Short answer: you need 27–30 bags of blown fiberglass to insulate a 1,000 sq ft attic to R-38. At typical manufacturer coverage of ~36 sq ft per bag at R-38, the exact math is 1,000 ÷ 36 = 27.8 → 28 bags, plus 1–2 spare for obstructions and touch-ups.
Targeting a different R-value? Our free Insulation Calculator gives bag counts for any R-value and attic size.
Worked Example: The Math for R-38
Step 1 — Confirm what R-38 means physically. R-38 in blown fiberglass installs at roughly 10–12 inches deep (check your product’s bag chart — depth varies slightly by manufacturer density). This is the depth you are buying.
Step 2 — Get per-bag coverage at R-38. Read the bag’s coverage chart — do not use the R-30 or R-13 figure. Typical: ~36 sq ft per bag at R-38 for standard blown fiberglass.
Step 3 — Divide the attic area: 1,000 ÷ 36 = 27.8 → 28 bags.
Step 4 — Add contingency: attics have eaves, hatches, HVAC platforms, and recessed lights that waste material. Add 1–2 bags: buy 29–30 bags total.
If you already have insulation: measure it. Six inches of old fiberglass ≈ R-19. To reach R-38 you only add R-19 — about 14 more bags at R-19 coverage (~70 sq ft/bag). Do not buy the full 28 if half the job is already done.
Why R-38 Specifically
R-38 is the Department of Energy’s recommended attic minimum for most US climate zones (zones 2–4) and the practical sweet spot of cost versus performance. Below R-38, heating/cooling losses climb steeply; above R-38 (to R-49/R-60), each extra bag buys diminishing returns. If you are in zones 5–8 (northern US), target R-49 instead — about 36 bags for this attic.
Energy savings context: upgrading a 1,000 sq ft attic from R-19 to R-38 typically trims heating/cooling bills 10–15%. At ~$1,200–$1,600 in DIY materials, payback is commonly 2–4 years depending on climate and energy prices.
What Changes the Bag Count
Product density. “High-performance” blown fiberglass achieves R-38 at slightly less depth, stretching coverage to ~40 sq ft/bag → 25 bags. Standard products sit at ~34–38. The bag chart is law — brand differences are real.
Cellulose instead of fiberglass. Blown cellulose at R-38 covers ~30 sq ft per ~25 lb bag → 1,000 ÷ 30 = 33.3 → 34 bags. Cellulose is denser, better at air-blocking, and often cheaper per bag ($30–$45), but heavier to handle.
Vaulted or obstructed sections. Sloped ceiling sections, skylight wells, and chimney chases cannot take full depth — you lose effective coverage. Walk the attic first and mentally add a bag per major obstruction cluster.
Metric Version
1,000 sq ft = 92.9 m². R-38 (US) = RSI 6.7, installed ~300 mm deep. At ~3.3 m² coverage per bag: 92.9 ÷ 3.3 = 28 bags.
Common Mistakes for R-38 Attics
Using the wrong coverage column. Bag charts show coverage at every R-value. Grabbing the R-30 figure (48 sq ft/bag → 21 bags) leaves the attic 30% under-insulated — the most expensive mistake on this list, because you will not know until energy bills disappoint.
Compressing insulation at the eaves. Stuffing 12 inches of insulation into a 6-inch eave gap crushes it to ~R-19 locally and blocks the soffit vent. Use baffles to hold the full depth back from the eaves.
Forgetting the attic hatch. An uninsulated hatch is a 6–10 sq ft hole in your R-38 blanket. Glue 2–3 layers of rigid foam to the hatch cover and weatherstrip the frame.
Blowing over knob-and-tube wiring. In older homes, insulation must not contact knob-and-tube wiring — it is a fire hazard. Have an electrician evaluate first; this is one case where “just blow over it” is dangerous and possibly a code violation. Defer to local code and a licensed electrician.
Frequently Asked Questions
How deep is R-38 blown insulation?
About 10–12 inches for fiberglass, 10–11 inches for cellulose. Depth varies by product density — always follow the installed-thickness column on your specific bag’s chart, verified with a ruler during installation.
How much does it cost to insulate a 1,000 sq ft attic to R-38?
DIY: roughly $1,000–$1,650 for 29–30 bags of fiberglass (blower often loaned free with 10+ bag purchase). Pro installation: typically $1,500–$3,000 including air sealing.
Is R-38 enough or should I go to R-49?
R-38 satisfies code minimums in zones 1–4 and is cost-effective everywhere. In zones 5–8, R-49 is recommended and worth the extra ~8 bags. Check the DOE zone map for your ZIP code.
Can I mix fiberglass and cellulose in one attic?
Yes — R-values add regardless of material. A common upgrade is blowing cellulose over existing fiberglass: the denser cellulose reduces air movement through the old layer. Just keep the total depth consistent and do not block soffit vents with either material.
Calculate any R-value with the Insulation Calculator, plus the Insulation Calculator Complete Guide and Insulation Cost Guide.
