Attic insulation calculator: bags and R-value

How it works

Enter the attic floor area in square feet, choose your IECC climate zone and estimate the R-value of the insulation already there. For existing insulation, select its material and enter its average depth. The calculator estimates the existing R-value from typical R per inch, then selects the next supported product layer that covers the gap to the zone reference.

The zone values shown are 2021 IECC recommendations for an uninsulated attic ceiling. They are a planning reference, the locally adopted energy code, amendments, building conditions and existing assembly may differ. This estimate does not determine code compliance or prescribe a safe retrofit design.

Bag counts use the manufacturer’s published minimum bags per 1,000 sq ft for the selected ProPink L77 R-value row, scaled to your area and rounded up to whole bags. The chart’s coverage and depth are estimates, check the current product label and installation instructions before buying.

Formula

• Estimate existing insulation

  Estimated existing R-value = average depth (in) × typical R per inch

  The selected material factor is a general estimate. Mixed, settled, wet or compressed material can perform differently.

• Find the remaining R-value gap

  R-value to add = max(0, zone ceiling reference − estimated existing R-value)

  If the entered insulation already meets the reference, the calculator estimates no additional product.

• Choose a product layer and count bags

  Selected layer = next supported product R-value at or above the gap Bags to buy = ceil(attic area ÷ 1,000 sq ft × published bags per 1,000 sq ft)

  The estimate uses the manufacturer’s bag-rate chart, not rounded displayed coverage.

Worked example

1. 1

   Use the zone reference

   IECC zone 2 reference = R-49

   This is the 2021 IECC uninsulated-attic ceiling reference used by the example.

2. 2

   Estimate the remaining insulation

   R-49 target − R-30 existing = R-19 to add

   Assume the existing material has already been assessed as about R-30.

3. 3

   Select product and calculate bags

   Next charted layer = R-19 1,000 sq ft ÷ 1,000 × 8.1 bags = 9 bags

   The charted layer adds approximately R-19, so the estimated total is R-49.

Assumptions and safety

This single-product example does not account for joist depth, existing material combinations, air sealing, ventilation, moisture, roof geometry, obstructions, settling, installation equipment or waste. Follow the product maker’s instructions and confirm that the assembly is appropriate for your home.

Do not disturb vermiculite unless an approved laboratory has confirmed that it does not contain asbestos. Protect yourself from dust, keep soffit ventilation clear and follow the cited safety guidance around recessed lights and other heat sources. Ask a qualified insulation professional if the existing attic conditions are uncertain.

Frequently asked questions