How Much Insulation Do I Need?

The amount of insulation depends on where you're insulating (attic, wall, crawl space), the target R-value for your climate zone, and the insulation type. Here's how to calculate it.

The quick formula

Bags needed = (Area sq ft × Bags per sq ft at target R-value)

Each bag of blown-in fiberglass (25 lb) covers a specific sq ft at a specific R-value — check the coverage chart on the bag label or use the calculator below.

R-value recommendations by climate zone

DOE Zone	States (examples)	Attic R-value	Wall R-value
Zone 1–2	Hawaii, Florida (south)	R-30	R-13
Zone 3	Texas, Georgia, Arizona	R-38	R-13 to R-15
Zone 4	Virginia, Missouri, Kansas	R-38 to R-49	R-13 to R-21
Zone 5	Ohio, Pennsylvania, Colorado	R-49	R-21
Zone 6	Minnesota, Michigan	R-49 to R-60	R-21
Zone 7	Montana, Alaska interior	R-60	R-21+

Blown-in fiberglass: bags per 1,000 sq ft

Target R-value	Depth needed	Bags per 1,000 sq ft
R-19	7–8 in	13–15 bags
R-30	11–12 in	20–22 bags
R-38	14–15 in	25–28 bags
R-49	18–19 in	32–36 bags
R-60	23–24 in	39–44 bags

Based on standard 25 lb bags of blown-in fiberglass. Actual coverage varies by brand — check the bag label.

Batt insulation: rolls per 1,000 sq ft

Type	R-value	Fits wall/cavity	Sq ft per bag/roll
Fiberglass batt	R-13	2×4 wall	40–50 sq ft
Fiberglass batt	R-15	2×4 wall	40–50 sq ft
Fiberglass batt	R-19	2×6 wall	40–50 sq ft
Fiberglass batt	R-21	2×6 wall	40–50 sq ft
Fiberglass batt	R-38	Attic floor (2 layers)	40 sq ft

How to measure your attic

Measure the floor area — length × width in feet (measure the attic floor, not the roof pitch)
Check existing depth — use a ruler to measure how deep any existing insulation is
Calculate the gap — subtract existing R-value from target R-value to find what you need to add
Account for joist framing — joists block some coverage; add 5–10% to your bag count

Attic insulation depth chart

Existing depth	Approx existing R-value (fiberglass)	Needs adding for R-49
0 in	R-0	Full R-49
3.5 in	R-11	R-38 more
6 in	R-19	R-30 more
10 in	R-30	R-19 more
14 in	R-44	R-5 more

Use the Insulation Calculator for your exact attic size, current R-value, and target R-value.

Frequently Asked Questions

How many bags of insulation do I need for a 1,000 sq ft attic?

It depends on the target R-value and insulation type. For R-38 blown-in fiberglass, you need roughly 25–28 bags (25 lb bags) for 1,000 sq ft. Use the Insulation Calculator for exact bag counts based on your attic size and target R-value.

How do I know what R-value I need?

The US Department of Energy recommends R-38 to R-60 for attics in most of the US (Climate Zones 3–7). Southern states (Florida, Texas coast) can use R-30. Northern states (Minnesota, Maine) should target R-60. Check the DOE Zone Map for your area.