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Calcimator

Attic Insulation Calculator

Insulation depth and material from current and target R-value.

About this calculator

This calculator works out how much new insulation an attic needs to close the gap between its current and target R-value, plus the material cost and estimated energy savings. Insulation material has a real effect on required thickness: at the default R-value gap, switching from closed-cell spray foam (R-6.5 per inch, the densest option) to blown fiberglass (R-2.5 per inch, the least dense) requires roughly 2.6 times the thickness to close the same R-value gap. The R-value gap itself — set by Target R-value and Current R-value — moves thickness even more across its full range, since a bigger gap simply needs more inches no matter which material is chosen; raising Target R-value from its low end to its high end can swing required thickness further than switching between the densest and least-dense material ever can.

Attic area is completely inert to thickness needed, R-value achieved, and estimated energy savings — it only scales the total volume, bag count, and material cost, since square footage has no bearing on how many inches of insulation close an R-value gap. Estimated annual savings depends only on the R-value gap (target minus current) and is capped at 50% of a $2,000/year baseline HVAC cost — at the calculator's default 38-point gap, savings are already at that 50% ceiling, so raising the target R-value further, or changing insulation material or attic area, produces zero additional savings in this estimate even though it keeps raising material cost. Total R-value achieved is a direct construction, not an independent estimate: thickness is solved backward from the R-value gap, so the result always comes out exactly equal to Target R-value (or to Current R-value if that's already at or above target) — it does not model any real-world loss or variance in how insulation performs once installed.

Inputs

sq ft

Results

Thickness needed

10.27 in

≈ 3 credit cards

Est. material cost

$4,108.00

≈ 4 smartphones

R-value to add38
Total R-value achieved49
Insulation volume855.9 cu ft
Bags needed (blown-in)32
Est. annual energy savings$1,000.00
How to Use This Calculator
  1. Enter Attic area — it scales volume, bag count, and cost, but never the required thickness or R-value.
  2. Enter Current R-value and Target R-value to set the R-value gap the new insulation needs to close.
  3. Choose Insulation material — R-value per inch ranges from 2.5 to 6.5 across the six options, a real 2.6x swing in thickness at a fixed R-value gap, though the gap itself (Target minus Current R-value) can move thickness even further across its own range.
  4. Review Thickness needed (in), R-value to add, and Est. material cost. Total R-value achieved is not an independent check — it's solved backward from Target R-value, so it will always match your target (or Current R-value if that's already higher).
  5. Check Est. annual energy savings, keeping in mind it's capped at 50% of a $2,000 baseline and may already be at that ceiling for a large R-value gap.

How the result changes with Target R-value

Target R-valueThickness neededEst. material cost
253.78 in$1,514.00
377.03 in$2,811.00
7417.03 in$6,811.00
10024.05 in$9,622.00

What each input means

Attic area
Total attic floor area to be insulated.
Current R-value
Existing insulation R-value (0 if bare). R-11 is typical for old homes.
Target R-value
Desired total R-value. DOE recommends R-49 to R-60 for most climates.
Insulation material
Type of insulation material to install.

What each result means

Thickness needed
Depth of new insulation required to reach target R-value.
R-value to add
Additional R-value needed beyond existing insulation.
Total R-value achieved
Combined R-value of existing plus new insulation. This always equals your Target R-value exactly (or Current R-value if it's already at or above target) — thickness is solved backward from the gap, not rounded to a stock size.
Insulation volume
Total volume of insulation material needed.
Bags needed (blown-in)
Number of bags for blown-in types. Shows 0 for batts/spray foam.
Est. material cost
Approximate material cost (labor not included).
Est. annual energy savings
Estimated yearly HVAC savings based on DOE guidelines.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Attic area = 1000, Current R-value = 11, Target R-value = 49, Insulation material = 2 = 4 input(s) provided
  2. Calculate Thickness needed
    10.27 = 10.27
  3. Calculate Est. material cost
    Est. material cost = atticArea * thicknessNeeded * costPerSqftInch
    4108 = $4,108
  4. Calculate R-value to add
    R-value to add
    38 = 38
  5. Calculate Total R-value achieved
    Total R-value achieved = currentRValue + thicknessNeeded * rPerInch
    49 = 49

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

How much does the insulation material I choose affect the required thickness?

Material sets R-value per inch, which ranges from 2.5 (blown fiberglass) to 6.5 (closed-cell spray foam) across the six options — over a 2.5x spread. At the default R-value gap, switching from the densest to the least dense material requires roughly 2.6 times the thickness. That's a real, meaningful effect, but the R-value gap itself (Target R-value minus Current R-value) moves thickness even more across its full range, since a bigger gap always needs more inches regardless of material.

Why does 'Total R-value achieved' always exactly equal my target?

Because it's solved backward from the target, not independently modeled. The calculator computes the thickness needed to close the R-value gap, then multiplies that thickness back out by R-value per inch — algebraically, that always returns exactly your Target R-value (or your Current R-value if it's already at or above target). It isn't a real-world verification that the installed insulation performs at its rated R-value; it's a restatement of the target you already entered.

Does a bigger attic need thicker insulation?

No. Attic area has zero effect on Thickness needed, R-value to add, Total R-value achieved, or Est. annual energy savings — it only scales how much material and how many bags that thickness requires across the floor area, and therefore the total cost.

Why doesn't raising my target R-value increase my estimated savings?

Estimated annual savings is capped at 50% of a $2,000/year baseline HVAC cost. At the calculator's default settings the R-value gap (38 points) is already large enough to hit that 50% ceiling, so raising the target further, switching material, or changing attic area adds no additional estimated savings even though material cost keeps rising.

What does 'Bags needed' show for spray foam or batt insulation?

Zero. The bag-count formula only applies to the two blown-in material options (blown fiberglass and blown cellulose); batts come in pre-cut panels and spray foam is applied wet, so neither is sold or estimated in bags. Use Insulation volume for those material types instead.

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