How Thick Is R-30 Insulation?
The short answer
R-30 insulation is roughly 9 to 10 inches (23-25 cm) thick as a standard fiberglass batt - the math is 30 ÷ R-3.2 per inch ≈ 9.4 in. Denser materials hit R-30 much thinner: cellulose needs about 8.6 in (22 cm), mineral wool 7.5 in (19 cm), and closed-cell spray foam only about 4.6 in (12 cm).
The rule works for any material and any target: thickness = target R-value ÷ the material’s R per inch. R-value measures resistance to heat flow, and it scales linearly with thickness, so the division is the whole calculation.
R-30 thickness by material
Nominal planning values (the same R-per-inch ratings the calculator uses - always confirm the manufacturer’s data sheet, since products vary by density):
| Material | R per inch | Thickness for R-30 |
|---|---|---|
| Blown fiberglass | R-2.5 | 12 in (30 cm) |
| Fiberglass batt | R-3.2 | 9.4 in (24 cm) |
| Cellulose | R-3.5 | 8.6 in (22 cm) |
| Open-cell spray foam | R-3.6 | 8.3 in (21 cm) |
| Mineral wool | R-4.0 | 7.5 in (19 cm) |
| EPS rigid board | R-4.0 | 7.5 in (19 cm) |
| XPS rigid board | R-5.0 | 6.0 in (15 cm) |
| Polyiso rigid board | R-6.0 | 5.0 in (13 cm) |
| Closed-cell spray foam | R-6.5 | 4.6 in (12 cm) |
Two takeaways from the table: blown fiberglass is the fluffiest route to R-30 (a full foot of depth), and closed-cell spray foam gets there in under 5 inches - which is why it shows up where space is tight.
Worked example - R-30 between 2×10 joists
A 2×10 joist is actually 9.25 in deep. Fill that cavity completely and multiply depth by R per inch:
- Fiberglass batt: 9.25 × 3.2 = R-29.6 - a hair under R-30.
- Cellulose: 9.25 × 3.5 = R-32.4 - clears it.
- Mineral wool: 9.25 × 4.0 = R-37.0 - clears it with margin.
And layers add - two materials in series simply sum their R-values. An existing 6-inch R-19 batt topped with 2 in of polyiso board (2 × 6.0 = R-12) gives 19 + 12 = R-31, past R-30 without touching the batt.
👉 The R-Value Calculator does this instantly - pick the material and enter a thickness, and it returns the assembly R-value and compares it to the recommended R for your attic, wall or floor in your climate zone.
Is R-30 actually the right target?
R-30 is not a universal goal - the recommended total depends on the assembly and your climate zone (Energy Star / IECC guidance, zones 1 hot to 7 cold):
| Assembly | Where R-30 is the recommended target |
|---|---|
| Attic | Zone 1 only - zones 2-3 want R-49, zones 4-7 want R-60 |
| Floor | Zones 5-6 - zones 1-2 want R-13, zones 3-4 R-19, zone 7 R-38 |
| Wall | Never - wall targets run R-13 to R-21, and a stud cavity can’t fit 9 in of batt anyway |
So if you’re insulating an attic in most of the US, R-30 is a starting point, not the finish line - the colder two-thirds of the country calls for R-49 to R-60, which in blown fiberglass means roughly 20 to 24 inches of depth.
Watch-outs
- Compression destroys R-value. Squeezing a 9.4-in R-30 batt into a 7.25-in (2×8) cavity does not give you R-30 - the trapped air does the insulating, and compressing the batt squeezes it out. Match the batt to the cavity depth.
- R per inch is nominal. Manufacturers vary density, so an off-the-shelf R-30 batt can sit anywhere in the roughly 9-to-10-inch range - the bag or data sheet states the exact loft.
- Blown insulation settles. Installers blow it deeper than the target so it settles to the rated depth - follow the coverage chart on the bag, not just the fresh-blown height.
- Air-sealing matters as much as R-value. Gaps and bypasses leak heat regardless of thickness - seal first, then insulate. And code minimums vary by jurisdiction, so confirm your local energy code.
Related
- R-Value Calculator - assembly R-value from material and thickness, checked against the recommended R for your climate zone.
- Walls tools - the full walls toolset.
- Recommended attic R-value by climate zone - the zone-by-zone targets R-30 gets compared against.
- What is the minimum roof pitch for shingles - another build number, worked out the same way.