R-value measures a material's resistance to heat flow. The higher the number, the better the insulation slows heat from passing through it. That's the whole idea in one sentence.
Everything else is detail, but the detail matters when you're buying. R-value is what you're really paying for, and knowing how it stacks up by inch and by space saves you from over-buying or under-insulating. Before you order anything, run your numbers through the insulation calculator so you know your target before you stand in the aisle.
What the number actually means
Heat always flows from warm to cold. In winter it leaks out of your house; in summer it pushes in. Insulation doesn't stop that flow, it slows it, and R-value is the unit that grades how well.
Because R-value is additive, two layers add up. An R-13 batt under an R-6 layer gives you R-19 total. That simple rule is why "topping up" an attic works: you're stacking new R-value on top of whatever is already there.
A few things R-value does not measure: it says nothing about air leaks, moisture resistance, or fire rating. A wall packed with high-R insulation still loses heat fast if air whistles around the gaps. Insulation and air-sealing are two separate jobs, and you want both.
R-value per inch, by material
Materials differ in how much resistance they pack into each inch. These are the working numbers our calculator uses:
- Fiberglass batt: about R-3.1 to R-3.4 per inch (plan on roughly R-3 per inch)
- Blown fiberglass: about R-2.5 per inch
- Blown cellulose: about R-3.5 per inch
- Closed-cell spray foam: about R-6 to R-7 per inch
So closed-cell foam delivers more than twice the R-value of blown fiberglass in the same thickness. That's why foam wins in tight spaces where you can't fit much depth, and why fluffy materials win in open attics where depth is cheap. For a full breakdown of trade-offs, see types of insulation compared.
How much R-value you actually need
The right target depends on where the insulation goes and how cold your climate runs.
- Attics: modern recommendations land at R-38 to R-49, climbing to R-49 to R-60 in cold climates.
- 2x4 walls: about R-13 to R-15, because the stud depth limits how much you can fit.
- 2x6 walls: about R-19 to R-21.
Walls are a hard ceiling. You can't squeeze R-30 into a 2x4 cavity no matter what you buy, because there isn't room for the loft. Attics are where you get the biggest return, since you can pile depth on cheaply. For the climate side of this, see insulation by climate zone.
The mistake that quietly kills R-value
Here's the rule people break: never compress insulation. The loft, all that trapped air, is what actually insulates. Cram an R-19 batt into a space meant for R-13 and you don't get R-19. You get something closer to R-13 or worse, because you crushed the air out of it.
The same goes for stuffing batts behind wiring or stacking boxes on attic insulation. Anywhere the material gets squashed, its R-value drops. Fluff batts out to fill the cavity without packing them, and keep storage off the attic floor.
Measure twice. Buy once.
And before you add a single batt, air-seal first. Caulk and foam the gaps, cracks, and penetrations, then insulate over the top. Insulation laid over leaky framing is like a sweater with the zipper open. A spray foam gun kit handles the bigger gaps around top plates and chases.
Reading the bag
Every bag and roll lists its R-value right on the label, along with a coverage chart telling you how many bags or square feet you need to hit a given R-value at a given depth. Trust that chart over any rule of thumb, because it's specific to that exact product. Two bags of blown fiberglass from different brands can settle to different depths.
The bottom line
R-value is resistance to heat flow, higher is better, and it adds up layer by layer. Pick a target for each space, choose a material that fits the depth you have, never compress it, and air-seal before you start. Plug your dimensions into the insulation calculator and it'll turn your target R-value into the actual bags or batts you need to buy.