The numbers R-19 and R-38 represent thermal resistance. The higher the number, the better the insulation stops heat from moving through it. But R-value alone does not tell you where to use which one.
The short answer: R-19 is for walls and floors. R-38 is for attics. Using R-19 in an attic is the single most common insulation mistake homeowners make, and it leaves a lot of energy savings on the table.
Here is why the difference matters and how to pick the right value for each part of your house.
What R-value actually means
R-value measures thermal resistance per inch of material. Higher resistance means heat moves through more slowly. The number is additive: two layers of R-19 batts stacked together give you R-38.
Different insulation materials provide different R-values per inch:
| Material | R-value per inch |
|---|---|
| Blown cellulose | 3.7 |
| Blown fiberglass | 2.5 |
| Fiberglass batts | 3.1 to 3.7 |
| Rigid foam (XPS) | 5.0 |
| Spray foam (closed cell) | 6.5 to 7.0 |
An R-38 attic using blown cellulose needs about 10.3 inches of material. The same R-38 using blown fiberglass requires about 15.2 inches.
Where R-19 belongs
R-19 is the standard for exterior walls framed with 2x6 studs. A 2x6 stud cavity is 5.5 inches deep. A standard R-19 batt fits exactly, filling the cavity without compressing. Compression reduces R-value, so stuffing an R-19 batt into a 2x4 wall cavity gives you less than R-13.
R-19 also appears in floor systems, specifically in floors over unheated spaces like crawl spaces, garages, and unfinished basements. The joists hang below the floor, creating cavities for batts.
Common R-19 locations:
- Exterior walls with 2x6 framing
- Floors over crawl spaces
- Floors over unheated garages
- Knee walls in finished attic spaces
Where R-38 belongs
R-38 is the DOE recommendation for attic floors in most US climate zones. Heat rises. Attics are the biggest source of heat loss in winter and heat gain in summer. The payback on good attic insulation is faster than almost any other home improvement.
Most attics do not have framing limiting the depth, so you can pile on as much insulation as you want. That is why attic insulation targets R-38 to R-60 while walls are limited to whatever fits in the stud cavity.
Common R-38 to R-60 locations:
- Attic floors in climate zones 3 through 8
- Vaulted ceiling assemblies with deep rafter bays
- Floors over open-air spaces in very cold climates
Recommended R-values by location in your house
| Location | 2x4 wall | 2x6 wall | Attic floor | Floor over crawl |
|---|---|---|---|---|
| Climate zone 1-2 (South, Gulf Coast) | R-13 | R-19 | R-30 to R-38 | R-13 |
| Climate zone 3-4 (Southeast, Mid-Atlantic) | R-13 | R-19 | R-38 | R-19 |
| Climate zone 5-6 (Midwest, Northeast) | R-15 to R-21 | R-21 | R-49 | R-25 to R-30 |
| Climate zone 7-8 (Mountain, Alaska) | R-21 | R-21 | R-49 to R-60 | R-30 |
The cost difference
Adding insulation to an attic floor is cheap per square foot because it is a simple, open space you can DIY with a rented blower. The material difference between R-19 and R-38 of blown cellulose over 1,000 square feet:
- R-19 (6 in of cellulose): roughly 24 bags, around $120 to $150 in materials
- R-38 (10.3 in of cellulose): roughly 48 bags, around $250 to $300 in materials
The extra $130 in materials adds the equivalent of another R-19 on top of what you already have. In cold climates, that investment pays back in reduced heating costs within 2 to 3 years.
Upgrading existing insulation
If your attic already has some insulation, you do not need to remove it first. Measure the depth of what is there, calculate the existing R-value using the table above, and calculate only what you need to add to reach your target.
For example: 4 inches of old fiberglass batts already in place equals about R-13. To reach R-38, you need to add R-25. Using blown cellulose, that is 6.8 more inches of insulation.
Our insulation calculator handles this automatically. Enter the area, target R-value, and any existing insulation depth, and it calculates only the additional material needed, with bag count and cost estimate. For attic-specific guidance including climate zone recommendations, the attic insulation calculator shows recommended R-values for your zone.
Frequently Asked Questions
Can I use R-19 batts in an attic?
Technically yes, but it is a significant under-insulation. The DOE recommends R-38 to R-60 for most US attic floors. Installing R-19 in an attic meets code in only the warmest climate zones (1 and 2). In most of the country, R-19 in an attic floor leaves a large amount of energy savings unrealized and will not pass an energy audit for newer homes.
What happens if I compress R-19 batts into a smaller cavity?
The R-value drops. Insulation works by trapping air in its fibers. Compressing batts reduces that air space and lowers the effective R-value. An R-19 batt compressed to fit in a 3.5-inch 2x4 wall cavity performs closer to R-13. Always match the batt thickness to the cavity depth, or use a product rated for that cavity width.
Do I need to remove old attic insulation before adding more?
Usually no. You can blow new insulation directly over existing batts or loose fill, regardless of type, as long as the existing material is dry and not contaminated with mold or rodent waste. If the old insulation has been wet or is moldy, it must be removed and the area treated before adding new material. Otherwise, simply add the additional depth needed to reach your target R-value on top.
Is spray foam better than blown insulation for attics?
Not for attic floors. Spray foam is more appropriate for the roof deck (creating a conditioned attic) and for air-sealing specific gaps. For an open attic floor where you just want a thick thermal layer, blown cellulose or blown fiberglass is significantly cheaper per square foot and just as effective at the same R-value. Spray foam earns its premium in areas where both air-sealing and insulation are needed in one product.
How can I tell if my attic has enough insulation?
Look at the attic floor joists. If you can see the tops of the joists, you need more insulation: the current depth is under about 6 inches (roughly R-19 or less). If the insulation is level with or above the joists, you are probably at R-30 or more. A proper R-38 to R-49 attic should have insulation significantly above the joists, often 10 to 14 inches deep.
Can I install blown-in attic insulation myself?
Yes, and it is one of the better DIY insulation projects. Most hardware stores offer free blower rental with the purchase of a minimum number of bags (typically 10 bags). The work is straightforward: staple baffles at the eaves to maintain ventilation, seal any penetrations and gaps first, then blow the insulation to the marked depth. The whole job on a typical attic takes 4 to 8 hours.