Insulation Calculator: 12 × 20 ft area
Find how many bags or bundles of insulation you need for an area at a given R-value, with a rough cost.
// Bill of materials
Costs are rough, materials only — edit the price to match your quote.
Pre-filled for a 12 × 20 ft area. Change any value to recalculate, or start from scratch.
How it's calculated
Area = length × width. Each bag or bundle covers less area at a higher R-value, so we divide the area (plus a little waste) by the coverage for your chosen R-value and round up.
Worked example — 12 × 20 ft area
- Area length
- 20 ft
- Area width
- 12 ft
- R-value target
- R-30 (attic)
- Waste allowance
- 5 %
- Price per bag/bundle
- 45 $
Nearby sizes
| Room size | Bags to buy | |
|---|---|---|
| 8 × 8 ft area | 2 bags | Open |
| 8 × 10 ft area | 2 bags | Open |
| 10 × 10 ft area | 2 bags | Open |
| 10 × 12 ft area | 3 bags | Open |
| 10 × 16 ft area | 3 bags | Open |
| 12 × 12 ft area | 3 bags | Open |
| 12 × 16 ft area | 4 bags | Open |
| 12 × 20 ft area (this page) | 5 bags | |
| 16 × 16 ft area | 5 bags | Open |
| 16 × 20 ft area | 6 bags | Open |
| 16 × 24 ft area | 7 bags | Open |
| 20 × 20 ft area | 8 bags | Open |
Tips for buying insulation
- Air-seal gaps and penetrations before you insulate — insulation slows heat, it doesn't stop drafts.
- Don't compress batts to fit; squashed insulation loses much of its R-value.
- Attics benefit most from a deep top-up; R-38 to R-49 is common in cold climates.
- Wear a respirator, gloves and long sleeves — fiberglass and mineral wool irritate skin and lungs.
- Keep insulation clear of recessed lights unless they're rated IC (insulation contact).
- Blown-in fills irregular attics better than batts; batts suit open, regular stud and joist bays.
Frequently asked questions
How much insulation do I need for a 20x20 attic?
A 20×20 attic is 400 sq ft. At R-30 (about 58 sq ft per bag) with a little waste, that's about 8 bags.
Why does higher R-value need more bags?
Higher R-value means thicker product, so each bag covers less floor area. We use realistic coverage per R-value and adjust automatically.
What R-value do I need?
It depends on climate and location — roughly R-13 to R-21 for walls and R-38 to R-49 for cold-climate attics. Check your local energy code.
Batts or blown-in?
Batts suit open, regular bays; blown-in fills irregular attics and hard-to-reach spaces more evenly. This tool estimates by coverage either way.