Insulation goes in once — after rough-in, before drywall — and mistakes get sealed inside the wall. This checklist compresses the Drywall Insulation Complete Guide into checkable actions for before you calculate, order, install, and close up. Run your numbers in the Drywall Insulation Calculator as you go.
Before You Calculate
- Measure every wall’s length × height at the plates — the batt fills the stud bay, not the finished room dimension.
- Deduct all openings (doors, windows, pass-throughs): width × height each. Don’t deduct outlet boxes — waste covers those.
- Measure actual stud spacing in several bays per wall: 16 in o.c. → 15-in batts; 24 in o.c. → 23-in batts (see how to measure bays).
- Confirm cavity depth: 3.5 in (89 mm) for 2×4, 5.5 in (140 mm) for 2×6 — the batt’s R-value label must match.
- Use one unit system for the whole estimate. Mixing feet and inches without converting is the classic input error.
- Confirm your R-value target with the local AHJ / energy code before ordering (see the code compliance guide).
Before You Order
- Apply waste last, as a multiplier: 5–10% for batts (10% default for cutout-heavy walls).
- Round batts UP to the next whole batt — 39.5 means 40. Then convert to bags and round bags up separately.
- Match facing to the assembly: faced batts toward the warm side in heating climates; unfaced + separate barrier where code requires one — never two vapor retarders.
- Cross-check metric vs. imperial: both runs should agree within rounding (RSI = R × 0.176).
- Include air-sealing supplies in the order: caulk, canned foam, tape — insulation without sealing underperforms.
- Don’t forget interior sound walls (bedrooms, baths, offices) and the ceiling/attic if they’re in scope.
Before You Install
- Rough-in complete and inspected: electrical, plumbing, HVAC done — insulation goes in after trades, not before.
- Air sealing done: plates, penetrations, and gaps caulked/foamed before the first batt.
- PPE ready: respirator/dust mask, goggles, gloves, long sleeves — non-negotiable for fiberglass and mineral wool.
- Batts dry and staged indoors: wet batts lose performance — don’t install into damp cavities.
- Sharp knife + straightedge on hand: cut pieces ~1 in oversize for a friction fit, split (don’t stuff) around wires.
- Recessed lights checked: IC-rated for insulation contact; non-IC fixtures need manufacturer-specified clearance.
Before the Drywall Crew Arrives
- Every bay filled, no gaps or voids: check tops of bays, narrow bays beside windows/doors, and behind electrical boxes.
- No compressed batts: the batt label matches the cavity depth everywhere — an R-19 squeezed into a 2×4 bay performs like R-13.
- Facing direction correct on every faced batt (warm side in heating climates).
- Ceiling/attic insulation complete to target depth — verify with a ruler in several spots, not just at the hatch.
- Photograph everything: bay fill, facing, air sealing, ceiling depth — once drywall is up, this is the only record.
- Pre-drywall inspection passed (where required): R-value labels and documentation on site for the inspector.
Common Last-Minute Catches
Catch these now, not after the drywall is hung:
- Gross vs. net area. Forgetting to deduct a 40 ft² patio door overbuys ~4 batts. Re-check the deductions — see common mistakes.
- Wrong batt width on site. 15-in batts bought for what turned out to be 24-in o.c. framing — measure before you buy, not after.
- Skipped air sealing. Batts are in, plates aren’t sealed — pull what you must and seal; insulation over air leaks is half an assembly.
- One bay short. The order came up one batt short and someone suggests leaving a closet bay empty — don’t. Buy the batt.
- Wet cavity or wet batts. A roof leak or plumbing drip discovered mid-install — dry everything before closing up.
Quick Reference
Formulas: Net area = Σ(L × H) − openings · Batts = ceil(net ÷ coverage × (1 + waste)) · RSI = R × 0.176
Worked example: 12 ft × 8 ft wall − 15 ft² window = 81 ft² net; × 1.10 = 89.1 ft²; ÷ 9.69 ft² per 15×93 batt = 9.2 → 10 batts (see the formula explained guide).
| Item | Imperial | Metric |
|---|---|---|
| Stud spacing → batt width | 16 in o.c. → 15 in; 24 in o.c. → 23 in | 406 mm → 381 mm; 610 mm → 584 mm |
| Cavity → batt R-value | 2×4 → R-13; 2×6 → R-19/R-21 | 89 mm → RSI 2.29; 140 mm → RSI 3.34–3.70 |
| Batt coverage (15×93) | 9.69 ft² | 0.90 m² |
| Waste allowance | 5–10% (10% default) | |
| Attic targets | R-38 to R-60 | RSI 6.69 to 10.56 |
FAQ
How many batts do I need for a standard 12×8 wall?
About 10: 96 ft² minus a 15 ft² window = 81 ft² net; with 10% waste that’s 89.1 ft² ÷ 9.69 ft² per batt = 9.2 → 10. Run your walls through the Drywall Insulation Calculator for exact counts.
What waste percentage should I add?
5–10% for batts: 5% for simple walls, 10% for walls with many outlets, wires, and openings. Apply it as a multiplier on the net area.
Faced or unfaced batts?
Faced (kraft vapor retarder) toward the warm/interior side in heating climates; unfaced plus a separate barrier where code requires one. Never two vapor retarders trapping the cavity.
Do I need to insulate interior walls?
For sound, yes — bedrooms, bathrooms, and offices benefit from unfaced batts (roughly STC 33 empty to low-40s filled). It’s now-or-never before drywall.
What R-value does code require?
It varies by climate zone and jurisdiction — commonly around R-13 for 2×4 walls in warm zones, higher in cold zones, with attics at R-38 to R-60. Confirm with your local AHJ (see the code compliance guide).
Should I air seal before insulating?
Always — seal plates and penetrations first. Insulation slows conducted heat; only air sealing stops air bypassing it.
Disclaimer: results are estimates for planning purposes. Actual material needs vary with framing, cavity conditions, and product packaging — confirm final quantities with your supplier and R-value, vapor-barrier, and fire-safety requirements with your local building authority.
Run your numbers through the Drywall Insulation Calculator, then work this checklist top to bottom before the drywall crew arrives.
