Insulation Calculator
Adding insulation is one of the most cost-effective home improvements for energy savings. This insulation calculator figures out how many bags of blown-in insulation you need to reach your target R-value across your total square footage, plus a rough cost estimate.
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Formula
Bags = ceil(Area / Coverage per Bag); Cost = Bags × $45
Total area divided by coverage per bag gives the number of bags needed, rounded up. Coverage per bag is printed on each bag's label and varies by target depth and product. Cost estimate uses $45 per bag as an average market price for blown-in fiberglass or cellulose insulation.
How to use the Insulation Calculator
- 1
Enter your area to insulate (sq ft)
Value should be in sq ft.
- 2
Enter your coverage per bag (sq ft)
Value should be in sq ft/bag.
- 3
Enter your target r-value
- 4
Read your results instantly
Results update in real time as you type.
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Understanding R-Values
R-value measures thermal resistance — the higher the number, the better the insulation. Energy Star recommends R-38 to R-60 for attics in most US climate zones. R-13 to R-21 is typical for exterior walls. Adding insulation above code minimums can reduce heating and cooling costs by 15–25%. The coverage per bag printed on insulation packaging varies by target depth — a bag at R-30 covers more area than the same bag at R-60.
Blown-In vs. Batt Insulation
Blown-in insulation (loose fill fiberglass or cellulose) is ideal for attics, as it fills around obstructions and achieves consistent coverage. Batt insulation (pre-cut fiberglass rolls) is better for walls and floors where you have open stud or joist bays. For existing walls, blown-in is the only practical option without major renovation. Most blown-in products can be rented from hardware stores — the blower comes free with purchase of a minimum number of bags.
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Air Sealing Before Insulating
Insulation slows heat transfer but doesn't stop air movement. Before adding blown-in insulation to an attic, seal all air leaks with caulk or spray foam — around ceiling penetrations, top plates, light fixtures, and chimneys. Air sealing is often more impactful than adding extra insulation depth. The Department of Energy estimates that sealing air leaks reduces energy use by 10–20% in typical homes.
Tips & Insights
Read the Coverage Chart
Every bag of blown-in insulation has a coverage chart on the label showing how many bags are needed per 1,000 sq ft at different R-values. Use that chart over any online calculator for the most accurate estimate.
Rent the Blower for Free
Most home improvement stores lend an insulation blower machine for free when you purchase a minimum number of bags (typically 10–15). This makes DIY attic insulation very economical.
Don't Block Soffit Vents
Maintain at least 1 inch of clearance at all soffit vents when insulating an attic. Use rafter baffles (insulation dams) to keep blown-in insulation from blocking ventilation — blocked soffits cause moisture problems and ice dams.
Worked Examples
Attic Insulation Project
30 bags needed — estimated cost of $1,350 for a 1,200 sq ft attic at R-38.
Small Ranch Home Attic
20 bags — about $900 in material to bring a small home to R-49 attic insulation.
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Frequently Asked Questions
What R-value do I need for my attic?
Energy Star recommends R-38 to R-60 for most US attics. Climate zone 5 and above (northern states) should target R-49 to R-60. Warmer southern zones may only need R-30 to R-38.
How much does blown-in insulation cost?
DIY blown-in insulation typically costs $0.25–$0.50 per sq ft in materials. Professional installation adds $0.50–$1.50 per sq ft in labor. A 1,000-sq-ft attic costs roughly $750–$2,000 professionally installed.
Is blown-in or batt insulation better?
Blown-in is better for attics because it fills gaps around obstructions and settles into a consistent layer. Batt insulation is more convenient for open wall and floor cavities during new construction.
How long does blown-in insulation last?
Fiberglass blown-in insulation lasts 40–80 years under normal conditions. Cellulose lasts 20–30 years and may need top-off applications as it settles. Neither requires replacement unless water damage occurs.
Can I add blown-in insulation over existing insulation?
Yes. You can blow additional insulation directly over existing batts or old blown-in insulation as long as there is no moisture damage or mold. Do not cover recessed lights that aren't rated IC (insulation contact).
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