How to Calculate Insulation Depth for a Target R-Value
Insulation depth is simply the target R-value divided by the R-value per inch of your chosen insulation material. For example, to achieve R-38 with blown fiberglass (R-2.2 per inch), you need 38 ÷ 2.2 = 17.3 inches of installed depth. This calculator does that arithmetic for you and also estimates how many bags of blown insulation you will need to cover the area.
Different insulation types have very different thermal resistances per inch. Spray foam provides the highest R-value per inch and also acts as an air barrier, but it costs significantly more than batts or blown materials. For most DIY attic projects, blown cellulose or fiberglass offers the best balance of cost and performance.
DOE Recommended R-Values by Climate Zone
- Zone 1–2 (hot, e.g. Florida, southern Texas): Attic R-30 to R-49.
- Zone 3–4 (mixed, e.g. Virginia, Kansas): Attic R-38 to R-60.
- Zone 5–6 (cold, e.g. Chicago, Denver): Attic R-49 to R-60.
- Zone 7–8 (very cold, e.g. Alaska, northern Minnesota): Attic R-49 to R-60.
- Wall cavities: R-13 for 2×4 studs, R-20 to R-21 for 2×6 studs.
Comparing Insulation Types
The most popular insulation materials for homes each have trade-offs in cost, performance, and installation method. Understanding these helps you make the right choice for your project.
- Fiberglass batts (R-3.14/in): DIY-friendly, cut to fit between studs or joists. Best for unfinished walls and floors.
- Blown fiberglass (R-2.2/in): Rented blower equipment needed; ideal for existing attic floors with obstacles.
- Cellulose — blown (R-3.7/in): Made from recycled paper; excellent for retrofitting existing walls and attics.
- Closed-cell spray foam (R-6.5/in): Highest R-value per inch; also seals air gaps. Requires professional installation in most cases.
Frequently Asked Questions
What R-value do I need in my attic?
The US Department of Energy recommends R-49 to R-60 for attics in most cold and mixed climates, and R-30 to R-49 for warmer regions. Check your local building code or the DOE's Zone Map for the exact recommendation for your location.
How many bags of blown insulation do I need?
Each bag of blown insulation covers approximately 40 square feet when installed to an R-38 rating (the coverage printed on the bag). For different R-values, the bag coverage changes proportionally — more depth for higher R-values means fewer square feet per bag. This calculator uses the 40 sq ft per bag guideline as a starting estimate; always check the coverage chart on the specific bags you purchase.
Can I mix insulation types?
Yes. A common approach is to install batts between joists first, then blow additional cellulose or fiberglass on top to reach the target R-value. R-values from different layers are simply added together, so a layer of R-19 batts plus 5 inches of R-3.7/in cellulose gives R-19 + R-18.5 = approximately R-38.
