one axis
2 · Walls	—	Framing cap, continuous insulation	Shared walls use the same arithmetic	—
3 · Ceiling	Heel taper, depth, attic strategies, summer gain	—	—	—
4 · Air and the boundary	The largest leakage surface	Penetrations, rim joist	Air and contaminant barrier	Edge at the garage line
5 · Moisture	Air-transported moisture into the attic	Drying direction	—	Slab moisture under finishes
6 · Ground and radiant	Backing behind a ceiling panel	—	—	Edge loss, heated slab
7 · Interior cavities	Partitions and the floor between stories: acoustic and fire properties of the fill

Table 1 maps the parts to the boundary surfaces. Subsections that cover one surface carry an Applies to tag; untagged subsections apply generally. An Informs tag marks the subsections behind each decision above.

1 · Heat flow: what R-value describes and what it omits

Bottom line. Resistances add in series and conductances add in parallel by area, and the number on a product label covers conduction through one uniform layer only.

Resistances add in series; conductances add in parallel

R_path = Σ Rᵢ U_assembly = Σ fⱼ / R_pathⱼ Q = U · A · ΔT

R resistance, h·ft²·°F/Btu · fⱼ area fraction of path j · Q heat flow, Btu/h

layers along one path stack their resistances; side-by-side paths share the area and the weakest dominates.

R-value is measured in steady state on a uniform layer. Four things that matter in a house are outside it:

Framing in parallel with the insulation (Part 2).
Non-uniform thickness, such as the taper at a truss heel (Part 3).
Air moving through or around air-permeable insulation (Part 4).
Radiation across air spaces and heat storage, which shape summer attic gains (Part 3).

Film and layer values used throughout: interior films R-0.68 (vertical), 0.61 (heat flow up), 0.92 (heat flow down); exterior film R-0.17; softwood R-1.25 per inch; RSI = R ÷ 5.678.

The envelope on one axis: UA by component

Putting every surface in UA (Btu/h·°F) shows where heat leaves before any cost or effort enters. Air leakage enters as a UA equivalent, 1.08 × cfm. For the reference house (2,300 sf, two stories, R-13 walls, 300 sf of original clear dual-pane glazing, 6 ACH₅₀, garage cavities taken as empty) the total is about 873 Btu/h·°F (Figure 1).

Table 2 lists the components and their bases.

Table 2 Reference house envelope, existing condition: windows lead, the ceiling as a conductor is near the bottom
Component	Area, sf	Coefficient	UA, Btu/h·°F	Share
Windows, existing clear dual pane	300	U 0.78	234	27%
Exterior walls, R-13 cavity	1,935	U 0.088	170	19%
Garage–house walls, uninsulated	405	U 0.268	108	12%
Floor over garage, uninsulated	500	U 0.214	107	12%
Air leakage, whole house (annual average)	—	92 cfm	99	11%
Slab edge, uninsulated	—	F 0.73 × 105 ft	77	9%
Ceiling, R-30 with heel taper	1,400	U 0.0341	48	5%
Exterior doors	60	U 0.5	30	3%

Reference geometry: 1,400 sf ceiling on a 150 ft perimeter, 1,935 sf net opaque wall, 405 sf of garage–house wall, 500 sf of floor over the garage, 105 ft of exposed slab edge. Air leakage is the annual-average natural rate, ACH₅₀ ÷ 20. Design heat loss is UA × design ΔT (HVAC · Design heating load).

Figure 1. Windows carry 27% of the UA on 6.5% of the envelope area, and the two garage surfaces together carry about 25%; ceiling conduction at R-30 carries 5%. Gray marks existing conditions; the hatched bar is a UA equivalent, not a conductance.

The ranking is on the magnitude axis only. Per square foot the glazing is about nine times worse than the wall; per dollar of change the order depends on access and price. Title 24 required R-13 walls and R-19 floors at the garage throughout this era, and many houses have them; filled to those levels the two garage rows total about 59 Btu/h·°F instead of 215, and the reference case takes them as empty to show what an unfilled bay costs.

2 · Walls: the framing caps a 2×4 assembly

Bottom line. Over a whole elevation about a quarter of a 2×4 wall is wood, and that path sets a floor on U that no cavity fill can pass.

Parallel paths: cavity and framing

Ceiling and atticExterior wallsGarage boundarySlab

Informs D2

Studs, plates, headers, corners and partition backing occupy about 25% of a 2×4 elevation (Title 24 Reference Joint Appendix JA4, ASHRAE 90.1 Appendix A), well above the 9% counted from studs alone. Each path carries the same exterior and interior layers, R-2.13 for stucco, paper, ½ in plywood, ½ in gypsum and films (Figure 2).

Figure 2. The stud path is fixed at R-6.50 whatever goes in the cavity, so it carries a growing share of the heat as the fill improves.

U = 0.75 / (2.13 + R_cavity) + 0.25 / 6.50 U → 0.0384 as R_cavity → ∞

framing fraction 25%; R_cavity installed fill R

the cavity term shrinks as fill improves, and the framing term stays put.

Table 3 gives the assembly values for the common fills.

Table 3 Reference 2×4 stucco wall: tripling the cavity R-value gains less than a third in assembly R
Cavity fill	Assembly U	Effective R	Share through framing
R-11 batt	0.096	10.5	40%
R-13 batt (reference existing)	0.088	11.4	44%
R-15 high-density batt	0.082	12.2	47%
Closed-cell foam, 3½ in	0.069	14.6	56%
Cavity R → ∞	0.0384	26	100%

Reaching U-0.048, the new-construction prescriptive wall value in climate zones 3–4, from inside the cavity would need R-76 in 3½ in. That value was set for 2×6 framing with continuous insulation. The 2025 mandatory value for a 2×4 wall, U-0.095, is met by R-13 or R-15 batts in this build-up (Figure 3).

Figure 3. The curve flattens onto the framing asymptote, and the prescriptive line sits below it; continuous insulation (dashed) is the only change that shifts the asymptote. Green points meet the 2025 mandatory 2×4 value; R-11 (gray) sits within input uncertainty of it.

Sensitivity to framing fraction

Table 4 Framing fraction moves the R-15 wall less than it moves the asymptote
Framing fraction	U with R-15	Asymptote
15%	0.073	0.023
20%	0.077	0.031
25%	0.082	0.038
30%	0.087	0.046

A measured framing take-off changes the numbers, not the conclusion: at 15% framing the R-15 wall is U-0.073, and the cavity-only route to U-0.048 still needs about R-42 (Table 4).

With the framing fixed, continuous insulation is the lever on the asymptote

Ceiling and atticExterior wallsGarage boundarySlab

Informs D2

With the framing fixed, a layer across the studs is the one change that adds its full R to both paths. With exterior work out of scope, it goes on the interior face under furring and new gypsum. Over an R-15 cavity, R-7.6 of continuous insulation reaches U-0.048.

Table 5 Interior continuous insulation to reach U-0.048 over R-15: thickness and vapor class decide between products
Board	R per in	Board, in	With furring and gypsum, in	Vapor behavior
Polyiso, foil-faced	5.9	1.3	2.7	Class I facer
XPS	5.0	1.5	2.9	Class II
Mineral wool board	4.2	1.8	3.2	Vapor-open
EPS	4.0	1.9	3.3	Class III

Room depth adds ¾ in furring and ⅝ in gypsum over the board; foam plastics also need a thermal barrier (CRC R316), which the gypsum provides.

Across the reference wall area the change from the existing R-13 to U-0.048 is a ΔUA of about 77 Btu/h·°F, the largest change available on the opaque exterior walls; replacing the windows (Windows & Doors · Energy value) and filling an empty garage floor are of similar or larger size. It is also the only measure here that consumes floor area, which partitions already draw on (Soundproofing · Partition assemblies). Foil-faced polyiso is the thinnest and reverses the wall's drying direction; mineral wool board keeps it (Part 5).

Watch out

Closed-cell foam in a 2×4 cavity gains little over a batt because the framing still governs (Table 3), and it blocks inward drying, bonds both faces of the wall together and is not reversible.

3 · Ceiling and attic: depth, the heel and summer gain

Bottom line. A vented truss attic can take deep insulation cheaply in the field and very little at the eave, and above about R-38 depth buys little winter energy.

A standard truss heel thins the insulation at the eave

Ceiling and atticExterior wallsGarage boundarySlab

Informs D1

Where a truss bears on the wall, the space between the top plate and the roof sheathing is the heel height, about the top-chord depth (3½–5½ in) for a standard heel. CRC R806.3 keeps 1 in clear under the sheathing at the vents, so a 4 in heel leaves 3 in of insulation at the wall line. Depth then grows at the roof pitch (Figure 4).

Figure 4. At 4:12, a 14 in field (R-49 at R-3.5 per in) is reached 2.75 ft in from the wall. Under a hip roof, with eaves on all four sides, 27% of a 1,400 sf ceiling on a 150 ft perimeter sits in the taper.

d(x) = (h − c) + s·x U_band = ln[(a + bL)/a] / (L·b) a = R_nc + r·(h − c), b = r·s

h heel height · c vent clearance · s roof slope · r R per inch · L band width · R_nc films and gypsum

insulation depth grows linearly from the wall, so the band's average U is a logarithm rather than the field value.

Table 6 carries the taper through the common nominal levels.

Table 6 Nominal against effective ceiling R at a 4 in heel: the taper costs more as the field deepens
Nominal	Field depth, in	Taper band, ft	Share of ceiling	Weighted U	Effective R
R-19	5.4	0.61	6%	0.0493	20
R-30	8.6	1.39	14%	0.0341	29
R-38	10.9	1.96	20%	0.0287	35
R-49	14.0	2.75	27%	0.0245	41
R-60	17.1	3.54	34%	0.0219	46
R-75	21.4	4.61	43%	0.0198	51

4:12 pitch, 1 in clearance, loose fill at R-3.5 per in, 1,400 sf on a 150 ft perimeter with eaves on all four sides (hip). A gable roof has eaves on the two long walls only, so the band scales with eave length: with 80 ft of eave, R-49 covers 16% of the ceiling in taper and performs near R-45.

Nominal R-49 performs at about R-41. Title 24 §150.2(b)1J offers R-49 for the insulation alone or a weighted U-0.020 as alternatives; at a 4 in heel under a hip roof the weighted route needs about R-73 nominal, a 21 in field, so the two are different requirements. An exception in the same section accepts filling the available space where it cannot hold the required value without blocking ventilation (Figure 5).

Figure 5. Heel height is the dominant input: a 16 in raised heel removes the taper, and R-49 then performs at its field value. The heel cannot be raised in an existing truss roof, so it is measured, not chosen.

Attic strategies compared

Ceiling and atticExterior wallsGarage boundarySlab

Informs D1

Table 7 compares the four approaches for a vented truss attic.

Table 7 Attic strategies: relocating the boundary to the deck solves a problem a hydronic house mostly does not have
Strategy	Strengths	Limits	Fit
A · Seal the ceiling plane, then blow fill to a marked settled depth	Acts on the two largest ceiling terms, leakage and conduction; keeps the roof deck outside and its drying path open; reversible	Truss webs limit access and depth checks; the eave taper remains; each fixture added later reopens the sealed plane	Default for a vented truss attic with nothing mechanical in it
B · Method A plus a radiant barrier	Cuts radiant exchange from the deck by about 94%; part of the Title 24 Option C package	Value falls as ceiling R rises and as attic ducts disappear; dust degrades an attic-floor barrier	Summer comfort upstairs, not annual energy
C · Unvented attic: insulation at the roof deck (CRC R806.5)	Brings the attic inside; removes the taper and the leaky ceiling plane from the boundary	Spray foam over a sloped area larger than the ceiling; encapsulates webs and connector plates; changes the deck's moisture regime; several times the cost	Justified mainly where ducts or equipment must sit in the attic
D · Mandatory minimum where the ceiling is opened	No added cost	Leaves the leakage and depth terms as they are, at the time access is easiest	Legal where §150.2(b)1J is not triggered

Depth has a sharply falling return on the energy axis because the taper and the field both flatten. In the reference house, R-30 to R-49 saves about $27 a year in heating and R-49 to R-60 about $7 (Figure 6). The case for depth past R-38 rests on summer comfort and on the roof-borne sound path (Soundproofing · Roof and attic path), not on payback.

Figure 6. Each step of nominal R saves less than the one before, and the taper widens the gap. Gray marks the typical existing R-30; teal bars are neutral quantities.

Watch out

Under R806.3 every vented bay keeps a baffle and 1 in of clearance. Insulation blown against the soffit closes the vents that are the attic's moisture control, and baffles and depth dams attach to sheathing, plates or chord faces, never to a cut web (Framing · No truss member is cut).

Summer gain under a tile roof depends on radiation and heat capacity

Ceiling and atticExterior wallsGarage boundarySlab

Informs D1

q_r = σ · F_e · (T₁⁴ − T₂⁴) F_e = 1 / (1/ε₁ + 1/ε₂ − 1) α = k / (ρ·c_p)

ε emittance of the two surfaces · α thermal diffusivity

radiant exchange across the attic falls with the lower emittance; a material with more heat capacity per volume delays the afternoon peak.

With both surfaces at ε 0.9, F_e = 0.82; with one foil face at ε 0.05 it is 0.050, a 94% cut in the radiant term only. Cellulose has about four times the heat capacity per volume of loose fiberglass (ρc_p 0.5–1.2 against 0.1–0.3 Btu/ft³·°F), which shifts and flattens the peak gain into upstairs rooms on hot afternoons with little effect on annual energy.

4 · Air and the boundary: where the envelope is, and where it leaks

Bottom line. The insulation and the air barrier have to sit on the same continuous surface; where they separate, the insulation is bypassed.

The air barrier is a continuous plane, coincident with the insulation

Fibrous insulation is air-permeable, so it insulates only where a continuous air barrier sits against it. At the ceiling the barrier is the gypsum plane with its penetrations sealed; in walls it is the gypsum and the sheathing with their edges and penetrations sealed; at the garage it is the gypsum fire membrane. An insulation layer with an open air space on its warm side forms a ventilated cavity with little resistance.

Leakage is rated by a blower-door test as ACH₅₀ (air changes per hour at 50 Pa). Houses of this era typically test at 5–8 ACH₅₀. The flow at other pressures follows a power law.

Q = C · ΔPⁿ C = Q₅₀ / 50ⁿ n ≈ 0.65

Q airflow, cfm · ΔP pressure difference, Pa · n flow exponent (0.5 for a sharp orifice, 1 for laminar)

leakage grows less than in proportion to the pressure driving it.

At the ceiling, stack-driven leakage rivals conduction

Ceiling and atticExterior wallsGarage boundarySlab

Informs D1

ΔP = 0.0342 · P_atm · h · (1/T_out − 1/T_in) UA_leak = 1.08 · Q

h height above the neutral pressure plane, m · T absolute temperatures, K · P_atm 101,325 Pa

warm air is lighter than outdoor air, so the top of the house is pushed outward harder the colder it is outside.

The neutral plane sits where inflow and outflow balance. With 35% of the leakage in the ceiling plane, typical for the vintage, and the rest spread over the walls, it rises to about 3.8 m in a 5.5 m two-story house, and at 70 °F inside and 35 °F outside the ceiling sees about 1.4 Pa (Figure 7). A 6 ACH₅₀ reference house then pushes about 63 cfm through the ceiling, a UA equivalent of 68 Btu/h·°F against 48 for R-30 conduction and 34 at nominal R-49. Placing the neutral plane at mid-height, a common simplification, gives 87 cfm and is an upper bound.

Figure 7. Air leaves at the top and enters low. Deepening loose fill does nothing to this flow, because the fill is air-permeable; it only hides the leaks.

Table 8 Ceiling leakage at winter design against ceiling conduction: the ratio rises as insulation deepens
House ACH₅₀	Ceiling leak UA	vs R-30 conduction	vs R-49 conduction
3	34	0.7×	1.0×
4.5	51	1.1×	1.5×
6	68	1.4×	2.0×
8	90	1.9×	2.6×
10	113	2.4×	3.3×

35% of leakage in the ceiling plane, n 0.65, neutral plane by mass balance, no wind. In mild weather the stack term falls with ΔT and the two terms converge; the annual-average whole-house rate in Table 2 is lower than this design-hour flow for that reason.

At the reference leakage the ceiling plane passes 1.4–2.0 times its R-30 conduction and 2.0–2.7 times its R-49 conduction at design conditions, the range spanning the two neutral-plane assumptions (Table 8). Deepening insulation raises the ratio because it lowers only the conduction term. Near 3 ACH₅₀, or with a much smaller ceiling share, conduction at R-49 becomes the larger term; a blower-door test with a zone-pressure diagnostic measures both.

Recessed downlights and partition top plates are usually the two largest categories of ceiling leakage (Figure 8). Fixture type and count are a lighting decision; the partition head under trusses is a joint designed to move (Framing · Truss uplift). Airtightness and stiffness are separate properties, so a flexible continuous seal closes that joint for air and sound at once (Soundproofing · The partition-head gap).

Figure 8. Two categories that are decided outside this guide, the fixture plan and the partition-head detail, usually carry half the ceiling leakage. Ranges are typical for the vintage, not measured; the ordering is more reliable than the sizes. Teal bars are neutral quantities.

The garage puts two boundary surfaces inside the footprint

Ceiling and atticExterior wallsGarage boundarySlab

Informs D3

The boundary is one closed surface between conditioned and unconditioned volume (Figure 9). Interior partitions and the floor between stories are inside it and carry no thermal requirement. The garage is inside the footprint and outside the boundary, so the walls it shares with the house and the floor of the rooms above it are boundary surfaces, mandated today at R-15 for 2×4 walls and R-19 for floors. A crew that treated the garage ceiling as an interior floor left it empty or part-filled, and nothing visible distinguishes the two.

Figure 9. Five boundary surfaces. Surfaces ③ and ④ read as interior construction and carry no visual sign of their role. Teal is annotation only.

Table 9 Garage boundary assemblies: filling an empty bay cuts the floor's U by about six
Assembly	U	Effective R
Floor over garage, empty bay	0.214	4.7
R-19 batt, held against the subfloor	0.047	21.3
R-30, bay filled	0.034	29.7
Garage–house wall, empty	0.268	3.7
Garage–house wall, R-15	0.081	12.3

Floor: 9¼ in sawn joists at 16 in, 10% framing, ¾ in subfloor, ⅝ in Type X below. Wall: 2×4, gypsum both sides, 25% framing.

R-19 in a 9¼ in bay leaves about 3¾ in of air above the batt. Held against the subfloor it behaves as calculated; left to sag, the air layer above it connects to the garage through gaps at the rim and the assembly matches neither row of Table 9. The gypsum is also the fire separation (CRC R302.6) and the barrier against garage CO and fuel vapors (R302.5), so every penetration is a contaminant path before it is an energy one.

5 · Moisture: drying direction and vapor control

Bottom line. In IECC zone 3 a stucco wall can dry toward the interior, and a low-perm interior layer removes that path; moisture reaches the attic by air, not by diffusion.

Two climate-zone systems share digits and mean different things

Title 24 energy climate zones (16 zones, by ZIP code in Reference Joint Appendix JA2) set energy values. The IECC/ASHRAE 169 zones set the CRC moisture rules: most of coastal and inland California is 3C (warm-marine) or 3B, with Marine 4 on parts of the north coast. CRC R702.7 requires an interior Class I or II vapor retarder only in Marine 4 and zones 5–8, so a Title 24 "zone 4" house is usually not a "zone 4" house for vapor purposes.

A vapor-open interior lets a stucco wall dry inward

Ceiling and atticExterior wallsGarage boundarySlab

Informs D2

Stucco stores rain and dew, and sun drives that moisture inward in warm weather. A vapor-open interior (unfaced fill, latex-painted gypsum, Class III at about 3–10 perms) lets it leave; a low-perm interior layer stops it at the back of the gypsum (Figure 10). In winter the outdoor side is not cold for long enough to drive significant outward condensation.

Figure 10. A vapor-open interior keeps the wall's drying path; foam with a foil facer leaves outward drying only, which works where the foam keeps the cavity side of the gypsum above the summer dew point; a polyethylene sheet behind bare gypsum leaves a cold condensing surface. Green dries, amber is conditional, red fails.

Table 10 sets out the vapor rule for each assembly.

Table 10 Vapor control by assembly: an assembly must dry in at least one direction
Assembly	Code	What the physics implies	What fails
Exterior wall, cavity fill only	No interior vapor retarder in 3B or 3C	Unfaced fill and latex paint keep inward drying	Kraft facing or polyethylene on the warm side
Exterior wall with interior continuous insulation	No retarder required	Mineral wool board keeps inward drying; foam leaves outward only and needs the foam face above dew point	Foam plus polyethylene or kraft: two low-perm layers with a wettable cavity between
Vented attic ceiling	Air sealing (CZ 2, 4, 8–16 when §150.2(b)1J applies); ventilation per CRC R806	Air sealing is the moisture control; ventilation removes what arrives	A vapor barrier at the ceiling with the leaks left open
Slab under a moisture-sensitive finish	A vapor retarder under new slabs; product limits for insulation under a heated topping	Slab relative humidity measured by ASTM F2170 shows whether a bonded finish will stay dry	An impermeable finish on an untested slab of unknown vapor retarder

6 · Ground and radiant: slab edge, heated slabs, emitter backing

Bottom line. An unheated slab loses mostly at its edge and needs little insulation in a mild climate; a heated slab or an emitter panel needs insulation behind it to deliver its heat to the room.

An unheated slab loses heat at its edge

Ceiling and atticExterior wallsGarage boundarySlab

Informs D4

Q = F · P · ΔT

F slab-edge coefficient, Btu/h·ft·°F · P exposed perimeter, ft

slab loss is set by the length of edge exposed to outdoor air, not by the area.

Soil under a slab warms toward the slab temperature, so the field loses little and the edge dominates. F is about 0.73 for an uninsulated unheated slab and about 1.35 for an uninsulated heated one (ASHRAE 90.1 Appendix A, typical). Title 24 Table 150.1-A requires no slab-edge insulation in climate zones 3–4 for an unheated slab, which matches the mechanism. A slab continuous under an attached garage also loses sideways into the garage slab, a path the perimeter coefficient does not include.

A heated slab or panel loses a share of its output through its back

Informs D4

parasitic = q_back / (q_back + q_room) q_back = (T_emitter − T_back) / R_back

R_back resistance from emitter to the cold side

the share lost behind an emitter is set by the resistance behind it relative to the resistance to the room.

A slab delivering 15 Btu/h·sf to a 70 °F room runs near 80 °F at the tubing. Over ground at about 60 °F the field of an uninsulated slab loses about a quarter of its output, and the perimeter band, which loses toward outdoor air (35 °F at design), about two thirds; a new topping over R-10 board loses under a tenth (Figure 11). A ceiling panel follows the same rule against the attic: insulation in contact keeps the loss to 8% or less, the eave taper raises it to 20%, and an air gap open to the attic to about 63%. Contact is a physical requirement rather than a code number; the inputs are in Table 11.

Figure 11. Contact matters more than depth: R-30 and R-49 in contact differ by three points, a gap behind R-49 costs over half the output. Slab values rest on a quasi-steady ground resistance (R≈4 field, R≈1.5 edge) and are the least certain numbers here; the order holds across plausible soil values.

Inputs and results for the back-loss cases

Table 11 Back loss by backing condition
Emitter and backing	R_back	q_back, Btu/h·sf	Share lost
Field, uninsulated	4.0	5.0	25%
Perimeter band, uninsulated	1.5	30.0	67%
Topping over R-5	9.0	2.2	13%
Topping over R-10	14.0	1.4	9%
R-49 in contact	49.0	1.1	5%
R-30 in contact	30.0	1.8	8%
Eave taper, at the wall line	10.5	5.2	20%
Air gap open to the attic	1.5	36.7	63%

Slab: 15 Btu/h·sf to the room; field toward 60 °F ground, edge toward 35 °F outdoor air. Ceiling panel: 90 °F surface, 21 Btu/h·sf to the room, attic at 35 °F. Emission coefficients from HVAC · Radiant emission.

Three slab configurations differ in which loss they treat (Figure 12). Vertical perimeter insulation acts on the edge only. A topping over continuous board treats both, separates the emitter from the old slab's mass, which shortens its response, and raises the finished floor about 3½ in, with consequences for doors, stair risers and thresholds, and a gypsum-concrete topping adds about 14 psf of dead load where it sits on framing (Framing · Added floor dead load). Which geometry the code accepts is in Part 8.

Figure 12. Only a layer under the emitter treats the field loss. The two code-listed perimeter geometries presuppose excavation or a slab being poured. Tubing is blue and foam blue-gray, as materials; red arrows are losses.

7 · Interior cavities: fill for sound and fire

Bottom line. Most new fill in an interior remodel goes into partitions and the floor between stories, where it serves acoustics and fire, not heat.

Cavity fill for acoustic assemblies: airflow resistivity, not density

Fibrous fill damps the cavity resonance of a double-leaf wall and absorbs standing waves (Soundproofing · Lowering f₀). The governing property is airflow resistivity σ: below about 5,000 Pa·s/m² sound passes through, above about 30,000 the surface reflects, and the common products sit inside that band.

Table 12 compares the candidate fills on this property.

Table 12 Cavity fill for acoustic assemblies: spray foam fails mainly on structure
Material	σ, Pa·s/m²	Behavior in a partition
Fiberglass batt	8,000–15,000	In band
Mineral wool batt	20,000–35,000	Upper band
Cellulose, dense-pack	10,000–25,000	In band; needs netting in open walls
Open-cell spray foam	far above the band	Air-impermeable at common thicknesses; bonds to both leaves
Closed-cell spray foam	closed cell	Not an absorber; bonds both leaves rigidly

Spray foam of either type glues the two leaves together and restores the bridge that decoupling removes (Soundproofing · Coupling). Mineral wool and glass fiber batts meet the acoustic requirement and are accepted as fireblocking in specified applications (CRC R302.11.1), where solid blocking across both leaves of a double-stud wall would bridge it (Soundproofing · What does bind).

Table 13 Insulation materials on the properties that separate them
Material	R per in	Vapor	Distinguishing property
Fiberglass batt	3.7–4.3	Open	Lowest cost per R; performance limited by fit around wiring and boxes
Mineral wool batt	4.0–4.3	Open	Holds shape, non-combustible (ASTM E136); 1.5–2× fiberglass cost
Blown cellulose	3.2–3.8	Open, hygroscopic	High heat capacity; settles 10–20% unless specified by settled depth
Loose-fill fiberglass	2.5–3.5	Open	Lowest cost for attic depth; little heat capacity
Open-cell spray foam	3.6–3.8	Semi-open	Air seal and fill in one; bonds to framing; thermal barrier required
Closed-cell spray foam	6.0–7.0	Class II at 2 in	Highest R per inch; blocks drying; bonds leaves; thermal barrier required

Table 13 collects the fill materials; rigid boards are in Table 5.

8 · Code, permits and documentation

Bottom line. Title 24 Part 6 binds insulation work, is inspected, and engages component by component as the work touches each one.

Which code, which cycle

The 2025 California Energy Code (Title 24 Part 6) applies to permit applications filed on or after 1 January 2026, with the CRC (Part 2.5) setting ventilation clearance, vapor retarders, fire separations and foam barriers. AB 130 (2025) bars changes to residential building standards until 1 June 2031, so this cycle is likely to govern through most remodels started now. Local reach codes adopted before the freeze may still apply.

Three tiers: mandatory, prescriptive, alteration

Informs D5

Table 14 sets out the three tiers. The mandatory tier always applies; prescriptive or performance compliance sits on top of it.

Table 14 Title 24 tiers for an alteration: the alteration provisions concede the 2×4 arithmetic
Tier	Requirement (as summarized)	Role here
Mandatory (§150.0, §110.7, §110.8)	Ceiling U-0.043 / R-22; walls U-0.095 / R-15 in 2×4 and U-0.069 / R-21 in 2×6 (2025); raised floors R-19; sealing; certified insulation, facings in contact	Applies to any altered component on any path
Prescriptive, new construction (Table 150.1-A)	Wall U-0.048 in CZ 3–4; vented-attic ceiling R-30 in CZ 2–7 (Option C, 2025); slab edge not required in CZ 3–4; QII	A benchmark, and the standard design on the performance path
Alteration (§150.2(b))	Altered 2×4 walls R-15, with an exception for existing R-11; vented attic R-49 or U-0.020 in CZ 1–4, 6 and 8–16, air sealing in CZ 2, 4 and 8–16, downlights covered; no continuous insulation where siding stays	The tier that governs an interior remodel

The alteration rules accept cavity-only insulation where cladding stays because of the framing cap in Part 2. The performance path (§150.2(b)2) models each altered component against the higher of code or its existing condition and lets surpluses offset deficits; Quality Insulation Installation (Reference Appendix RA3.5) then becomes a modeled credit verified by an ECC rater (the 2025 title for the former HERS rater).

Each provision engages only when the work touches its component

Informs D1 D5

Figure 13 draws the triggers.

Figure 13. The questions are independent; an interior remodel can engage some and not others. Amber marks a requirement that engages on a yes.

The ceiling provision is the one most open to reading. §150.2(b)1J applies to altered vented-attic ceilings, and the HVAC alteration rules cross-reference it when a new or complete replacement duct system is placed in the attic; a hydronic system with no attic ducts may leave it unengaged, while radiant panels fixed to the ceiling may count as altering it. The reading belongs to the building department, and it decides whether R-49 and ceiling air sealing are required or voluntary.

The heated-slab rule assumes a slab being poured

Ceiling and atticExterior wallsGarage boundarySlab

Informs D4

§110.8(g) makes heated slab floors subject to Table 110.8-A on every compliance path, with R-5 in climate zones 1–15. The listed geometries are vertical insulation at the slab edge to 16 in or the frost line, or vertical plus 4 ft horizontal between slab and foundation; both presuppose excavation or a new pour. Insulation in contact with the ground must also limit water absorption (0.3%, ASTM C272, tested without facings) and vapor permeance (2.0 perm/in), which XPS and some high-density products meet and ordinary EPS does not; board laid over an existing slab under a topping is not in ground contact.

A topping over continuous board is thermally stronger than either listed geometry but is not one of them, so its acceptance is an equivalence question for the building department, settled at plan check rather than at inspection.

Verify

Table 110.8-A values, geometries and the ground-contact limits are from the 2022 text; the §150.2(b)1J trigger, zones and exceptions and the mandatory ceiling value are from 2025 secondary sources, which do not fully agree. They are confirmed against the adopted 2025 text and the building department's reading before they decide a scope.

CRC provisions that limit what may be installed

Table 15 CRC provisions an insulation change engages, with the reason for each
Section	Requirement	Why
R806.2, R806.3	Attic vent area 1/150 (1/300 with balanced upper vents); 1 in clear at eave vents; baffles	Creates the eave taper; vent paths are the attic's moisture control
R806.5	Unvented attics: air-impermeable insulation at the deck to a minimum R; no interior Class I retarder	The deck must stay above dew point because it cannot dry to outdoors
R702.7	Interior Class I or II retarders only in Marine 4 and zones 5–8	Keeps inward drying in zone 3
R302.6, R302.5	Garage separation gypsum (⅝ in Type X under habitable rooms); limits on openings and ducts	Fire and CO; insulation fills the rated assembly, not the reverse
R302.11	Fireblocking at floors, ceilings and 10 ft; mineral wool or glass fiber accepted in specified uses	Stops concealed spaces acting as chimneys
R316	Foam plastic separated from the interior by a thermal barrier, ½ in gypsum or equivalent	Foam ignites readily and smokes heavily

Table 15 lists the CRC constraints. The energy compliance documents are the CF1R (design, at plan check), CF2R (installation, signed by the installer) and CF3R (field verification by the ECC rater where a measure requires it, such as QII); the inspector checks the CF2R against the work before drywall.

Where it goes wrong

Insulating over an unsealed ceiling. Loose fill does not stop air; it buries the leaks and makes the attic surfaces above them colder, where exfiltrating air leaves its moisture (Part 4).
Sealing the partition head rigidly, or not at all. Hard caulk cracks with the first winter's truss movement; an open joint is a leak and a sound path (Part 4).
Blocking eave vents with new depth. Without baffles the fill closes the soffit vents and violates the 1 in clearance (Part 3).
Reading nominal R as assembly performance. R-15 batts make an R-12 wall; R-49 at a standard heel is about R-40 (Part 2, Part 3).
Batts compressed or gapped. A ¼ in gap at a stud or a batt split behind a box sets up convection; field studies place poor installations 20–30% below nominal, which is what QII inspects.
A vapor retarder because of "zone 4". The Title 24 zone is not the IECC zone; kraft or polyethylene on the warm side of a zone 3 stucco wall removes its drying path (Part 5).
Missing the garage boundary. Two surfaces inside the footprint carry about a quarter of the reference UA and are a CO path (Part 4).
A gap behind a radiant panel. Insulation present but not in contact costs over half the output and passes inspection (Part 6).
Spray foam in a decoupled partition. It bonds the leaves and gives back the decoupling (Part 7).
Ordinary EPS as perimeter insulation in ground contact. It fails the §110.8(g) absorption limit (Part 8).
Exposed kraft facings in an attic or garage fail the §110.8(c) flame-spread limits unless in contact with a finish; inspectors check this at the insulation inspection.
Closing the ceiling before the fixture plan is final. Every later downlight is a new hole in a buried plane (Part 4).

Reference

Glossary

ACH₅₀: Air changes per hour at a 50 Pa blower-door pressure.
Air barrier: The continuous plane that stops bulk air movement through an assembly.
ECC rater: Energy Code Compliance rater; the 2025 Title 24 name for the third-party verifier formerly called a HERS rater.
F-factor: Slab-edge heat loss per foot of exposed perimeter per °F.
Framing fraction: Share of an assembly's area occupied by framing.
Heel height: Vertical space between the wall top plate and the roof sheathing where a truss bears.
Neutral pressure plane: Height at which indoor and outdoor pressure are equal under stack effect.
Parasitic fraction: Share of an emitter's output lost through its back.
Perm: Unit of water-vapor permeance; Class I ≤ 0.1, Class II 0.1–1, Class III 1–10.
QII: Quality Insulation Installation, Reference Appendix RA3.5.
Settled depth: Depth of blown fill after settling; the specification for cellulose.
UA: Area times U-factor; heat flow per °F of temperature difference.

Formula sheet

Series R = Σ Rᵢ Parallel U = Σ fⱼ / Rⱼ Q = U·A·ΔT Annual conduction kWh = UA × 24 × HDD / 3,412 / COP Taper band U_band = ln[(a + bL)/a] / (L·b) Leakage power law Q = C·ΔPⁿ, C = Q₅₀ / 50ⁿ Stack pressure ΔP = 0.0342·P_atm·h·(1/T_out − 1/T_in) (Pa, m, K) Sensible air load q = 1.08·cfm·ΔT UA_leak = 1.08·cfm Natural infiltration ACH_nat ≈ ACH₅₀ / N, N ≈ 15–25 Slab edge Q = F·P·ΔT Back loss parasitic = q_back / (q_back + q_room) Radiant exchange q = σ·F_e·(T₁⁴ − T₂⁴), F_e = 1/(1/ε₁ + 1/ε₂ − 1)

Figure color legend

Red: deficient, or fails the stated criterionAmber: marginal or conditionalGreen: meets the criterionGray: existing or baselineTeal: neutral quantity, reference line or annotation; never a grade

Insulation is drawn in tan, rigid foam in blue-gray, wood, concrete and soil in their material colors; these are materials, not grades.

Code points to confirm

§150.0(a) mandatory ceiling value in the 2025 text
§150.2(b)1J trigger, climate zones, air-sealing zones and exceptions (existing R-38; limited space); the building department's reading for a project with no attic ducts
Altered-wall requirement and the existing-R-11 exception in §150.2(b)1
Table 110.8-A values, geometries and material limits; whether a topping over continuous insulation is accepted as equivalent
Table 150.1-A ceiling Option C and radiant-barrier requirement by zone
Climate zone of the parcel in Reference Joint Appendix JA2

Sources

California Energy Commission, What's New for 2025: Single-Family (2025) — §150.0(c) 2×4 U-0.095 / R-15, 2×6 U-0.069 / R-21; Option C ceiling R-30 in CZ 2–7.
3C-REN, 2025 Energy Code in Practice: Single Family Additions and Alterations (March 2026) — altered walls, §150.2(b)1J R-49 or U-0.020, air sealing and downlight rules.
California Energy Commission, Residential Compliance Manual, Chapter 3 (building envelope) and Reference Appendices JA2, JA4, RA3.5.
2025 California Residential Code, R302.5, R302.6, R302.11, R316, R702.7, R806.
ASHRAE 90.1 Appendix A (slab F-factors, framing fractions); ASHRAE Handbook — Fundamentals (stack effect, film coefficients); ISO 11855 (radiant surface coefficients).
California Building Standards Commission, Information Bulletin 25-03 (AB 130).
Companion guides: Framing, Soundproofing, Windows & Exterior Doors, HVAC, DHW & Ventilation (set v2).

Set v2 draft 1, 2026-10-23: rewritten as a generic theory guide to spec set v2 from the v1 guide (archived). Updated to the 2025 wall values; leakage now uses n 0.65, a mass-balance neutral plane and a 35 °F winter design, which lowers the ceiling leakage ratio from v1's 2.9× to 1.4–2.0×; continuous insulation recomputed by parallel paths (R-7.6, not R-8.7); hip and gable taper separated; heated-slab material limits limited to ground contact after review. Values from one script.

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