R-value requirements by climate zone and state

It depends on your DOE climate zone and which part of the house you are insulating. Attic recommendations range from R-30 in zone 1 to R-49 in zones 4 through 7; exterior walls from R-13 to R-21; crawl spaces from R-13 to R-25. Find your state in the table on this page to get your zone, then read across the zone row for the assembly you are insulating.

R-value measures resistance to heat flow through a material. Higher is better, and the values add together, R-19 of existing insulation plus R-30 of new gives you R-49. It measures conduction only, so it says nothing about air leakage, which is frequently the larger problem in an older house.

The Department of Energy defines eight climate zones for the United States. The 50 states span zones 1 through 7. Zone 8 covers only the far northern reaches of Alaska. Zone 1 is Hawaii and southern Florida; zone 7 covers Alaska and the coldest northern interior counties.

Yes, for layers in the same assembly. If you have R-19 in the attic and add R-30 on top, you have R-49 in total. This is why topping up existing insulation is normally more sensible than removing it first, unless the existing material is wet, mold-affected or rodent-contaminated.

Up to a point. Returns diminish sharply, and the numbers make the case better than the principle does: taking a 1,200 sq ft attic from bare joists to R-49 saves about $610 a year, while the last stretch of that same climb, from R-19 to R-49, saves roughly $63. The first few inches eliminate most of the heat flow. Past the DOE recommendation for your zone, the money almost always does more elsewhere, air sealing, duct sealing, or windows.

No. R-19 batts are made for 2x6 framing, and a 2x4 stud bay gives you 3.5 inches. Compressed into that space the batt delivers roughly R-13, so you pay for R-19 and get about a third less, while the squashed edges make gaps at the studs more likely. Use R-13 or R-15 batts, which are sized for the cavity. If you genuinely need a high R-value in 3.5 inches, closed-cell spray foam at R-6.5 per inch reaches R-22.75 in the same bay without being compressed at all.

Rarely, and almost never by depth alone. Most US homes are still well under the Department of Energy recommendation for their zone. Where excess causes real problems is mechanical rather than thermal: insulation packed over the soffit vents blocks the airflow that keeps the roof deck dry, and a second vapor barrier added partway through an assembly traps moisture inside it. Baffles at the eaves and unfaced material solve both. The more common limit is economic, on a 1,200 sq ft attic, closing an R-0 gap to R-49 saves roughly $610 a year while the last stretch from R-19 saves about $63, so past your zone's recommendation the money almost always does more on air sealing.

Not necessarily. The figures here are DOE recommendations for existing homes, not code. New construction follows whichever IECC code cycle your jurisdiction has adopted. Two states set their own targets above the federal recommendation: California through Title 24 and Washington through the Washington State Energy Code.