Insulation is rated in R-value — the single number that determines how well a given material resists heat flow. Understanding what R-value actually measures, how it scales, and where it matters most helps you make better sense of any insulation project, from a small attic top-up to a whole-house retrofit.
What R-value measures
R-value is a measure of thermal resistance. Higher R-value = more resistance to heat flow. The 'R' stands for 'Resistance.'
Concretely: an R-10 layer of insulation holds back about twice as much heat flow as an R-5 layer of the same area, under the same temperature difference.
The R-value of a particular material relates to its thickness and conductivity. A 6-inch-thick fiberglass batt has roughly the same R-value as 1 inch of closed-cell spray foam — different materials at different thicknesses reach the same R.
How R-values stack
R-values add linearly when layers are stacked. An R-13 batt plus an R-19 batt on top gives you R-32 total. That's important for attic top-ups: you don't have to remove existing insulation to add more — you just stack new on top of old.
For composite walls (drywall + cavity insulation + exterior sheathing), you add the R-value of each layer to get a total wall R-value. The total is what determines heat loss through the assembly.
Where the highest R-value matters most
In a typical US home, the most cost-effective places to add insulation are:
- Attic / ceiling — heat rises; in cold climates, 25 to 35 percent of whole-house heat loss goes through the ceiling. Adding attic insulation is usually the single best-bang-for-buck insulation project.
- Floors over unconditioned spaces (over a crawlspace, garage, or unheated basement) — typically about 15 percent of whole-house heat loss.
- Walls — significant heat loss but harder to retrofit. Easier to address during a major renovation or with exterior insulation.
- Foundation / basement walls — moderate, more about comfort and moisture control than energy loss.
The ceiling is usually first because it's accessible (you can roll or blow insulation over the existing), low cost per R-value, and the heat-loss savings is large. For most homes, going from R-11 to R-49 in the attic is the single highest-ROI upgrade.
What R-value can and can't do
R-value measures conductive heat flow through solid material — the kind where heat passes through insulation. It doesn't measure:
- Convective heat loss — air moving through gaps. A house with R-49 attic insulation but big air leaks around the attic hatch or recessed lights won't get the R-49's worth; the air bypasses the insulation.
- Radiant heat gain — heat radiated by the sun onto a roof or wall. Reflective barriers address this; they have low R-value on their own but can be effective when paired with conventional insulation.
This is why 'air sealing' and 'insulation' are usually discussed together. Sealing air leaks before adding insulation gets you much more value from the insulation.
For more on this, read our weatherstripping and air sealing basics guide.
Recommended R-values
US Department of Energy recommended attic insulation levels by climate zone:
- Hot climates (Zone 1 to 2): R-30 to R-49
- Mixed climates (Zone 3 to 4): R-38 to R-60
- Cold climates (Zone 5 to 8): R-49 to R-60
Most older US homes have R-11 to R-19 in the attic; the recommendation in nearly every climate is to top up to R-49 or higher. That single project typically reduces heating and cooling costs 10 to 20 percent.
How thickness relates to R-value
For any given material, R-value scales roughly linearly with thickness. A 6-inch fiberglass batt is R-19; a 12-inch fiberglass batt is R-38. The relevant spec on the label is 'thickness required for R-XX.'
Different materials have different R-values per inch:
- Fiberglass batt: ~3.0 to 3.5 R/inch
- Cellulose loose-fill: ~3.5 to 3.8 R/inch
- Open-cell spray foam: ~3.5 R/inch
- Closed-cell spray foam: ~6.0 to 7.0 R/inch
- Polyiso rigid board: ~5.5 to 6.5 R/inch
- XPS rigid board: ~5.0 R/inch
- Reflective foil (alone, with air gap): ~1.0 R/inch (but with a 3/4 inch air gap facing a heat source, can block an additional 5 to 10 R-equivalent of radiant heat — see our reflective foil vs fiberglass insulation guide)
Use the calculator
The insulation calculator on the home page takes your climate, heating source, and the R-values you have and target, and estimates annual savings and rough payback. Run a few scenarios (current R-11 to target R-30 vs R-49 vs R-60) before you commit to a material choice or contractor quote.