Thermal Inertia

R-value measures steady-state resistance. Thermal diffusivity tells you how fast heat actually moves — and how much a material can buffer daily temperature swings.

What is Thermal Diffusivity?

Thermal diffusivity (α) describes how quickly a temperature change propagates through a material. It is the ratio of how fast heat conducts to how much heat the material can store per unit volume:

α = k / (ρ · c_p)

k — Conductivity

How readily heat flows through the material (W/m·K). Higher means heat flows faster.

ρ — Density

Mass per unit volume (kg/m³). More mass means more thermal storage capacity.

c_p — Specific Heat

Energy absorbed per kilogram per degree (J/kg·K). Higher means each kilogram stores more energy.

A low α means the material is slow to respond to temperature swings — it absorbs heat during the hot part of the day and releases it slowly at night. This is called thermal inertia.

Standard R-value describes steady-state performance: how much heat flows per hour when the temperature difference is held constant. Real buildings never experience steady-state — outdoor temperatures swing 20–40°F between day and night. Two walls with the same R-value but different thermal diffusivities will perform very differently under real climate conditions.

Thermal Diffusivity by Insulation Type

α in mm²/s — lower values indicate better thermal inertia. Sorted from highest (poorest) to lowest (best).

Excellent ≤ 0.60

Good 0.60 – 1.20

Fair 1.20 – 4.00

Poor > 4.00

01234 mm²/s →

Glass Fiber

3.87 mm²/s

Open Cell SPF

3.48 mm²/s

EPS

1.17 mm²/s

Havelock Wool

0.91 mm²/s

Mineral Wool

0.89 mm²/s

Cellulose

0.58 mm²/s

XPS

0.57 mm²/s

Gutex

0.52 mm²/s

Closed Cell SPF

0.46 mm²/s

TimberHP

0.38 mm²/s

Values are representative midpoints computed from α = k / (ρ·c_p). Actual performance varies by product density, moisture content, and installation method.

Key Observations

Density is the hidden variable. Glass fiber and open-cell spray foam have nearly the same R-value as cellulose — but 6–7× higher thermal diffusivity. At 8–12 kg/m³ (0.5–0.75 lb/ft³), these materials are mostly air. Any heat that enters the assembly stores almost nothing in the fibers themselves and travels through quickly.
Wood fiber stands apart. Gutex and TimberHP wood fiber insulations have a specific heat capacity of 2,100 J/kg·K — more than 2.5× higher than glass fiber or mineral wool (840 J/kg·K). Combined with moderate density, their volumetric heat capacity is roughly 7–10× greater than fiberglass. This is why wood fiber is prized in passive solar and hygrothermal design.
Cellulose punches above its R-value. Cellulose has similar R-values to fiberglass (~R-3.6–3.7/in) but roughly 6× better thermal inertia. Its density of ~43 kg/m³ (versus ~12 kg/m³ for fiberglass) is entirely responsible. Dense-pack cellulose significantly delays the daily heat wave.
High R-value ≠ high thermal inertia. Closed cell spray foam achieves the best R-value per inch of any insulation here (R-6.5/in) but lands in the same thermal inertia tier as cellulose and XPS. Thermal diffusivity and R-value are independent properties measuring different things — one matters for hourly comfort, the other for annual energy loads.

Material Properties

Includes insulation and common structural materials for comparison. R-value and U-value shown per inch of thickness (IP units).

Material	Density kg/m³	Specific Heat J/kg·K	Diffusivity mm²/s	R-Value per inch	U-Value per inch
Insulation
Glass Fiber Batt Standard batt, ~0.75 lb/ft³	12	840	3.87	R‑3.7	0.270
Open Cell Spray Foam 0.5 lb/ft³ half-pound foam	8	1,400	3.48	R‑3.7	0.270
EPS (Expanded Polystyrene) Type II, 1.5 lb/ft³	24	1,250	1.17	R‑4.2	0.238
Havelock Wool Insulation Blown-in; batt version ~R-3.6/in	22	1,700	0.91	R‑4.3	0.233
Mineral Wool Batt Stone/rock wool, ~3 lb/ft³	48	840	0.89	R‑4.0	0.250
Cellulose (Blown-in) Dense-pack; recycled paper fiber	43	1,600	0.58	R‑3.6	0.278
XPS (Extruded Polystyrene) R-5/in new; ages to ~R-4.2–4.5 over time	35	1,450	0.57	R‑5.0	0.200
Gutex Wood Fiber Insulation Gutex Thermofibre blown-in product	35	2,100	0.52	R‑3.8	0.263
Closed Cell Spray Foam 2 lb/ft³; R-6.5/in initial value	32	1,500	0.46	R‑6.5	0.154
TimberHP Wood Fiber Insulation TimberBatt; Maine-made wood fiber	48	2,100	0.38	R‑3.8	0.263
Framing
Dimensional Lumber Douglas fir, ~12% moisture content	505	1,700	0.14	R‑1.3	0.800
Sheathing
Plywood Sheathing Typical structural plywood	540	1,300	0.17	R‑1.2	0.820
OSB Sheathing Oriented strand board	650	1,300	0.15	R‑1.1	0.901
Masonry & Concrete
CMU (8" Normal Weight) 8" hollow block ≈ R-1.11 for entire assembly; per-inch not meaningful†	—	—	—	—	—
Concrete (3,000 psi) Normal weight, ~143 lb/ft³	2300	880	0.86	R‑0.082	12.2
Metals
Mild Steel (1018) AISI 1018 carbon steel	7870	486	13.60	R‑0.0028	357
Structural Steel (A36) ASTM A36	7850	460	13.80	R‑0.0029	345
Stainless Steel (304) AISI 304 austenitic SS	7960	500	4.07	R‑0.0089	112

† CMU: An 8" normal-weight hollow concrete block has an assembly R-value of ~R-1.11. Because the hollow cores dominate the heat path, a per-inch value is not a useful comparison to solid materials.

All values are representative midpoints. Sources: ASHRAE Handbook of Fundamentals, NIST NSRDB, manufacturer data sheets (Havelock, Gutex, TimberHP).