Why one R-value-per-inch number is not enough
Cellulose is often listed near R-3.2 to R-3.8 per inch, while loose-fill fiberglass spans a broad product-dependent range that commonly overlaps R-3. Mineral wool loose-fill also varies. ENERGY STAR uses roughly R-3 to R-3.5 per inch as a practical estimate for common attic materials.
Density changes the result. Loose-fill can be installed at different weights per square foot, and cellulose settles after installation. Manufacturers account for those effects in coverage charts that connect target R-value, bags, area, and thickness.
Estimate existing R-value carefully
Take several depth measurements because attic insulation is rarely level. Average the undisturbed areas and note thin spots near eaves, hatches, and service paths. Multiplying depth by a planning R-value gives a screening estimate, not a certificate of performance.
Mixed layers need separate calculations. For example, estimate an older batt layer from its label or thickness and material, then estimate loose-fill above it. Do not count badly compressed, wet, or missing sections as though they perform like intact insulation.
Use bag count to catch a thin installation
- Ask for the product name, bag weight, target R-value, and total bags on the quote.
- Compare the promised area with the manufacturer's maximum coverage per bag.
- Photograph the bag labels and count empty bags during a DIY or contracted job.
- Check depth markers across the attic after the material has been distributed evenly.
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Product details, health guidance, and code references were checked against these sources.
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