Why Settling Happens — The Physics
Loose-fill insulation — cellulose or fiberglass — is composed of millions of tiny fibers that interlock with each other like a house of cards. These structures are inherently unstable. Over time, the constant pull of gravity causes fibers to rearrange, filling the air gaps between them. The entire mass compacts downward.
The amount of settling depends on material density:
Loose Cellulose
0.5–0.7 lb/cu ft
10–15% settling
Loose Fiberglass
0.3–0.5 lb/cu ft
5–10% settling
Dense-Pack Cellulose
3.0–3.5 lb/cu ft
<3% settling
💡 Dense-pack magic: At 3.0–3.5 lb/cu ft, dense-pack cellulose is 5–7× denser than loose-fill. Fibers are already fully compressed during installation — there's nowhere left to settle. That's why it barely moves after install.
Settling Timeline — Months Matter
Settling follows an exponential decay curve, not a straight line. 60% of total settling happens in the first 3 months. By 6 months, 90% is done. By 12 months, it's 100% complete. The formula the calculator uses:
For loose cellulose (12-month settling period): 1 month → 63% settled, 3 months → 95%, 6 months → 99.7%, 12+ months → 100%. The calculator uses this exact formula for the most accurate prediction.
How to Measure Your Actual Settled Depth
Grab a wooden ruler or tape measure
Avoid flexible rulers — they'll bend in the insulation.
Push straight down through insulation
Keep the ruler perpendicular to the attic floor, not at an angle.
Stop when it touches the floor
You'll feel a hard stop when it hits the attic deck.
Take 3–5 measurements
Each corner + center spot. Settling can be uneven.
Average the numbers
Add all measurements and divide by count — that's your true settled depth.
When Do You Need to Top Up?
The decision is simple: Is your settled R-value below your zone's target? If yes, top up. If no, you're fine.
Typical topping-up cost: DIY loose-fill = $500–$1,000 for a 1200 sq ft attic. Professional = $1,000–$1,500. Payback is usually 2–5 years from energy savings.
Go to Quick Mode for topping-up calculationFrequently Asked Questions
What is settling and why does it happen?▼
Blown-in insulation is composed of loose fibers that settle under their own weight over time. Loose-fill materials (cellulose, fiberglass) are light — 0.3–0.7 lb/cu ft — and the interlocking fibers gradually rearrange, reducing air voids and overall thickness. Settling is not a defect; it's normal physics that every blown-in insulation install must account for.
How much settling is normal?▼
Loose cellulose settles 10–15% (average 12.5%), completing 90% of that settling in the first 6 months and 100% by 12 months. Loose fiberglass settles 5–10% (average 7.5%), completing fully by 6 months. Dense-pack cellulose settles less than 3% — almost negligible. Batts and spray foam don't settle at all.
Why do I need to account for settling?▼
If you blow exactly 14 inches of loose cellulose (targeting R-49 at R-3.5/inch), it settles to 12.25 inches → only R-42.9 delivered. That's a 13% R-value gap you didn't plan for, and it happens automatically whether you want it or not. Professional installers compensate by blowing 15% extra. DIYers often forget this step.
How do I measure my actual settled depth?▼
Grab a wooden ruler or tape measure, go up into the attic, and push the ruler straight down through the insulation until it touches the attic floor. Do this in 3–5 different spots (each corner + center) and average the numbers. This is your real settled depth — compare it to the nominal install depth your contractor told you.
What's the settling exponential curve?▼
Settling follows exponential decay: 60% of total settling in the first 3 months, 90% by 6 months, 100% by 12 months. This calculator uses the formula: Effective settling % = Total settling % × (1 − e^(−months ÷ material's settlingMonths)). At 1 month, you've seen ~63% of settling; at 3 months, ~95% of loose cellulose settling is complete.
When should I top up settled insulation?▼
Top up when your settled R-value is below your zone's target. If you have a 1200 sq ft Zone 4B attic (target R-49) and measure 12 inches of settled loose cellulose (R-42), you're 7 R-short. Adding 2 more inches (after new settling: 1.7 inches) gets you back to R-49. Typically costs $500–$1,000 for DIY topping up.
Can I calculate settling for spray foam?▼
No need — spray foam (open-cell or closed-cell) doesn't settle. It expands in place, cures rigid, and maintains its installed thickness indefinitely. The settling corrector is for loose-fill materials only: cellulose and fiberglass blown into attics.