R-38 Insulation Calculator
R-38 fiberglass batts are 12 to 12.5 inches thick; R-38C for cathedral ceilings is 10.25 inches and fits a 2×12 rafter with an inch to spare for venting. A bag of 16 × 48 in batts covers 42.66 sq ft, so a 1,000 sq ft attic with 10% waste takes 26 bags. Enter your area and climate zone to see what it takes to reach the code level.
Your project
Covers cuts, compression and odd bays. 10% is the usual default.
Your results
- Bags to buy
- 26
- 42.66 sq ft per bag · 1,100 sq ft with waste
- Batts
- 207
- 5.33 sq ft each
- Thickness
- 12.5 in
- R-3.04 per inch
- Fits without compressing
- Deeper than a 2×12 — attic floor
- 2021 IECC attic level (zone 5)
- R-60: R-22 short
- Top up with about 8.8 in of blown fiberglass, or a second layer of unfaced batts across the joists
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
Gear for this job
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Prices as of Oct 1, 2026, 4:38 AM UTC. Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
How this calculator works
- Coverage = area × (1 + waste %)
- Bags = ceil(coverage ÷ sq ft per bag); batts = ceil(coverage ÷ (width × length ÷ 144))
- R per inch = R-value ÷ thickness
- Attic shortfall = 2021 IECC ceiling R for the zone − batt R; blown depth = shortfall ÷ R-2.5 per inch
R-38 fiberglass batts and rolls: sizes, thickness and bag coverage
| Product | Thickness | Pieces per bag | Sq ft per bag |
|---|---|---|---|
| 16" × 48" batts, 12.5" (16" o.c.) | 12.5 in | 8 | 42.66 |
| 24" × 48" batts, 12.5" (24" o.c.) | 12.5 in | 8 | 64 |
| 16" × 48" batts, 12" (9 per bag) | 12 in | 9 | 48 |
| R-38C 15.5" × 48" (cathedral, 16" o.c.) | 10.25 in | 8 | 41.33 |
| R-38C 23.75" × 48" (cathedral, 24" o.c.) | 10.25 in | 8 | 63.33 |
Johns Manville Fiberglass Manufacturers Fact Sheet (wood framing, kraft-faced and unfaced). Other brands are close but not identical — use the square feet printed on your bag.
Bags of R-38 for common areas (10% waste)
| Area | 16" × 48" batts, 12.5" | 24" × 48" batts, 12.5" |
|---|---|---|
| 100 sq ft | 3 bags | 2 bags |
| 250 sq ft | 7 bags | 5 bags |
| 500 sq ft | 13 bags | 9 bags |
| 1,000 sq ft | 26 bags | 18 bags |
| 1,500 sq ft | 39 bags | 26 bags |
Bags = area × 1.10 ÷ sq ft per bag, rounded up.
Fiberglass batt R-values and thickness
| R-value | Thickness | R per inch | Fits |
|---|---|---|---|
| R-11 | 3.5 in | 3.14 | 2×4 wall |
| R-13 | 3.5 in | 3.71 | 2×4 wall |
| R-15 | 3.5 in | 4.29 | 2×4 wall (high density) |
| R-19 | 6.5 in | 2.92 | 2×8 floor/ceiling; compresses in 2×6 |
| R-21 | 5.5 in | 3.82 | 2×6 wall (high density) |
| R-30 | 10.25 in | 2.93 | 2×12 or attic floor |
| R-38 | 12.5 in | 3.04 | Attic floor |
| R-49 | 13.5 in | 3.63 | Attic floor |
Johns Manville fact sheet thicknesses; lumber depths 2×4 = 3.5 in, 2×6 = 5.5 in, 2×8 = 7.25 in, 2×10 = 9.25 in, 2×12 = 11.25 in.
R-38 in an attic
The 2021 IECC asks for R-49 in zones 2–3 and R-60 in zones 4–8, so R-38 is short almost everywhere except zone 1 — R-11 short of R-49 and R-22 short of R-60. Laying R-38 unfaced batts across existing insulation is a common way to get there; blown fiberglass at about R-2.5 per inch needs another 4.4 in for R-49 or 8.8 in for R-60.
Older codes asked for R-38 in many zones, so it is also the level many homes already have. If yours is at R-38, the extra R-11 to R-22 has a smaller payback than the first R-30 — air sealing the ceiling first usually pays faster.
R-38C for cathedral ceilings
R-38C is a high-density batt 10.25 in thick. In a 2×12 rafter (11.25 in) it leaves an inch for a vent baffle under the roof deck. Use it where standard R-38 at 12–12.5 in won't fit.
Frequently asked questions
How thick is R-38 insulation?
12 to 12.5 inches for fiberglass batts and 10.25 inches for high-density R-38C (Johns Manville). Blown fiberglass at about R-2.5 per inch needs roughly 15 inches.
Is R-38 enough for an attic?
In climate zone 1, yes (the 2021 IECC asks for R-30). Zones 2–3 call for R-49 and zones 4–8 for R-60, so R-38 falls R-11 to R-22 short there.
How many square feet are in a bag of R-38?
Johns Manville lists 42.66 sq ft for eight 16 × 48 in batts (12.5 in), 48 sq ft for nine 12 in batts and 64 sq ft for eight 24 × 48 in batts. R-38C bags cover 41.33 and 63.33 sq ft.
R-38 vs R-49: what's the difference?
About an inch of batt and R-11. R-49 batts are 13.5 in thick; in zones 2–3 R-49 is the 2021 IECC attic level, so R-38 needs topping up there.
Can I lay R-38 over old insulation?
Yes, if the old insulation is dry. Use unfaced R-38 and lay it at right angles across the joists so it covers their tops — a paper facing between layers can trap moisture.
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Bags to buy
26