Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate additional attic insulation depth and bags needed to reach a target R-value from your current level.
Adding attic insulation is usually a question of "how much more" rather than starting from scratch -- most attics already have some insulation, and the goal is topping it up to a recommended target R-value (commonly R-49 to R-60 for many US climate zones, though this varies significantly by region). This calculator works out the additional depth and bags needed using your specific product's own R-value-per-inch and bag-coverage ratings, both of which vary meaningfully between blown-in fiberglass, cellulose, and other products.
Enter your attic's floor area, your current R-value (an energy audit or visual depth-to-R-value chart for your existing insulation type can estimate this), your target R-value (check your climate zone's recommendation), your chosen product's R-value per inch, and its bag coverage per inch of depth (both printed on the product's coverage chart).
Additional R-Value = Target R-Value − Current R-Value
Additional Depth = Additional R-Value ÷ R-Value per Inch
Bags Needed = (Attic Area × Additional Depth) ÷ Bag Coverage per Inch
R-value-per-inch and bag coverage both vary significantly by product and even by target R-value within the same product line (blown-in insulation settles over time, so manufacturers publish coverage charts calibrated for specific installed R-values, not a single flat rate) -- always use your specific bag's printed coverage chart rather than a generic estimate.
Say an 800 square foot attic has R-30 of existing insulation and you want to reach R-60, using a product rated at 2.6 R per inch with bags that cover 50 square feet per inch of depth. The additional R-value is 30. Additional depth is 30 divided by 2.6, about 11.5 inches. Bags needed are 800 times 11.5 divided by 50, roughly 184.6, which rounds up to 185. If your existing R-value is already at or above the target, the tool shows no additional R-value needed instead of a negative number.
This is the input people struggle with most. Measure the depth of the existing insulation in several places, then multiply by the R-value per inch for that material type. Loose fill that has compacted over the years has less R per inch than fresh material, so a visual chart can overestimate. If you are unsure, use a conservative number, or ask an energy auditor. If the attic has patchy coverage, calculate the areas separately, since they will have different depths.
The bag count is only as good as the numbers you enter from the product label. Two brands of loose fill can differ significantly in coverage at the same R-value. Consider buying a few extra bags, since unopened bags are often returnable at retailers that allow it. Confirm your climate zone's recommended R-value with your local energy office or utility, and consider that some utilities publish rebates for attic upgrades.
A rough estimate: measure the existing insulation's depth in inches and multiply by that material's typical R-value per inch (loose-fill fiberglass is roughly 2.2-2.7 per inch, cellulose roughly 3.2-3.8 per inch) -- for precision, a home energy audit can assess it directly.
US Department of Energy recommendations range from about R-30 in warmer southern climate zones up to R-60 in the coldest northern zones -- check your specific climate zone's current recommendation.
Generally no -- blown-in insulation is typically added directly on top of existing insulation (as long as it's dry and not compressed/damaged), which is exactly what this "additional depth" calculation assumes.
Split it into sections with different current R-values, run the calculator for each, and add the bag counts.
That is a product and code question. Unfaced batts are commonly laid across existing insulation, but check the manufacturer's guidance for your situation.
Returns diminish as you add depth. The real gains are usually in reaching the recommended level and in sealing air leaks.