R-value measures how well insulation resists heat flow — higher is better. But the right number is not "as high as possible." It depends on where you are insulating and your climate, and it is capped by how deep the cavity is. This guide gives the working ranges, then sends you to the insulation calculator for batts or bags.
Recommended R-values by location
The US Department of Energy publishes target ranges by climate zone. These are the common residential ranges:
| Where | Typical R-value | Notes |
|---|---|---|
| Attic | R-38 to R-60 | Highest payback; go higher in cold climates |
| Walls (2x4 / 2x6) | R-13 to R-21 | Limited by stud cavity depth |
| Floors over crawlspace | R-25 to R-30 | Match the batt to the joist depth |
Cold northern zones aim at the top of each range; warm southern zones can sit lower. Your local code minimum is the floor, not the goal.
R-value vs thickness
Each insulation type has an R-value per inch, so thickness and R-value are linked. Fiberglass batts run about R-3 per inch, so an R-13 batt is roughly 3.5 inches (a 2x4 cavity) and an R-19 to R-21 batt fills a 2x6 cavity. Blown-in fiberglass and cellulose are rated by installed depth, which is why attic bags list a coverage area per bag at a target R-value.
Do not compress a thick batt into a shallow cavity to make it fit — compression lowers the real R-value. Use a batt sized for the framing instead.
Batts, rolls, or blown-in?
- Batts and rolls suit open stud and joist bays — walls, accessible floors, and new framing.
- Blown-in loose fill is fast over a large attic and fills around obstructions, but needs a blower (often rented with a bag purchase).
The insulation calculator takes your square footage, form, and target R-value and returns the number of batt packages or blown-in bags, with coverage that already reflects the R-value you pick.
Air-seal first
Insulation slows heat conduction, but air leaks bypass it. Sealing top plates, can lights, and the attic hatch before adding insulation usually does more per dollar than chasing the last few R-value points. Then add insulation to the recommended range for your zone.
FAQ
What R-value do I need for an attic?
Most US attics are recommended at about R-38 to R-60. Colder northern climates target the high end (R-49 to R-60), while warm southern climates can be satisfied near R-30 to R-38. Check your climate zone, since attics are where added R-value pays back the fastest.
What R-value do I need for walls?
Wood-framed walls are typically R-13 to R-21. A 2x4 wall usually holds R-13 to R-15 batts; a 2x6 wall holds R-19 to R-21. You are limited by the cavity depth, so the framing decides the practical maximum.
What R-value do I need for floors?
Floors over unconditioned space such as a crawlspace or garage are usually R-25 to R-30. Match the batt to the joist depth and support it so it stays in contact with the subfloor above.
Is higher R-value always better?
More R-value slows heat flow, but returns shrink as you go up, and the cavity depth caps what fits. The cost-effective move is to hit the recommended range for your climate, air-seal first, and not over-compress batts (compressing them lowers the actual R-value).