Insulation Guide: Everything You Need to Estimate
To estimate insulation, multiply the length × width of the surface for the square footage, then multiply by the installed cost per square foot for your insulation type. Fiberglass batts run about $1–$2.40/ft², blown-in $1–$2.50, and spray foam $3–$7. A 600 ft² area in batts costs roughly $600–$1,440.
Get exact numbers
Use the free Insulation Calculator to estimate your project.
How to estimate insulation
Formula: Cost = Length × Width × installed price per ft² (by insulation type)
- Measure the surface. Multiply length × width for walls, or the attic floor area.
- Pick an insulation type. Batts are cheapest; spray foam gives the highest R-value per inch and air-seals.
- Check the R-value target. Aim for R-38 to R-60 in attics and R-13 to R-21 in walls, depending on climate.
- Estimate cost. Multiply the area by the installed price per square foot for your type.
Example: A 30 ft × 20 ft attic = 600 ft². With fiberglass batts at ~$1.50/ft², that's about $900 installed.
Common mistakes to avoid
Under-insulating the attic
The attic is where most heat is lost. Hitting R-38 to R-60 there gives the biggest energy savings.
Ignoring air sealing
Insulation slows heat flow but doesn't stop air leaks. Seal gaps and penetrations first for full performance.
Compressing batts
Squeezing batts into tight cavities lowers their R-value. Use the right thickness for the cavity.
Skipping the vapor barrier
In cold climates, a vapor barrier on the warm side prevents condensation inside walls.