04 / Guidance and practical checks
How the gap and order are built
The zone step shows ENERGY STAR's current retrofit reference for an uninsulated attic or floor alongside the broader range in the Department of Energy's older guide. You choose the total R-value to plan for; the tool subtracts the effective R-value already present, then divides that difference by the selected product's installed R per inch. Packages are rounded up from the coverage on that product's label at the additional R. When existing effective R already meets or exceeds the chosen target, the result recommends no added layer or package purchase and needs no product record. A wall has no single numeric cavity target in the cited ENERGY STAR table: its advice depends on the framed cavity and exterior sheathing, so the wall target remains your assembly-specific entry.
Worked insulation gaps
The loaded example supplies zone 5, 1,000 sq ft of attic floor, sound existing R11, chosen target R60, and R3 per installed inch. It deliberately leaves the product and coverage blank, so the tool reports the missing label record instead of a package order. To reproduce the arithmetic only, enter an illustrative product label and 40 sq ft per bag at added R49: expect an R49 gap, about 16.3 in added depth, and 25 bags. Replace that illustrative coverage with the exact current product chart before planning a purchase.
A zone 3 floor with no existing layer uses ENERGY STAR's R19 reference. At a selected batt's labeled R3.2 per inch, the estimated added depth is about 5.9 in. For 600 sq ft and a product bundle covering 48 sq ft at R19, round 600 ÷ 48 up to 13 bundles. Check whether the floor cavity and facing actually permit that assembly.
Inspect before counting packages
Measure attic floor area and inspect the condition of the existing layer; choose an effective R credit only after checking gaps, compression, and dampness. Verify air sealing, baffles, clearances, and assembly suitability, then read coverage at the calculated added R from the selected product's bag chart. Compare the displayed package count with that same chart before buying.
Common insulation planning mistakes
- Do not credit compressed, wet, or displaced insulation with its full printed R without checking its condition.
- Use the bag chart's coverage at the calculated added R, not a tempting coverage figure at a thinner layer.
- Air-seal penetrations and check ventilation baffles before burying an attic floor.
- Confirm vapor-control and facing details for your climate and assembly; the tool cannot decide them from a zone alone.
Questions before buying insulation
Why is wall mode different from attic mode?
Heat flow through studs and sheathing makes a wall an assembly problem. ENERGY STAR describes cavity fill and exterior sheathing by zone instead of one total cavity number for every existing wall, so a wall R target must come from the actual wall design.
Can I count existing batts by their printed R?
Only if their installed condition supports it. Gaps, compression, dampness, and inconsistent depth can lower the effective layer. An unknown condition should be inspected before using that credit in the gap.
Why does changing product coverage change packages but not depth?
Depth follows the entered R per inch. Package count follows labeled coverage at the added R. Those are separate product properties; neither can be inferred safely from the words “batt” or “blown.”
Source and planning boundary
ENERGY STAR's retrofit R-value table supplies the current point targets and wall notes; it cites the 2021 IECC as background. The DOE Guide to Home Insulation, published October 2010, supplies the historical broad ranges shown for context. Both sources were reviewed September 24, 2026. This planning display does not establish a local code requirement, safe fire clearance, moisture control, or product compatibility. Verify the zone, substrate, label coverage, and assembly with a qualified installer before work.