Pink fiberglass insulation batts fitted between wall studs

Reference

Insulation R-value chart: R-value per inch and how thick R-30, R-38 and R-49 are

R-value is resistance to heat flow, and it adds up by the inch. The same R-38 is about 13.5 inches of blown fiberglass, 12 of cellulose or 6 of closed-cell foam — so the question is always two questions: what R do you need, and how many inches of what gets you there.

How thick is R-30, R-38 and R-49? About 10.75, 13.5 and 17 inches of blown fiberglass; 9.6, 12 and 15.2 inches of cellulose. Most US attics need R-38 to R-60.

Insulation R-value chart: R-value per inch by insulation type

Insulation	R per inch	Form
Blown-in fiberglass (loose fill)	R-2.9 per inch brand chart; loose fill runs R-2.2–2.7	Blown
Blown-in cellulose	R-3.5 per inch brand chart; loose fill runs R-3.2–3.8	Blown
Fiberglass batts and rolls	R-3.1 per inch	Batt
Mineral wool batts	R-4.2 per inch	Batt
Open-cell spray foam	R-3.6 per inch	Spray
Closed-cell spray foam	R-6.5 per inch	Spray
Rigid foam, EPS	R-3.8 per inch	Rigid
Rigid foam, XPS	R-5 per inch	Rigid
Rigid foam, polyiso	R-6 per inch	Rigid

How thick is R-13, R-19, R-21, R-30, R-38, R-49 and R-60

Inches of each material for a given R. Blown-in rows are the depths printed on the manufacturer's coverage chart — cellulose as blown, before it settles about 10%; batt rows are the product thickness where that R is made, and two layers where no single batt reaches it. Everything else is R ÷ R-per-inch.

Insulation	R-13	R-19	R-21	R-30	R-38	R-49	R-60
Blown-in fiberglass (loose fill)	4.5 in	7 in	7.2 in	10.75 in	13.5 in	17 in	20.5 in
Blown-in cellulose	4.3 in	6.2 in	6.0 in	9.6 in	12 in	15.2 in	18.4 in
Fiberglass batts and rolls	3.5 in	6.25 in	5.5 in	10 in	12 in	2 layers	2 layers
Mineral wool batts	3.1 in	4.5 in	5.0 in	7.25 in	2 layers	2 layers	2 layers
Open-cell spray foam	3.6 in	5.3 in	5.8 in	8.3 in	10.6 in	13.6 in	16.7 in
Closed-cell spray foam	2.0 in	2.9 in	3.2 in	4.6 in	5.8 in	7.5 in	9.2 in
Rigid foam, EPS	3.4 in	5.0 in	5.5 in	7.9 in	10.0 in	12.9 in	15.8 in
Rigid foam, XPS	2.6 in	3.8 in	4.2 in	6.0 in	7.6 in	9.8 in	12.0 in
Rigid foam, polyiso	2.2 in	3.2 in	3.5 in	5.0 in	6.3 in	8.2 in	10.0 in

Recommended R-value by climate zone

Zone 1 is south Florida and Hawaii; 2 the Gulf Coast and Arizona; 3 the South and coastal California; 4 the mid-Atlantic and Pacific Northwest; 5 the lower Midwest and Northeast; 6 the upper Midwest and northern New England; 7 northern Minnesota and the Rockies. Figures are the 2021 code for new homes. From zone 4 north a wall needs R-20 in the cavity plus R-5 of foam board over the studs (or R-13 plus R-10), which means 2×6 framing; a 2×4 wall tops out at R-15. When topping up an attic that already has 3–4 inches, ENERGY STAR sets the bar one step lower — R-38 in zones 2–4, R-49 in 5–7.

Zone	Attic	Walls	Floors
Zone 1	R-30	R-13	R-13
Zone 2	R-49	R-13	R-13
Zone 3	R-49	R-20	R-19
Zone 4	R-60	R-20 + R-5 board	R-19
Zone 5	R-60	R-20 + R-5 board	R-30
Zone 6	R-60	R-20 + R-5 board	R-30
Zone 7	R-60	R-20 + R-5 board	R-38

Batt and roll sizes and how much a bag of blown-in covers

Blown-in	One bag at R-30	One bag at R-38	One bag at R-49	One bag at R-60
Blown-in fiberglass (loose fill) (27.5 lb bag)	66.7 sq ft	52.6 sq ft	40 sq ft	31.3 sq ft
Blown-in cellulose (19 lb bag)	21.7 sq ft	16.4 sq ft	12 sq ft	9.3 sq ft

Batts	Widths	R-values	Sq ft per package
Fiberglass batts and rolls	15 and 23 in (16 and 24 in framing)	R-11, R-13, R-15, R-19, R-21, R-25, R-30, R-38	R-13: 106.56 · R-19: 77.5 · R-21: 67.81 · R-30: 58.67 · R-38: 42.67
Mineral wool batts	15.25 and 23 in (16 and 24 in framing)	R-15, R-23, R-30	R-15: 59.7 · R-23: 39.8 · R-30: 29.9

How to measure what you have

Push a ruler through the attic insulation to the drywall and read the depth, then multiply by the R per inch for what you see: pink or yellow fluff is fiberglass, gray newsprint-looking material is cellulose, batts have a paper or foil face. Cellulose settles 10–20% in its first years, so an old layer reads at its settled depth and a new one is blown deeper than the chart's settled figure. Loose fiberglass from different brands runs R-2.2–2.7 per inch, so the same R can need a few more inches — follow the chart on your own bag. Subtract what you have from what your zone needs; the difference is what to add on top. The attic insulation calculator turns that into bags or batts.

Work out your insulation →

Recommended R-values by climate zone: IRC 2021 Table N1102.1.3 and U.S. Department of Energy / ENERGY STAR, Recommended Home Insulation R-Values. Blown fiberglass: Owens Corning AttiCat coverage chart (27.5 lb bag, open attic: 15, 19, 25 and 32 bags per 1,000 sq ft at R-30, 38, 49 and 60). Cellulose: Greenfiber INS541LD coverage chart (19 lb bag) for bags and installed depth; settled depth at 90% of installed, the ratio on Greenfiber's own INS515LD card (11.60 in installed → 10.44 in settled at R-38). Batts: Owens Corning EcoTouch and Johns Manville fiberglass batt data (R-13 at 3½ in, R-19 at 6¼ in, R-21 at 5½ in, R-30 at 10 in, R-38 at 12 in); Rockwool Comfortbatt data. Spray foam and rigid foam: U.S. Department of Energy insulation materials guide and manufacturer data. Generic loose-fill ranges (fiberglass R-2.2–2.7 per inch, cellulose R-3.2–3.8 settled) and cellulose settling of 10–20%: NREL insulation fact sheet and PNNL Building America Solution Center; existing layers are read at the middle of those ranges (rock wool loose fill R-2.2–3.3, DOE). Wall requirements: IRC 2021 Table N1102.1.3 via PNNL BASC. Vermiculite: U.S. EPA, Protect Your Family from Asbestos-Contaminated Vermiculite Insulation. Batt lengths: Owens Corning EcoTouch (93 in wall, 48 in attic) and Rockwool Comfortbatt (47 in); 24 in batts: Owens Corning R-30 W67A 80 sq ft, R-38 W65A 64 sq ft per bag. Attic prep — air sealing first, dams taller than the finished depth around the hatch and flues, baffles with a 1 in air gap, one depth ruler per 300 sq ft: U.S. Department of Energy attic floor preparation job aid. Accessed 2026-10-17. Coverage per bag varies by brand and the installed density; the chart printed on the bag is the one to follow.

R-value questions

How thick is R-38 insulation?

About 13.5 in of blown fiberglass, 12 in of cellulose, or a 12 in fiberglass batt.

What R-value do I need in my attic?

R-30 in south Florida, R-49 across the South and R-60 from zone 4 north under the 2021 code. Topping up an attic that already has 3–4 inches, ENERGY STAR accepts one step lower: R-25 to R-30 in zone 1, R-38 in zones 2–4 and R-49 in zones 5–7.

What is the R-value of closed-cell spray foam?

About R-6.5 per inch — the highest of common insulations; open-cell is about R-3.6.

Can I add new insulation over old?

Yes, as long as the old is dry. Lay unfaced batts or blow loose fill on top; the R-values add.

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