Insulation R-Value Calculator — Thickness & Bags Estimator
Calculate the R-value of your existing insulation, determine how much more you need to meet DOE recommendations, and estimate bags of blown-in or rolls of batt insulation required.
How to Use This Calculator
- Enter the length and width of the area you need to insulate
- Select where you are insulating (attic, wall, floor, etc.)
- Choose your climate zone to get DOE-recommended target R-values
- Enter your existing insulation type and current depth
- Select the material you plan to add
- Click "Calculate Insulation Needed" for depth, bags, and recommendations
How the Calculation Works
current_r = existing_depth_in × r_per_inch[insulation_type]
r_needed = doe_target_r - current_r
added_depth_in = r_needed / r_per_inch[new_material]
area_sqft = length_ft × width_ft
// Blown-in bags: 1 bag covers ~40 sq ft at 3.5" deep (cellulose) or ~48 sq ft at 6" (fiberglass)
bags_needed = ceil(area_sqft / coverage_per_bag[material][depth]) R-value is calculated by multiplying the insulation depth (in inches) by the R-value per inch for that material type. The DOE target R-value minus your current R-value gives the additional R-value needed. Dividing by the R-per-inch of the new material gives the required additional depth. Coverage calculations are based on manufacturer bag coverage charts for blown-in insulation, or roll/batt dimensions for batts.
Variables:
- current_r: Current total R-value of existing insulation
- r_needed: Additional R-value required to meet the target
- added_depth_in: Inches of new insulation needed to achieve target R-value
- area_sqft: Total area to be insulated in square feet
- bags_needed: Number of bags of blown-in insulation required
Pro Tips
- DOE recommends R-49 to R-60 for attic insulation in most of the US — check your climate zone for exact requirements
- Adding insulation is one of the highest ROI home improvements — it typically pays back in 3–7 years through energy savings
- Blown-in cellulose (R-3.7/inch) is excellent for topping off existing attic insulation over batts
- Air sealing before adding insulation is critical — gaps and penetrations cause more heat loss than thin insulation
- Insulation R-values are additive — existing R-11 + new R-30 batts = R-41 total
Recommended Products
Frequently Asked Questions
What R-value do I need in my attic?
The DOE recommends R-49 to R-60 for attics in most US climate zones (Zones 4–8). Warmer climates (Zones 1–3, southern US) need R-30 to R-49. If you currently have 3–4 inches of old insulation (R-11 to R-15), you likely need significant additions to reach modern standards.
What is R-value?
R-value measures an insulating material's resistance to heat flow. Higher R-values mean better insulation. It is additive — stacking R-15 batts plus R-30 blown-in gives R-45. R-value per inch varies by material: fiberglass batts (R-3.1–3.7/in), blown cellulose (R-3.5–3.7/in), spray foam open-cell (R-3.5–3.8/in), spray foam closed-cell (R-6.0–7.0/in).
How many bags of blown insulation do I need for a 1,200 sq ft attic?
To achieve R-38 from scratch using blown fiberglass: approximately 12–13 inches needed. At roughly 40 sq ft coverage per bag at proper depth, you would need around 30 bags. Always follow the coverage chart printed on the bag — it specifies bags needed per 1,000 sq ft at each R-value target.
Can I add insulation over existing insulation?
Yes — you can add blown-in insulation over existing batts in an attic without removing the old material. The R-values are additive. If installing new batts over old fiberglass batts, use unfaced batts to avoid moisture trapping. Never add insulation over wet, damaged, or moldy existing insulation — address moisture problems first.
What is the best insulation for an attic?
Blown-in cellulose or blown fiberglass is ideal for existing attics because it easily fills irregular spaces and installs over existing material. For new construction attics, you can use batts first, then cover with blown-in. In extreme climates or for maximum performance, spray foam on the roof deck creates a conditioned attic space, but is significantly more expensive.