Fiberglass Insulation Roll and Batt Calculator: Walls, Attic, and Floor Joists with
Calculate bags of fiberglass batt insulation needed for three US project zones: walls (2×4 or 2×6 framing), attic and ceiling, and floor joists over unconditioned space. Select your US DOE climate zone for recommended R-values. Choose your framing spacing for a net cavity area deduction. Results show bags per zone and total bags for a complete project order.
Enable up to three project zones: walls, attic, and floor joists. Enter net area for each zone and select your target R-value. Set your US climate zone to see DOE recommended R-values. Choose framing spacing for a net cavity deduction (16-inch OC removes 15% for studs). Results show bags per zone and total bags for your complete project order.
| Zone | R-Value | Area | Net Area | Bags |
|---|
Fiberglass Batt Insulation R-Values, US Framing Sizes, and DOE Climate Zone Recommendations
Fiberglass batt and roll insulation is the most commonly used residential insulation product in the United States, installed in hundreds of thousands of homes annually. It is available in standard widths that match US framing dimensions (15 inches for 16-inch on-center framing, 23 inches for 24-inch on-center framing) and in R-values calibrated to standard framing depths. The most important thing to understand about fiberglass batts is that the R-value you need varies significantly by where in the house you are insulating and what US climate zone you live in.
The US Department of Energy (DOE) publishes recommended R-values for existing homes and new construction that vary by climate zone (zones 1 through 8, from hottest to coldest), by application (walls, attic, floors), and by the existing insulation level. This calculator uses DOE zone-specific guidance to recommend the appropriate R-value for each zone in your project and then calculates the number of bags of fiberglass batt insulation needed to complete the work.
Standard US Fiberglass Batt R-Values and Applications
| R-Value | Thickness | Width | Application | Coverage per Bag (approx) |
|---|---|---|---|---|
| R-11 | 3.5 in | 15 or 23 in | 2×4 walls (older code minimum) | ~75 sq ft |
| R-13 | 3.5 in | 15 or 23 in | 2×4 walls (current standard) | ~73 sq ft |
| R-15 | 3.5 in | 15 in | 2×4 walls (premium / high density) | ~73 sq ft |
| R-19 | 6.25 in | 15 or 23 in | 2×6 walls, floor joists | ~56 sq ft |
| R-21 | 5.5 in | 15 in | 2×6 walls (high density) | ~60 sq ft |
| R-30 | 9.5 in | 15 or 16 in | Attic, floor joists | ~40 sq ft |
| R-38 | 12 in | 16 in | Attic (most US new construction) | ~32 sq ft |
| R-49 | 15 in | 16 in | Attic (northern US, cold climates) | ~24 sq ft |
| R-60 | 19 in | 16 in | Attic (severe cold, AK, zone 7-8) | ~20 sq ft |
How the Fiberglass Insulation Calculator Works
For each enabled zone, net area with framing deduction = gross_area x framing_factor (0.85 for 16-inch OC, 0.91 for 24-inch OC, 1.00 for no deduction). Net area with waste = net_area x waste_factor. Bags = ceil(net_area_with_waste / coverage_per_bag), where coverage_per_bag is the standard US manufacturer coverage for the selected R-value. Total bags = sum of all active zones. Coverage values per bag are approximate manufacturer specifications; always verify with your specific product label before ordering.
3 Real Fiberglass Insulation Estimates
Project: 1,400 sq ft ranch house, walls plus attic. Wall area (gross): 1,100 sq ft. Attic: 1,400 sq ft. Framing: 16-inch OC. Waste: 10%.
Walls (R-13, 16-inch OC): 1,100 x 0.85 = 935 sq ft net. With 10% waste: 1,029 sq ft. Bags: ceil(1,029/73) = 15 bags R-13.
Attic (R-38, no framing deduction for open attic floor): 1,400 x 1.00 = 1,400 sq ft. With 10% waste: 1,540 sq ft. Bags: ceil(1,540/32) = 49 bags R-38.
DOE Zone 4 recommendation: R-13 to R-15 for walls (matched), R-38 to R-60 for attic (R-38 meets minimum). Total: 64 bags.
Project: 1,800 sq ft two-story, 2×6 exterior walls plus attic. Wall area: 2,200 sq ft gross. Attic: 900 sq ft. Framing: 16-inch OC. Waste: 10%.
Walls (R-21, 2×6, 16-inch OC): 2,200 x 0.85 = 1,870 sq ft net. With waste: 2,057 sq ft. Bags: ceil(2,057/60) = 35 bags R-21.
Attic (R-49, Zone 6 requirement): 900 sq ft. With 10% waste: 990 sq ft. Bags: ceil(990/24) = 42 bags R-49.
DOE Zone 6 recommendation: R-13 to R-21 walls (R-21 meets upper recommendation), R-49 to R-60 attic (R-49 meets minimum). Total: 77 bags.
Project: Adding floor joist insulation over an unvented crawlspace. Floor area: 1,200 sq ft. Framing: 16-inch OC (2×10 joists, R-30 fits). Waste: 10%.
Floor joists (R-30, 16-inch OC): 1,200 x 0.85 = 1,020 sq ft net. With waste: 1,122 sq ft. Bags: ceil(1,122/40) = 29 bags R-30.
DOE Zone 4 recommendation for floors: R-25 to R-30. R-30 meets the upper end of the recommendation. Wire hangers (wire insulation supports, also called insulation supports or tiger claws) are needed to hold batts up against floor sheathing — plan for 1 bag of wire supports per 100 sq ft of floor joist area.
3 Expert Tips for Fiberglass Batt Insulation
When insulating stud walls, the studs themselves occupy floor-to-ceiling space that does not receive insulation — a batt cannot be installed inside a stud. For standard 2×4 walls framed at 16 inches on center, approximately 15 percent of the gross wall area consists of stud material. This means a 1,000-square-foot gross wall area has only about 850 square feet of actual cavity space available for insulation. This calculator applies a 15 percent deduction for 16-inch OC framing and a 9 percent deduction for 24-inch OC framing (which uses fewer studs per linear foot of wall). For attic applications where insulation is laid directly on the attic floor between joists, the deduction is smaller and many contractors apply no framing deduction at all for attic floor installations. For wall calculations, the framing deduction is important to avoid both over-ordering (if you forget the deduction) and under-ordering (if you apply the deduction but forget that doubled studs at corners and rough openings add back some solid material). The 15 percent figure for 16-inch OC framing is a standard US industry estimate that accounts for corner framing, headers, and typical door and window rough openings.
The R-value of fiberglass batt insulation is rated at its labeled thickness. When a batt is compressed to fit a shallower cavity than its rated thickness, the R-value drops proportionally because the insulating air pockets within the fiberglass matrix are collapsed. A common US installation error is using R-19 batts (rated for a 6.25-inch cavity) in a 5.5-inch deep 2×6 stud bay by compressing the extra 0.75 inches. The compressed R-19 in a 5.5-inch cavity achieves approximately R-17.5, not R-19. The correct product for a 2×6 stud bay is R-21 (rated at 5.5 inches, the exact depth of a 2×6 stud bay). Similarly, using R-30 batts (9.5 inches) in an 8-inch deep ceiling joist bay compresses the batt and reduces performance. The correct product for an 8-inch bay is R-25 or R-26. Always match the batt thickness to the cavity depth exactly, or use a product specifically rated for that cavity depth. Faced fiberglass batts installed in wall cavities should be installed with the kraft paper or foil face oriented toward the warm-in-winter side of the wall (toward the interior in cold climates). In mixed and hot climates in the southern US, the vapor retarder orientation recommendation differs — consult your local building code or building science consultant for mixed-climate vapor control requirements.
This is the most important building science principle for US homeowners doing DIY insulation projects, and it is ignored in virtually every mainstream US insulation tutorial. Fiberglass batt insulation is an excellent thermal resistor but a very poor air barrier. Air can move freely through the open fiber matrix of a fiberglass batt. If there are gaps, holes, or bypasses around the insulation (between the top plate and the ceiling drywall, around electrical boxes, at the sill plate, around pipe penetrations, or at the attic access hatch), outside air moves through those gaps and around the insulation, dramatically reducing the effective thermal performance of the insulated assembly. The DOE and the Building Performance Institute (BPI) both recommend air sealing before adding or upgrading insulation, especially in attics where bypasses are most common. In an attic, air sealing means: caulking or foaming all top plate gaps between the ceiling plane and the attic floor, sealing all electrical fixture boxes (using air-tight IC-rated fixtures or fire-rated air sealing covers), foaming around all penetrations (plumbing, HVAC ducts, chimney chases), and insulating and weatherstripping the attic access hatch. A properly air-sealed and insulated attic can reduce a US home’s heating and cooling energy use by 15 to 25 percent annually — the air sealing step alone accounts for much of that improvement. Fiberglass batts without air sealing provide roughly 60 to 70 percent of their rated thermal performance in a typical US house with average air leakage. For the DOE air sealing guide, visit energy.gov.
Frequently Asked Questions About Fiberglass Insulation
Related Insulation and Drywall Calculators
Net area = gross_area x framing_factor (16-inch OC: 0.85, 24-inch OC: 0.91, none: 1.00). Area with waste = net_area x waste_factor. Bags = ceil(area_with_waste / coverage_per_bag). Coverage per bag values: R-13 73 sq ft, R-15 73 sq ft, R-19 56 sq ft, R-21 60 sq ft, R-25 45 sq ft, R-30 40 sq ft, R-38 32 sq ft, R-49 24 sq ft, R-60 20 sq ft. These are approximate standard US manufacturer values; verify with your specific product label before ordering. DOE R-value recommendations based on US DOE Energy Saver guidelines for existing homes as of 2026. For current recommendations by ZIP code see energystar.gov. For DOE air sealing guidance see energy.gov/energysaver. Always wear personal protective equipment (N95 respirator, safety glasses, gloves, long sleeves) when handling fiberglass insulation. Last reviewed: August 2026.