What R-value do I need for exterior walls?

The IRC prescriptive requirements vary by climate zone: R-13 for zones 1-3 (southern US), R-20 or R-13 plus R-5 continuous insulation for zone 4, and R-20 or R-21 for zones 5-8 (northern US and Canada). Many states adopt the IECC energy code, which may require higher values. Check the energy code for your specific zip code.

Fiberglass vs. mineral wool batts — which is better?

Mineral wool (Rockwool) is denser, provides better soundproofing (STC 45+ vs. STC 35 for fiberglass), is non-combustible (melting point above 2,000 degrees F), and resists moisture without losing R-value. Fiberglass is 30-40% less expensive and lighter to handle. Both provide approximately R-3.2 to R-3.8 per inch of thickness.

How many batts of insulation do I need?

Divide the total wall or ceiling area by the coverage per batt (check the package label — a standard 15-inch by 93-inch fiberglass batt covers about 40 sq ft per bag of 10 batts). For 1,200 sq ft of wall area, you need about 30 bags. Add 10% for waste from cutting around outlets, pipes, and irregular framing.

Can I use R-13 insulation in a 2x6 wall?

You can, but it would leave a 2-inch air gap in the cavity and would not meet energy code requirements in most climate zones. The R-13 batt is designed for 2x4 walls (3.5-inch cavity). For 2x6 walls, use R-19 or R-21 batts that fill the full 5.5-inch cavity depth.

Do I need a vapor barrier with insulation?

In cold climates (zones 5-8), a Class I or Class II vapor retarder (kraft-faced batts or a separate polyethylene sheet) is required on the warm side of the insulation to prevent moisture from condensing inside the wall. In warm, humid climates (zones 1-3), a vapor retarder is generally not recommended on the interior side, as it can trap moisture inside the wall.

What R-value insulation do I need for my attic?

Attic insulation requirements are the highest in the home because heat rises and the attic is the primary heat loss pathway. The IECC recommends R-38 for zones 1-3, R-49 for zones 4-5, and R-60 for zones 6-8. Many homeowners install blown-in fiberglass or cellulose over existing batts to reach the target R-value.

Cavity Insulation Calculator

How many insulation batts do I need? Free calculator for mineral wool and PIR insulation — covers standard wall cavities.

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How to Use This Calculator

1Measure total wall or ceiling area to be insulated.
2Click Calculate.

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About This Material

Cavity insulation fills the space between studs, joists, or rafters to slow heat transfer through the building envelope. The two most common batt insulation materials are fiberglass (Owens Corning, Johns Manville, Knauf) and mineral wool (Rockwool). Standard R-values for common wall cavities are: R-13 or R-15 for 2x4 walls (3.5-inch cavity), R-19 or R-21 for 2x6 walls (5.5-inch cavity), and R-30 to R-38 for floor and ceiling cavities. Attic insulation requirements range from R-38 in mild climate zones to R-60 in cold zones (5-8). Fiberglass batts are the least expensive option at $0.50 to $1.00 per sq ft, while mineral wool batts cost $1.00 to $1.50 per sq ft but offer better soundproofing, fire resistance (non-combustible), and moisture resistance. Batts are available in 15-inch widths for 16-inch framing and 23-inch widths for 24-inch framing. Spray foam insulation is an alternative that fills cavities completely and air-seals simultaneously but costs 2 to 4 times more than batts.

Next Steps:

Drywall Sheet Calculator →Exterior Sheathing Calculator →

Installation Tips

•Fill cavities completely — compressed or gapped insulation loses R-value.
•Do not leave gaps around electrical boxes and pipes.
•Wear a respirator when handling fiberglass batts.

Common Mistakes to Avoid

Compressing batts — compression significantly reduces R-value.
Leaving air gaps at top plates and bottom plates.
Not installing vapor retarder in cold climates.

Cavity Insulation Calculator

How to Use This Calculator

About This Material

Installation Tips

Common Mistakes to Avoid

Frequently Asked Questions

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