Is R19 or R30 better in the attic?

R30 is significantly better than R19 in an attic and is almost always the better choice if you have to choose between the two. R-value measures resistance to heat flow, so higher is better. R19 provides 19 hours of resistance, while R30 provides about 58 percent more resistance to heat loss and gain. The Department of Energy recommends attic insulation levels of R38 to R60 in most of the United States, with R49 being the most common target for new construction. R19 alone is far below code in every climate zone for attics and was only acceptable for floors or crawl spaces decades ago. In practical terms, an attic with only R19 will lose a lot of heat in winter, cause ice dams, and make upstairs bedrooms hot in summer. If you already have R19 in your attic and are wondering whether to add more, you should add R30 on top of it, not replace it. Insulation R-values are additive when you stack layers perpendicularly, so R19 plus R19 cross-hatched or R19 plus R30 will get you to R38 to R49. If you are starting from scratch and your joists are only 2x6, you cannot fit R30 batts properly because R30 batts need about 9.5 inches of depth and will be compressed, losing performance. In that case, use blown-in fiberglass or cellulose to reach R38 to R60. So R30 is better than R19, but neither is enough by itself for a modern attic.