INSULATION BATTS

Enter your total wall length in feet (the perimeter of the area you're insulating, NOT the room area) and the wall height. Pick the R-value to match your wall or ceiling cavity: R-13 for standard 2×4 walls (the most common residential framing), R-19 for 2×6 walls (newer construction with deeper cavities for more insulation), R-30 or R-38 for ceilings and attics where building codes require higher R-values.

The result is the gross batt count (no subtraction for studs, doors, or windows). For ordering, this is what you want — the extra absorbs any edge cuts. The bag estimate matches the typical retail packaging of each R-value (R-13 sells 8 batts/bag; R-38 sells 4).

For a 1,500 ft² house with about 60 linear feet of exterior wall and 8-foot ceilings, using R-13 batts:

Wall area = 60 × 8 = 480 ft². At 32 ft²/batt: 480 ÷ 32 = 15 batts.

Bags needed: 15 ÷ 8 batts/bag = 2 bags.

At ~$50/bag for R-13 ($6.25/batt), materials cost $100. The same house with R-19 batts (24 ft²/batt) needs 20 batts in 4 bags at ~$60/bag = $240.

For ceilings: 1,500 ft² of ceiling at R-30 (24 ft²/batt) = 63 batts in 16 bags at ~$45/bag = $720. Ceiling insulation alone is typically the biggest insulation line item — and the highest-impact one for energy bills.

Stuffing batts. Compressing R-13 to fit a tight cavity drops the actual R-value to R-9 or R-10. Cut batts to fit; don't squish them.

Leaving gaps around outlets, switches, and windows. Air leaks defeat the insulation. Use spray foam (one can per 8–10 outlets) to seal small gaps before installing batts.

Insulating without a vapor barrier in cold climates. R-13 unfaced batts in a wall in Minnesota will trap moisture inside the cavity and rot the studs within a few years. Use kraft-faced batts or add a separate poly vapor barrier on the warm side.

Mixing R-values to save money. The lowest R-value in your wall determines effective performance. R-19 in 80% of the wall and R-13 in 20% performs like R-13 across the whole wall once thermal bridging is considered.

R-13: 32 ft²/batt for 16" o.c. 2×4 walls, 8 ft tall. R-19: 24 ft²/batt for 2×6 walls. R-30: 24 ft²/batt for ceilings. R-38: 16 ft²/batt for high-R ceilings.

Always wear a respirator (N95 or better) and long sleeves when handling fiberglass — itch lasts for days.

Code minimums vary by climate: ceiling R-38 in cold zones, R-30 in warm; walls R-13 minimum, R-19 in cold zones.

Spray foam outperforms batts (about R-6 per inch closed-cell vs R-3.5 per inch fiberglass) but costs 3–5× as much per ft².

Old fiberglass loses about 5% R-value per decade as it settles and absorbs dust. Worth replacing in attics older than 30 years.