Attic Insulation Math: Turn a Recommended R-Value Into Inches, Bags and a Comparable Quote
Updated October 8, 2026
ENERGY STAR publishes a zone-by-zone table of recommended attic R-values for existing homes. Read whether a recommendation describes insulation to add or a final total before converting it into inches, bags and a comparable quote.
R-value measures how well insulation resists heat flow, and a higher R-value means better resistance. The math below separates an added layer from a final total.
Read the recommendation before doing the math
ENERGY STAR labels these columns as insulation to add. Select your zone and existing-depth column; do not subtract existing insulation from an already-additional recommendation.
| Zone | Add to uninsulated attic | Add when 3–4 inches already exist |
|---|---|---|
| 1 | R30 | R25 |
| 2 | R49 | R38 |
| 3 | R49 | R38 |
| 4A and 4B | R60 | R49 |
| 4C, 5 and 6 | R60 | R49 |
| 7 and 8 | R60 | R49 |
Subtract only from a separately stated final total
If a contractor specifies a final combined R-value, subtract the measured existing value once. Ask the installer to identify the material, inspect its condition and explain how existing insulation is counted.
Additional R-value = final combined target − measured existing R-value
That formula does not apply to the table above: its existing-depth column already specifies the added layer.
Turn the gap into inches and bags
Every number below is invented to show the steps. Replace each one with your own measurements and the figures printed on the product you choose.
Zone 4A home, attic already has 3–4 inches. The table calls for adding R49.
With a hypothetical existing R11 layer, adding R49 gives a nominal combined R60. Do not calculate 49 − 11.
The new loose-fill label says R3.0 per installed inch. 49 ÷ 3.0 = 16.33, so round up to 17 inches of added material. Check: 17 × 3.0 = R51.
The attic floor measures 1,200 square feet, and the bag label says one bag covers 40 square feet at that 17-inch thickness. 1,200 ÷ 40 = 30 bags. At a made-up $22 per bag, materials come to 30 × $22 = $660, before tools, labor or other items.
Real labels differ by product and by the thickness you need, so use the manufacturer’s coverage chart, not these numbers.
A second example: an uninsulated attic
Take a hypothetical Zone 3 home with no attic insulation. For this empty attic, the table calls for adding R49. The new material’s label says R3.5 per installed inch.
49 ÷ 3.5 = 14 inches exactly. The attic floor is 900 square feet, and the label says a bag covers 40 square feet at that depth. 900 ÷ 40 = 22.5, so round up to 23 bags. At the same made-up $22 per bag, 23 × $22 = $506 in materials.
Both examples add R49. Their different thicknesses reflect the hypothetical products’ R-values per inch. Bag coverage must be taken from each product’s chart at the required installed depth.
Seal and ventilate before you bury it
ENERGY STAR treats air sealing as its own attic project, separate from adding insulation. Ask whether gaps around pipes, wires and the attic hatch will be sealed first, and whether you need to keep insulation clear of recessed lights, flues and vents under the product instructions and local code. One manufacturer’s installer estimating form lists vent baffles and a barrier around the attic access as separate items, a good reminder that a quote should account for more than the blown-in material.
When to pause and call a professional
Stop and ask a qualified contractor or inspector before adding insulation if you see signs of moisture or mold, active roof leaks, bath fans or dryer vents that end in the attic, or old wiring and recessed lights that are not rated to be covered. These problems are not fixed by adding R-value, and insulation can make some of them harder to see.
Put these lines in every quote
| Line item | Why it matters | Quote A / Quote B |
|---|---|---|
| Added R-value, final combined R-value and zone | Prevents confusing the new layer with the final total | ________ |
| Existing R-value and how it was measured | Shows what is retained and counted | ________ |
| Material and R-value per inch | Lets you check thickness and cost | ________ |
| Installed thickness and area | Confirms the quantity | ________ |
| Air sealing included? | Often priced separately | ________ |
| Baffles and attic access treatment | Protect airflow and the hatch | ________ |
| Price, exclusions and cleanup | Prevents surprise add-ons | ________ |
Read the answers, not just the price
If two quotes differ widely, compare their added R-value, final combined value and thickness first. One may be adding far less than the other. A lower price for a thinner layer is not a bargain on a like-for-like basis.
Ask each contractor to explain how the quoted new layer reaches the stated added R-value and combined total. A clear answer with measurements is a better sign than a confident round number.
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