Insulation R-Value Guide

R-value measures thermal resistance — the higher the R-value, the better the insulation. What you need depends on your climate zone and where in the house you're insulating.

What is R-value?

R-value is a material’s resistance to heat flow. A wall with R-19 loses heat at roughly half the rate of a wall with R-10. Higher is better — but the location (attic, wall, floor) and your DOE climate zone determine the target.

DOE recommended R-values by zone

Attic insulation

Zone	States	Uninsulated attic	Add to existing
1–2	Hawaii, S. Florida	R-30	R-25 to R-30
3	Texas, Georgia, AZ	R-38	R-25 to R-38
4	Virginia, MO, OR	R-38 to R-49	R-38
5	OH, PA, CO, IL	R-49	R-38 to R-49
6	MN, MI, WI	R-49 to R-60	R-49
7	MT, ND, Alaska	R-60	R-49 to R-60

Wall insulation (new construction)

Zone	Wood-frame wall	Continuous rigid foam
1–3	R-13	Not required
4	R-13 to R-15	R-5
5–8	R-20 or R-13+R-5	R-10

Floor and crawl space

Zone	Over unheated crawl	Crawl space walls
1–3	R-13	R-5
4–6	R-19 to R-25	R-10
7–8	R-30	R-15

R-value by insulation material

Material	R-value per inch	Best for
Blown-in fiberglass	R-2.5	Attic top-up
Blown-in cellulose	R-3.2 to R-3.8	Attic, walls
Fiberglass batt	R-3.1 to R-3.8	Walls, floors
Mineral wool batt	R-3.7 to R-4.2	Walls, fire resistance
Open-cell spray foam	R-3.5 to R-4	Walls, rim joists
Closed-cell spray foam	R-6 to R-7	Crawl spaces, tight areas
Rigid foam (EPS)	R-3.8 to R-4.2	Continuous exterior
Rigid foam (XPS)	R-5	Below-grade, moisture
Rigid foam (polyiso)	R-6 to R-6.5	Roofs, walls

Why R-value matters more in cold climates

Going from an uninsulated attic (R-0) to R-19 saves roughly 50% of heat loss through the attic. Going from R-19 to R-38 saves another 25%. Each additional increment saves less. In Climate Zone 6, the payback period for going from R-38 to R-60 is typically 4–6 years in energy savings.

Use the Insulation Calculator to calculate exactly how many bags or rolls you need to hit your target R-value.

Frequently Asked Questions

What R-value do I need for my attic?

The DOE recommends R-38 to R-60 for attics depending on your climate zone. Most of the continental US (zones 3–6) needs R-38 to R-49. Cold northern states like Minnesota and Montana should target R-60.

Is R-13 or R-15 better for 2×4 walls?

Both fit a 2×4 wall cavity (3.5 inches). R-15 provides about 15% more thermal resistance, which can meaningfully reduce heating costs in colder climates. The price difference is usually small, so R-15 is often worth it in Zones 4–7.

Does higher R-value always mean better insulation?

Higher R-value means more thermal resistance per inch, but diminishing returns apply. Going from R-0 to R-19 saves far more energy than going from R-38 to R-49. Focus on hitting minimum DOE recommendations before over-insulating.

What R-value does spray foam have?

Closed-cell spray foam delivers R-6 to R-7 per inch — the highest of any common insulation. Open-cell spray foam is R-3.5 to R-4 per inch. This makes closed-cell useful in tight spaces like rim joists or crawl space walls where depth is limited.

Can I mix insulation types in my attic?

Yes. A common approach is to lay fiberglass batts between joists to R-19, then blow cellulose or fiberglass on top to reach R-49. This is often the most cost-effective method for deep attic insulation.