Fiberglass, Blown-In, or Spray Foam: What R-Value Means for Your Claim

R-value: the number that matters

R-value measures resistance to heat flow through a material. Higher R-value means better thermal performance. Your climate zone determines the minimum R-value required by the International Energy Conservation Code (IECC).

In warm climates like Florida (Zone 2), walls require R-13 and attics R-30 to R-38. In moderate climates (Zone 4), walls need R-13 to R-15 and attics R-38 to R-49. In cold climates (Zones 5-7), walls may require R-20 to R-21 and attics R-49 to R-60.

When insulation is replaced during a repair, it must meet the current code requirement, not just match what was there before. If your 1995 home had R-11 wall insulation and the current code requires R-13, the replacement goes in at R-13. This is a code upgrade covered under your Ordinance or LawYour Walls Are Open. Now the Inspector Wants $5,000 in Upgrades.Nobody warned me about this one. When the drywall came down on my claim, I thought we were just replacing what got damaged. Then the building inspe...
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Fiberglass batts: the standard

Fiberglass batts are pre-cut blankets of glass fiber insulation. They come in standard widths to fit 2x4 walls (R-13) and 2x6 walls (R-19). Material costs are $0.

50-$1. 00 per square foot. Installation adds $1.

00-$2. 00 per square foot. Batts are straightforward to install in open stud cavities, which makes them the most common and least expensive option.

But batts leave gaps around pipes, wires, electrical boxes, and any irregular framing. These gaps reduce the effective R-value because heat finds the path of least resistance. Batts also lose R-value if they are compressed or installed without filling the full cavity depth.

For a kitchen wall with 150 square feet of exposed cavity, fiberglass batt replacement runs $225-$450 installed.

Blown-in cellulose and fiberglass

Blown-in insulation is pumped into cavities through a hose. Cellulose, made from recycled paper treated with boric acid fire retardant, fills around pipes and wires better than batts because it flows into irregular spaces. Blown-in fiberglass works similarly but uses glass fibers instead of cellulose.

Cost is $1. 00-$2. 50 per square foot installed.

Blown-in is excellent for attics where you need to fill a large open space to a consistent depth. For retrofit wall cavities, it can be dense-packed through small holes without removing all the drywall. The tradeoff: blown-in cellulose can settle over time, losing 10-20% of its initial R-value in attic applications.

It also absorbs moisture more readily than fiberglass, which matters in humid climates.

Spray foam: premium performance

Spray foam is the highest-performing insulation per inch of thickness. Open-cell spray foam costs $1. 50-$2.

50 per square foot installed and provides R-3. 7 per inch. It expands to fill every gap and crack, creating an air barrier as well as a thermal barrier.

Closed-cell spray foam costs $2. 50-$4. 00 per square foot installed and provides R-6 to R-7 per inch.

Closed-cell also adds structural rigidity to walls and acts as a moisture barrier. In hurricane zones and flood-prone areas, closed-cell spray foam is increasingly specified because it resists water damage where fiberglass and cellulose would be ruined. If your existing insulation was spray foam, the replacement should be spray foam.

Not fiberglass. Not cellulose. Spray foam.

The cost difference between fiberglass batts and closed-cell spray foam on 150 square feet of wall cavity is $300-$600. That's enough to matter on your estimate.