Insulation Calculator

Every bag of blown-in insulation prints a coverage chart that lists how many square feet one bag covers at each R-value. Higher R-values mean a deeper layer, so one bag covers fewer square feet. Always read the bag for the R-value you want before counting bags.

Batts and rolls list coverage on the package too, usually as the total square feet per package. Match the batt width to your joist or stud spacing (16 or 24 inches on center) so the pieces fit without gaps.

Because coverage shifts with R-value and product type, treat 40 sq ft per bag as a starting example. Plug in the actual coverage from the bag you buy to get an accurate count for your project.

R-value describes resistance to heat flow. This calculator does not calculate the R-value of a cavity or add the thermal resistance of layers. Product labels connect bag count, installed thickness, settled thickness, density, and coverage for a stated R-value. Use the row for the exact product and target rather than transferring a coverage figure from another brand or material.

Measure the surface that will receive insulation. A rectangular attic floor can use length times width. Walls may need each section measured separately, and sloped roof decks have more surface area than their horizontal footprint. Openings, kneewalls, rim joists, vaulted areas, and additions should be handled as separate rectangles when their products or depths differ.

Whether to subtract windows, doors, hatches, ducts, or framing depends on the material and estimate. Batts occupy cavities between framing, while continuous boards cover framing differently. Small openings and cutting loss can offset each other. For a close purchase estimate, create an area schedule and apply the actual package coverage to each material group.

Installed performance depends on workmanship. Compression, gaps, wind washing, moisture, missing air sealing, blocked soffit vents, and insulation placed against heat-producing equipment can cause poor performance or hazards. Electrical, combustion, recessed-light, chimney, and ventilation clearances need the applicable product and code instructions.

Climate and assembly design determine the appropriate insulation level. Existing insulation, vapor retarders, roof ventilation, conditioned attics, wall type, and local moisture conditions can change the correct approach. More insulation is not automatically safe if it traps moisture or blocks required airflow. Check current local requirements or obtain a building-science assessment.

The result rounds upward to a whole bag after applying the entered waste percentage. It assumes every package has identical coverage and that partial packages cannot be purchased. It does not optimize package sizes, account for damaged material, or show leftover quantity. Keep receipts and follow retailer policies if unopened packages may be returned.