Costs & R-Value
What R-Value Do I Need?
R-value measures resistance to heat flow: higher is better, and the right target depends on where you live and which surface you are insulating.
Updated January 2026 · about 7 min read
Targets by climate zone
Zones 1–2 (Gulf Coast, south Florida, south Texas): attic R-30 to R-49, walls R-13 to R-15, floors R-13.
Zone 3 (Southeast, southern California): attic R-38 to R-49, walls R-13 to R-20, floors R-19.
Zone 4 (mid-Atlantic, mid-South): attic R-49, walls R-20 or R-13 plus R-5 continuous, floors R-19 to R-25.
Zones 5–6 (Midwest, Northeast, mountain West): attic R-49 to R-60, walls R-20 to R-21 plus continuous foam, floors R-30, basement walls R-15 to R-19.
Zones 7–8 (northern Minnesota, Maine, Alaska): attic R-60, walls R-21 plus R-10 continuous, floors R-38.
R-value to inches
Divide your target by the material's R per inch. R-49 in cellulose (R-3.6/in) is about 14 inches; in blown fiberglass (R-2.5/in) about 19 inches; in closed-cell spray foam (R-6.5/in) about 7.5 inches.
- Fiberglass batt: 3.1–3.8 per inch
- Blown fiberglass: 2.5 per inch
- Blown cellulose: 3.6 per inch
- Mineral wool: 4.2 per inch
- EPS board: 3.9 per inch
- XPS board: 5.0 per inch
- Polyiso board: 6.0 per inch
- Open-cell foam: 3.7 per inch
- Closed-cell foam: 6.5 per inch
Gear for this job
Owens Corning R-19 Unfaced Fiberglass Insulation 77.5 Sq.ft (15 in x 93 in) 1 Batt
Check price on Amazon
Owens Corning R-19 Fiberglass Batt Insulation Fits 2x6 Floor or Walls a Total of 10 Bags and Square Footage of 775 FT
Check price on Amazon
Owens Corning R-19 Unfaced Fiberglass Insulation 128-sq ft (24-in W x 96-in L)
Check price on Amazon
Owens Corning R-19 Faced 23 by 48 Fiberglass Batt Insulation Fits 2x6 Floor or Walls a Total of 10 Bags and Square Footage of 1280 FT
Check price on Amazon
As an Amazon Associate we earn from qualifying purchases. Product links on this page may earn us a commission at no extra cost to you.
Diminishing returns
Going from R-0 to R-13 eliminates about 92% of conductive loss through that surface. R-13 to R-38 picks up another 5%. Beyond R-60 the change is barely measurable. Spend the first dollars on the worst surfaces, not the last inch of an already-good attic.
Effective vs nominal R-value
Wood framing is about R-1.25 per inch, so a wall of R-21 batts between studs performs closer to R-16 overall. Continuous exterior foam is how you close that gap — an inch of polyiso over the sheathing does more than upgrading the cavity batt.
Run the numbers
Frequently asked questions
- What R-value is required by code?
- The IECC sets minimums by climate zone — commonly R-49 to R-60 in attics for zones 4 and up, and R-20 or R-13+5 in walls. Local amendments vary, so check with your building department.
- Can you have too much insulation?
- Financially yes, past about R-60 in an attic. Structurally, only if you block ventilation or overload a ceiling.
- How do I find my climate zone?
- The DOE/IECC map assigns a zone 1–8 by county. Our R-value calculator asks for your zone and returns targets for every surface.
Insulation gear worth owning
The tools and materials that show up on almost every insulation job.
Owens Corning R-19 Unfaced Fiberglass Insulation 77.5 Sq.ft (15 in x 93 in) 1 Batt
Check price on Amazon
Owens Corning R-19 Unfaced Fiberglass Insulation 77.5 Sq.ft (15 in x 93 in) 1 Batt
Check price on Amazon
Multi Purpose XPS Foam Board,2 Pack 15.6x11.7X 2in Polystyrene Craft Foam Sheets,Rigid and Easy to Shape,DIY Modeling Art Projects, Party Decoration, Cosplay Prop Production, Carve,Heat Insulation
Check price on Amazon
Etekcity Infrared Thermometer Laser Temperature Gun 774 | Cooking, Pizza Oven, Griddle, BBQ, Meat, HVAC, Engine, Adjustable Emissivity, MAX Display, Surface Measurement
Check price on Amazon
As an Amazon Associate we earn from qualifying purchases. Product links on this page may earn us a commission at no extra cost to you.