How to calculate insulation
Insulation is estimated by area and the coverage of each bag or roll:
Bags or rolls = area ÷ coverage per bag/roll (rounded up)
For batts, coverage is the square footage one roll or bundle covers. For blown-in insulation, the coverage per bag drops as the R-value rises — a deeper layer covers fewer square feet, so always read the bag's coverage chart for your target R-value.
Recommended R-values by climate
The U.S. Department of Energy sets target R-values by climate zone — colder regions need more. Typical recommendations:
| Climate | Attic | Walls | Floors |
|---|---|---|---|
| Warm (South, zones 1–2) | R-30 to R-49 | R-13 to R-15 | R-13 |
| Mixed (zones 3–4) | R-38 to R-60 | R-13 to R-21 | R-19 to R-25 |
| Cold (North, zones 5–7) | R-49 to R-60 | R-19 to R-21 | R-25 to R-30 |
R-value per inch by type
Which material you pick changes how thick the layer has to be to hit your target R-value:
| Insulation | R per inch |
|---|---|
| Fiberglass batts | ~3.1–3.4 |
| Blown-in fiberglass | ~2.5 |
| Blown-in cellulose | ~3.2–3.7 |
| Mineral wool (rock wool) | ~3.0–3.3 |
| Open-cell spray foam | ~3.5–3.7 |
| Closed-cell spray foam | ~6.0–7.0 |
| Rigid foam board (XPS / polyiso) | ~5.0–6.5 |
Which insulation should you use?
- Fiberglass batts/rolls. Cheapest and most DIY-friendly for open walls, floors, and standard joist spacing — cut to fit snugly between framing with no gaps.
- Blown-in (loose-fill). Fiberglass or cellulose blown over an attic floor; fast coverage that fills awkward gaps. Blowers are often free to rent with a bulk purchase.
- Mineral wool. Denser batts with better fire resistance and sound damping, at a higher price.
- Spray foam. Air-seals as it insulates and gives the most R per inch (closed-cell), but it's a pro job and costs the most.
- Rigid foam board. High R per inch for basement walls, rim joists, and under exterior sheathing.
- Wear protection. Gloves, a mask, and goggles are a must with any insulation.
How much does insulation cost?
Bags or rolls run about $15–$60 each depending on type and R-value. As an installed ballpark, blowing in an attic runs roughly $1–$2 per sq ft DIY and $2–$4 with a pro; fiberglass batts are similar, while spray foam costs several times more. Adding attic insulation is one of the highest-return home upgrades — it pays back in heating and cooling savings.